# Dwarfs in nearby voids: results of SALT spectroscopy

S.A. Pustilnik,1 A.Y. Kniazev,2,3,4 A.L. Tepliakova,1 Y.A. Perepelitsyna,1
E.S. Egorova4,5
1 Special Astrophysical Observatory of RAS, Nizhnij Arkhyz,Karachai-Circassia
369167, Russia
2 South African Astronomical Observatory, PO Box 9, 7935 Observatory, Cape
Town, South Africa
3 Southern African Large Telescope Foundation, PO Box 9, 7935 Observatory,
Cape Town, South Africa
4 Sternberg Astronomical Institute, Lomonosov Moscow State University,
Universitetskij Pr. 13, Moscow 119992, Russia
5 Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität
Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany E-mail<EMAIL_ADDRESS>(SAP)

(Accepted December ??, 2023, Received August 20, 2023)

###### Abstract

In the framework of the ongoing project, aimed at the systematical studying
galaxies in nearby voids, we conducted spectroscopy with the Southern African
Large Telescope (SALT) of 62 objects from the Nearby Void Galaxy (NVG) sample.
They include 8 remaining objects of the 60 preselected candidates to eXtremely
Metal-Poor (XMP) dwarfs, two known void XMP dwarfs and 52 void dwarfs residing
within the Local Volume. For 47 galaxies residing in the nearby voids, we
obtained spectra of the diverse quality. For 42 of them, we detected the
Hydrogen and Oxygen lines that allowed us to get estimates of O/H in the
observed Hii regions. For 12 of the 42 objects, we detected the faint line
[Oiii]$\lambda$4363, that allowed us to directly derive the electron
temperature Te and obtain their gas O/H by the direct method. 14 objects with
the undetected [Oiii]$\lambda$4363 line fall to the lowest metallicities range
(12+log(O/H) $\lesssim$ 7.5 dex). For them, we use a carefully checked new
empirical ’Strong line’ method of Izotov et al. For 14 other objects with only
strong lines detected and with 12+log(O/H) of $\sim$7.5–8.0 dex, we used the
modified version of ’semi-empirical’ method of Izotov and Thuan. It accounts
for effect of the excitation parameter O32 on Te. 16 new galaxies are found
with parameter 12+log(O/H) $\lesssim$ 7.39 dex. Of them, four have 12+log(O/H)
= 7.07 – 7.20 dex. Of the 60 observed NVG objects, 15 have mistaken radial
velocities in HyperLEDA. They do not reside in the nearby voids.

###### keywords:

galaxies: dwarf – galaxies: evolution – galaxies: redshifts – galaxies:
abundances – cosmology: large-scale structure of Universe

††pagerange: Dwarfs in nearby voids: results of SALT
spectroscopy–References††pubyear: 2023

##  1 Introduction

Voids represent the most rarefied and the largest volume elements of the
Large-Scale Structure of the Universe. Despite galaxies residing in voids
comprise a relatively small fraction of the whole realm of galaxies (van de
Weygaert, 2016), they are the important ’bricks’ of our picture of the general
galaxy evolution. Thus, Aragon-Calvo & Szalay (2013) predict the unusual
properties of void structures and their galaxies due to the ’time machine and
cosmic microscope’ effects of the void environment.

A more detailed modelling of the void galaxy evolution is necessary to compare
their predicted properties with the observed ones. In particular, one needs
the high resolution simulations in order to address the small dwarfs with the
baryonic mass of 106 – some 108 M⊙. Namely for this mass range, the
observations (see below) demonstrate significant differences with the
properties of galaxies in denser environments.

The effect of environment on evolution is expected to be stronger for the
lower mass objects. Since all known wide-area spectral sky surveys have the
similar limiting apparent magnitude of B (or $r$) of $\sim$18.5, to have a
representative galaxy sample to, e.g., MB = –10 to –12, one should explore the
volume with the distances of R $\lesssim$ 20–25 Mpc. This choice differs
substantially from many studies of void galaxy samples in more distant voids
($\sim$80–200 Mpc), which probe properties of only the upper part of the
galaxy luminosity function (e.g., Rojas et al., 2005; Patiri et al., 2006;
Kreckel et al., 2012, and references therein).

An alternative approach related to the study of galaxies in the nearby voids,
was suggested and partly realised by our group. The first step in this
direction was the formation and studying of the galaxy sample in the nearby
void Lynx-Cancer (Pustilnik & Tepliakova, 2011; Pustilnik et al., 2016, and
references therein). In particular, it was found that void galaxies as a whole
are less evolved in average as compared to the reference sample of the Local
Volume (LV) late-type galaxies of Berg et al. (2012). They have elevated, in
average, ratio of gas mass to luminosity (M(Hi)/LB) (Pustilnik, Martin, 2016)
and the reduced gas metallicity (Pustilnik et al., 2016, 2021).

A surprising result of the above unbiased study of the whole Lynx-Cancer void
sample was the discovery of several low-luminosity dwarfs with the eXtremely
Metal-Poor (XMP) Hii regions and the extremely-high gas mass fraction, fg =
Mgas/Mbaryon, up to 0.97–0.99 (Pustilnik et al., 2010, 2011; Chengalur &
Pustilnik, 2013; Chengalur et al., 2017). Besides, an independently discovered
XMP dwarf AGC198691 (Hirschauer et al., 2016; Aver et al., 2022) appeared to
reside in the same void. We adopt here the definition of XMP galaxies as
objects with 12+log(O/H) $\lesssim$ 7.21 dex, or with Z(gas) $\lesssim$
Z⊙/30111 The solar value of 12+log(O/H) is adopted to be 8.69 dex after
Asplund et al. (2009).. This metallicity is close to that of the prototype of
such unusual galaxies, the famous blue compact dwarf IZw18 (Searle & Sargent,
1972).

In addition, these unusual dwarfs reveal blue colours of the outer parts of a
galaxy (e.g. Perepelitsyna et al., 2014; Chengalur et al., 2017), where, in
the framework of the common paradigm, the most aged stellar population
resides. The colours, being compared to the evolutionary tracks from PEGASE2
package (Fioc & Volmerange, 1999), correspond to the ages of the related
stellar population of about one to several Gyr. That hints, along with the
extremely high gas mass fraction, to the early stages of galaxy evolution.

These properties, in turn, may connect the unusual void dwarfs with the
predicted so-called Very Young Galaxies (VYG) (Tweed et al., 2018), defined as
objects, formed more than a half of stellar mass within the last 1 Gyr. While
the other their properties are not specified in simulations, the first
attempts to find such a rare population are presented by Mamon et al. (2020).
Meanwhile, the comprehensive study of a void XMP dwarf IZw18 and its companion
IZw18C gives a clear indication on their VYG nature (Papaderos & Östlin,
2012).

The seach for and the study of XMP dwarf galaxies was in focus of many groups
since the discovery of the record low metallicity of IZw18. We already
discussed recently the continuing efforts and findings of such unusual
galaxies in the previous papers of this series. Therefore we refer a reader to
these texts. The only remarks we would add to this issue, are the recent
results of the search for new XMP dwarfs within the nearby voids (Pustilnik et
al., 2020b, 2021). They appeared very promising, since the number of known
such objects was doubled thanks to our conducted program during several last
years.

However, one should note that the prominent XMP dwarfs in the local Universe
are occasionally found also in a less rarefied environments. The known
examples include ’Little Cub’ galaxy (Hsyu et al., 2017) and ’Peekaboo’ galaxy
(Karachentsev et al., 2023) with 12+log(O/H) = 7.11 and 6.98 dex,
respectively. This is not clear yet, whether their evolutionary paths differ
from those for the similar objects in voids.

Table 1: Journal of SALT spectral observations. Main sample No | Name | Date | Expos. | PA |  $\theta$″ | Air
---|---|---|---|---|---|---
|  |  | time, s |  |  | mass
1 | J0015+0104 | 2019.10.01 | 2$\times$1200 | 346.0 | 1.6 | 1.22
2 | ESO294-010+ | 2020.12.18 | 2$\times$1200 | 64.0 | 1.4 | 1.21
3 | PGC004055+ | 2022.06.27 | 2$\times$1200 | 66.0 | 1.7 | 1.27
4 | UGC01085+ | 2022.11.22 | 2$\times$1100 | 169.0 | 1.6 | 1.32
5 | AGC124137∗ | 2018.11.10 | 2$\times$1200 | 238.0 | 1.6 | 1.34
| -#- | 2019.09.02 | 2$\times$1200 | 238.0 | 1.5 | 1.35
| -#- | 2019.10.25 | 2$\times$1200 | 238.0 | 1.7 | 1.34
6 | ESO199-007+ | 2021.02.03 | 2$\times$1200 | 13.0 | 1.5 | 1.33
7 | PGC1166738${}^{*}+$ | 2019.09.06 | 2$\times$1150 | 41.5 | 1.6 | 1.22
| -#- | 2019.12.25 | 2$\times$1100 | 41.5 | 1.6 | 1.29
| -#- | 2019.12.29 | 2$\times$1100 | 41.5 | 1.7 | 1.29
8 | PGC013294${}^{*}+$ | 2019.09.01 | 2$\times$1200 | –24.0 | 1.1 | 1.27
9 | PGC712531+ | 2021.07.31 | 2$\times$1200 | 101.5 | 2.0 | 1.22
10 | PGC681755+ | 2022.12.26 | 2$\times$1300 | 51.0 | 1.2 | 1.27
11 | ESO359-024+ | 2023.01.24 | 2$\times$1200 | 255.5 | 1.7 | 1.28
12 | PGC016389+ | 2023.02.22 | 2$\times$1200 | 92.0 | 1.0 | 1.19
13 | HIJ0517-32$\dagger$+ | 2022.01.06 | 2$\times$1200 | 345.0 | 2.3 | 1.31
14 | ESO553-046+ | 2023.01.25 | 2$\times$1250 | 101.0 | 2.0 | 1.27
15 | PGC138836+ | 2021.02.04 | 2$\times$1150 | 161.0 | 2.0 | 1.31
| -#- | 2021.02.07 | 2$\times$1150 | 161.0 | 2.0 | 1.30
16 | PGC018431+ | 2023.04.11 | 2$\times$1200 | 87.0 | 1.8 | 1.23
17 | ESO308-022+ | 2023.02.21 | 2$\times$1200 | 331.0 | 1.9 | 1.28
18 | PGC020125+ | 2022.12.18 | 2$\times$1200 | 70.0 | 1.1 | 1.18
| -#- | 2022.12.26 | 2$\times$1200 | 70.0 | 1.2 | 1.21
19 | J0935-1348 | 2013.12.28 | 3$\times$800 | 0.0 | 1.7 | 1.25
20 | PGC029033+ | 2023.01.21 | 2$\times$1100 | 33.0 | 1.8 | 1.23
21 | PGC041667+ | 2022.04.27 | 2$\times$1100 | 293.5 | 1.5 | 1.27
22 | PGC091215+ | 2022.03.05 | 2$\times$1300 | 5.0 | 1.3 | 1.23
23 | AGC227970+ | 2022.05.03 | 2$\times$1200 | 40.0 | 1.9 | 1.30
24 | AGC225197+ | 2022.03.05 | 2$\times$1200 | 352.0 | 1.3 | 1.27
25 | UGC07983+ | 2022.04.27 | 2$\times$1100 | 5.0 | 1.1 | 1.33
26 | PGC1289726+ | 2022.04.25 | 2$\times$1100 | 38.0 | 1.3 | 1.27
27 | AGC227972+ | 2022.04.26 | 2$\times$1100 | 134.5 | 1.9 | 1.29
28 | AGC227973+ | 2021.04.07 | 2$\times$1100 | 326.0 | 1.4 | 1.28
| -#- | 2021.06.13 | 2$\times$1100 | 326.0 | 1.6 | 1.26
| -#- | 2022.03.09 | 2$\times$1100 | 326.0 | 1.3 | 1.28
29 | AGC226122+ | 2023.01.31 | 2$\times$1100 | 46.5 | 1.6 | 1.29
30 | PGC1264260+ | 2021.02.06 | 2$\times$1100 | 23.0 | 1.6 | 1.24
31 | UGC08055+ | 2022.04.30 | 2$\times$1100 | 97.0 | 1.3 | 1.33
32 | PGC044681${}^{*}+$ | 2023.02.23 | 2$\times$1100 | 30.5 | 1.8 | 1.30
33 | KKH86+ | 2022.05.31 | 2$\times$1100 | 324.5 | 1.3 | 1.26
34 | AGC716018∗ | 2020.03.16 | 2$\times$1150 | 41.0 | 1.6 | 1.30
35 | AGC249197∗ | 2019.05.29 | 2$\times$1150 | 190.0 | 1.0 | 1.36
36 | HIJ1738-57$\dagger$${}^{*}+$ | 2019.05.03 | 2$\times$1150 | 84.5 | 1.2 | 1.23
37 | PGC408791+ | 2021.07.29 | 2$\times$1150 | 279.5 | 1.4 | 1.27
| -#- | 2021.11.06 | 2$\times$1150 | 279.5 | 2.0 | 1.27
38 | PGC129680+ | 2022.04.28 | 2$\times$1100 | 45.5 | 1.0 | 1.29
39 | PGC1016598∗ | 2019.07.11 | 2$\times$1300 | 103.0 | 1.8 | 1.30
40 | ESO289-020+ | 2022.04.28 | 2$\times$1100 | 331.5 | 1.4 | 1.23
41 | ESO238-005${}^{*}+$ | 2019.05.11 | 2$\times$1100 | 47.0 | 1.6 | 1.22
42 | PGC1028063+ | 2022.05.26 | 2$\times$1100 | 300.0 | 1.2 | 1.20
43 | ESO347-017+ | 2022.05.27 | 2$\times$1100 | 90.0 | 1.2 | 1.21
44 | PGC3192333+ | 2020.12.17 | 2$\times$1100 | 265.5 | 1.4 | 1.22
45 | PGC680341+ | 2020.12.15 | 2$\times$1100 | 51.5 | 1.2 | 1.24
46 | ESO348-009+ | 2022.05.31 | 2$\times$1200 | 90.0 | 1.3 | 1.31
47 | ESO149-003+ | 2022.06.06 | 2$\times$1200 | 146.5 | 1.6 | 1.30
+ LV void galaxies; ∗ XMP candidates from PEPK20;
$\dagger$ ’HIJ’ means ’HIPASSJ’

The voids themselves are not simple and homogeneous. They have the fine sub-
structure resembling the Large-Scale Structure of the Universe (e.g. Gottlöber
et al., 2003; Aragon-Calvo & Szalay, 2013). This sub-structure contains nodes,
filaments and sub-voids. Therefore, one expects that properties of void
galaxies can vary, in particular due to the different effects of their in-void
local environments.

The first results, briefly summarised above, appeal to a much more massive
studying of void dwarf galaxies. To address the diversity of void galaxies and
to have an opportunity to compare cosmological simulations with the real void
objects, one needs properties for a sample of many hundreds to thousands
galaxies within the limited volume, unbiased as much as possible.

The natural questions related to such studies are the following: a) how much
properties of void galaxies can differ from those in a more typical
environment? b) how often properties of void galaxies significantly differ
from those of a more common population? c) what empirical relations exist
between the properties of void galaxies and other factors. The answers to
these questions can help us to understand possible differences in formation
and evolution scenarios of void galaxies from those in a more common
environment.

As a development of the observational study of dwarf void galaxies, in order
to increase substantially the statistics of such objects with known
evolutionary parameters, Z(gas) and gas mass fraction, we formed a sample of
25 voids in the volume with R $<$ 25 Mpc from the Local Group. We separate
1354 galaxies within this volume ($\sim$20 percent of the total number known
in this volume) which fall within these nearby voids. We refer to this sample
as the ’Nearby Void Galaxy’ (NVG) sample (Pustilnik, Tepliakova & Makarov,
2019, hereafter PTM19). The sample galaxies were collected mostly from the
HyperLEDA data base (Makarov et al., 2014).

The main parameters of the parent NVG sample are as follows. The range of the
absolute blue magnitudes is of MB = –7.5 to –20.5 mag, with the median value
of about –15.2 mag. The great majority of the NVG sample are the late-type
galaxies (late spirals and dwarf irregulars), with the major fraction of the
low surface brightness (LSB) galaxies. Similar to the wide range of
luminosities, the distribution on the hydrogen mass in void galaxies is also
very broad, with the full range of M(Hi) of 105.5 – 109.7 M⊙, with the median
M(Hi) $\sim$108.5 M⊙. The parameter of gas richness, the ratio M(Hi)/LB of
about a thousand NVG sample galaxies with known Hi data, spreads from 0.03 to
26, with the median of $\sim$1.

In the procedure of separation of void galaxies, all objects, appeared in the
preselected empty spheres, were assigned to the void sample. They were further
divided to the ’inner’ void subsample ($\sim$80 %), and the ’outer’ subsample.
The ’inner’ 1088 galaxies reside in the ’inner’ parts of voids. Their
distances to the nearest luminous neighbours, delineating voids, D${}_{\rm
NN}\geq$ 2.0 Mpc, with the median value of $\sim$3.4 Mpc.

The ’outer’ 242 galaxies have D${}_{\rm NN}<$ 2.0 Mpc, with the median of 1.65
Mpc. The separation of the void subsamples at D${}_{\rm NN}=$ 2.0 Mpc was
chosen to be consistent with the definition of galaxies residing in the nearby
Lynx-Cancer void from our previous study (Pustilnik & Tepliakova, 2011;
Pustilnik et al., 2016). In that work, the border of D${}_{\rm NN}>$ 2.0 Mpc
was intentively chosen so that to study the well defined void objects and to
exclude possible interlopers. The study of both, the ’inner’ and the ’outer’
subsamples, will allow one to understand how many of the ’outer’ void galaxies
represent on their properties an ’intermediate’ kind of the galaxy population
in the context of transition between the lowest density regions and the walls,
with a roughly mean matter density.

We then began two projects based on this sample. One of them is the search for
new XMP void dwarfs. We selected 60 candidate XMP dwarfs from the whole NVG
sample based on their known properties in the open data bases and in the
literature (Pustilnik et al., 2020a). The results of spectroscopy of $\sim$3/4
of them are presented in papers Pustilnik et al. (2020b, 2021).

Another project is devoted to the study of the subsample of the NVG objects
which fall within the LV. This consists of $\sim$260 objects from the NVG at
the distances closer than 11 Mpc. First results on this part were presented in
Pustilnik et al. (2022a, b). This is an analogue of our unbiased study of the
galaxy sample in the nearby void Lynx-Cancer. The northern part of void
galaxies in this project is studied at the SAO 6-m telescope BTA. The detailed
description of the LV void sample, with some intermediate results, will appear
in a paper in preparation.

Here we present the results of the Southern African Large Telescope (SALT;
Buckley, Swart & Meiring, 2006; O’Donoghue et al., 2006) spectroscopy from the
both projects. The first part includes eight of the remaining 14 dwarfs
selected in Pustilnik et al. (2020a) as the void XMP candidates. The second
part includes 49 galaxies selected as residing in voids within the LV, mainly
the least luminous dwarfs. Also, a known XMP dwarf J0015+0104 from the
Eridanus void (Guseva et al., 2009; Pustilnik et al., 2013) was observed to
improve the accuracy of its O/H.

The content of the paper is arranged as follows: the description of the SALT
spectral observations and data processing is presented in Sec. 2. In Sec. 3 we
give the description of emission line measurements and methods used for O/H
determination. The estimates of O/H for the observed galaxies are shown in
Sec. 4. In Sec. 5 we discuss the obtained results along with other available
information. In Sec. 6 we summarise and conclude. In the on-line supplementary
materials we present the following data. In Appendix A we provide finding
charts of all observed galaxies with the spectrograph long slit position
superimposed. In Appendix B, plots of 1D spectra for the observed galaxies are
presented. Appendix C presents the tables with line intensities, derived
physical parameters and the values of 12+log(O/H).

##  2 SALT observations and data processing

For our spectral observations we used the Southern African Large Telescope
(SALT; Buckley, Swart & Meiring, 2006; O’Donoghue et al., 2006) in service
mode in the period from May 2019 to April 2023. Several of the 64 target
galaxies were observed from two to three times in order to increase the S-to-N
ratio of the ’strong’ lines in the potentially very low-metallicity objects.
The obtained spectra were averaged for the subsequent analysis. See Tables 1,
2. The SALT Robert Stobie Spectrograph (RSS; Burgh et al., 2003; Kobulnicky et
al., 2003) was used with VPH grating PG0900 with the long slit of 1.5″ by 8′.
This option provides the range from 3600 Å to 6700 Å and the spectral
resolution of FWHM$\sim$6.0 Å. For the obtained 2D spectra, we used a binning
factor of four for the spatial scale and factor of two for the spectral
coordinate. This gives us a final spatial sampling of
0$\aas@@fstack{\prime\prime}$51 pixel-1 and spectral sampling of 0.97 Å
pixel-1 .

Spectral observations with the sufficiently small entrance hole or with the
narrow slit are the subject of the differential atmospheric refraction (e.g.,
Filippenko, 1982). To minimise its effect on the wavelength-dependent light
loss, the long slit position angle (PA) is recommended to keep close to the
direction of the atmospheric refraction (parallactic angle). In many cases,
this can substantially limit the efficiency of observations. The RSS is
equipped with an Atmospheric Dispersion Compensator (ADC), what allowed us not
to worry about the effect of atmospheric refraction at arbitrary long-slit
PAs. Spectrophotometric standards were observed during every night as a part
of the SALT standard calibrations program.

In order not to repeat literally the description of specifics of SALT
observations and flux calibration, we refer a reader to Section 2 of our
recent paper (Pustilnik et al., 2020b).

The majority of Hii-regions in the observed galaxies are faint and low-
contrast. For the pointing, we used nearby offset stars. In most cases, we
selected position angles (PAs, in degrees) of the slit so that to include a
nearby offset star and to cover a faint Hii-knot in a program galaxy. See the
journals of observation in Tables 1, 2. They present the main information on
each observation: the dates of observations, exposure times, seeing $\theta$
(in arc seconds) and air mass.

Since many of the observed dwarfs are faint and of low surface brightness, an
attempt to obtain their independent spectra can be a problem. To ease
independent checks of our data, we present in Tables 1, 2 the used long slit
position angles (PA). In Figures A1 – A4 of the on-line materials, we show
their images with superimposed long slit positions. All but a few images are
taken from the Legacy surveys database (Dey et al., 2019) The remaining
several images which are unavailable in the Legacy database, are taken from
the ESO Online archive (https://archive.eso.org/dss/dss).

Similar to our previous paper (Pustilnik et al., 2020b), the SALT science
pipelines (Crawford et al., 2010) were used for the primary data reduction.
For each CCD amplifier, they include bias and overscan subtraction, gain and
cross-talk corrections and finally, mosaicing. The following long-slit
reduction was conducted as described in the paper by Kniazev (2022).

Table 2: Journal of SALT spectral observations. Objects with wrong velocities No. | Name | Date | Expos. | PA |  $\theta$″ | Air
---|---|---|---|---|---|---
|  |  | time, s |  |  | mass
1 | PGC3197756+ | 2021.09.10 | 2$\times$1200 | 295.0 | 1.8 | 1.27
2 | PGC901638+ | 2021.08.03 | 2$\times$1200 | 314.5 | 1.3 | 1.23
3 | PGC3207684+ | 2021.10.08 | 2$\times$1150 | 259.0 | 1.3 | 1.22
4 | PGC3210819+ | 2021.08.04 | 2$\times$1200 | 317.0 | 1.8 | 1.22
5 | PGC504827+ | 2021.08.01 | 2$\times$1200 | 282.5 | 1.3 | 1.25
6 | PGC645417+ | 2022.01.23 | 2$\times$1150 | 48.0 | 2.2 | 1.28
7 | PGC016383 | 2023.02.22 | 2$\times$1200 | 92.0 | 1.0 | 1.19
8 | AGC208329+ | 2019.01.06 | 2$\times$1200 | 94.5 | 1.5 | 1.29
9 | J1355+04B$\dagger$ | 2022.05.31 | 2$\times$1100 | 73.0 | 1.0 | 1.31
10 | PGC1069207+ | 2021.04.14 | 2$\times$1100 | 137.0 | 1.2 | 1.21
11 | PGC264615+ | 2021.04.04 | 2$\times$1200 | 21.0 | 1.3 | 1.32
| -#- | 2022.04.12 | 2$\times$1200 | 21.0 | 1.5 | 1.35
12 | PGC064718+ | 2023.04.02 | 2$\times$1200 | 52.0 | 1.4 | 1.31
13 | PGC162688+ | 2021.08.02 | 2$\times$1200 | 107.0 | 1.6 | 1.26
14 | PGC163318+ | 2021.07.17 | 2$\times$1200 | 43.0 | 2.0 | 1.23
15 | 6dFJ2226-2916+ | 2020.11.13 | 2$\times$1200 | 23.0 | 1.8 | 1.21
16 | ABELL3888.14+ | 2021.07.30 | 2$\times$1200 | 337.5 | 1.3 | 1.24
17 | ABELL3888.12+ | 2021.08.03 | 2$\times$1200 | 134.0 | 1.5 | 1.23
+ selected as the LV void galaxies
$\dagger$ Makarov & Uklein (2012) adopt it as a companion of KKH86

##  3 Line measurements and O/H determination

The emission line fluxes obtained from 1D spectra were measured as described
in detail in Kniazev et al. (2004) and briefly summarised in Pustilnik et al.
(2020b). In order not to repeat this description, we refer a reader to these
papers. Very briefly, the procedures include the robust drawing of continuum
with the subsequent use of MIDAS222MIDAS is an acronym for the European
Southern Observatory package – Munich Image Data Analysis System.-based
programs for determination of parameters of emission lines.

Giving the line fluxes measured, we utilise an iterative procedure from Izotov
et al. (1994), which accounts for the total sight-line dust extinction and the
underlying Balmer line absorptions originating in the related young stellar
clusters. As a result, it provides the simultaneous estimate of the equivalent
width of absorption Balmer lines $EW(abs)$ and the extinction coefficient
$C(H\beta)$. The relevant equation (1) from Izotov et al. (1994) was used:

$I(\lambda)/I(H\beta)=[EW_{e}(\lambda)+EW_{a}(\lambda)]/EW_{e}(\lambda)\times$
$EW_{e}(H\beta)/[EW_{e}(H\beta)+EW_{a}(H\beta)]\times$
$F(\lambda)/F(H\beta)~{}exp[C(H\beta)f(\lambda)]~{}~{}~{}~{}~{}~{}~{}~{}(1)$

Here $I(\lambda)$ is the intrinsic line flux corrected for the overall
extinction (both in the Milky Way and internal to a particular galaxy or Hii
region) and the underlying Balmer absorption, while $F(\lambda)$ is the
measured line flux. Here, as in Izotov et al. (1994), $EW_{e}(\lambda)$ are
equivalent widths of used emission lines. $EW_{a}(\lambda)$ is the adopted
value of the underlying Balmer absorptions. The theoretical Balmer line ratios
I($\lambda$)/I(H$\beta$) for Case B from Brocklehurst (1971) were used for
iterative procedure with equation (1).

Following to Izotov et al. (1994), this term is used to estimate intrinsic
fluxes only for Balmer emission lines. Similar to this paper, we adopt the
reddening function $f(\lambda)$ from Whitford (1958). It is normalised so that
$f(H\beta)$ = 0. For this $f(\lambda)$, there is a relation between the excess
E($B-V$) and $C(H\beta)$: E($B-V$) = 0.68 $\times$ $C(H\beta)$.

In the classic (Te) method of the oxygen abundance determination, they use the
standard two-zone model and method of Aller (1984). Since the electron
temperature Te is different in high and low-ionisation Hii regions (e.g.
Stasińska, 1990), to calculate abundances of ions O++ and O+, one needs to
know temperature Te in both parts of an Hii region. The estimate of Te(O++)
uses the flux ratio of [Oiii] lines: I($\lambda$4363)/I([$\lambda$4959 +
$\lambda$5007) and the five-level atom model (Aller, 1984), with the electron
density Ne derived from the line ratio of the [Sii] doublet:
I($\lambda$6717)/I($\lambda$6731). Since in our spectra this doublet was
outside the range, we adopted for further calculations N${}_{\rm e}=$ 10 cm-3,
a typical electron density in Hii regions of dIrr galaxies. To derive Te(O+),
we used the fit for the relation between Te(O+) and Te(O++) from Izotov et al.
(1994), based on the models of photoionised Hii regions by Stasińska (1990).
This fit is read as follows:

te(Oii) = 0.243 + te(Oiii)[1.031 - 0.184 te(Oiii])] (2)

Here te = 10-4 Te. The resulting rms uncertainty of te(Oii) is transferred
from the corresponding rms error of te(Oiii) as derived with use of the above
relation. The ionic abundances of O+/H+ and O++/H+ are calculated with
formulae (3) and (5) from Izotov et al. (2006).

The major part of Hii regions in the studied galaxies are rather faint.
Accordingly, the fluxes of their emission lines are low. The weak auroral
line, [Oiii]$\lambda$4363 Å, which is used in the ’direct’ (Te) method for the
determination of the electron temperature Te, was detected only in the
minority of our targets. Therefore, for the estimate of O/H in the remaining
galaxies, we applied the semi-empirical method of Izotov & Thuan (2007).
However, as was shown in Pustilnik et al. (2021), this method provides the
reliable estimates of Te in rather limited range of parameters of Hii regions.
We modified their method, as described in Pustilnik et al. (2021), in order to
expand its applicability to the wider range of physical conditions.

The basement of method of Izotov & Thuan (2007) is the fitted empirical
dependence between the values of the electron temperature Te and parameter
$R_{\mathrm{2}3}$. It was derived from the analysis of the grid of models of
Hii-regions in Stasińska & Izotov (2003). As the authors find, for the large
representative sample of extragalactic Hii-regions, which cover the whole
range of observed O/H, the models approximate well the apparent relations of
the strong line intensities versus EW(H$\beta$). The parameter
$R_{\mathrm{2}3}$ is the ratio of the sum of fluxes of strong oxygen lines
[Oii]$\lambda$3727 Å, [Oiii]$\lambda$4959 Å, [Oiii]$\lambda$5007 Å to that of
H$\beta$. When Te is estimated via $R_{\mathrm{2}3}$, the rest of the
calculations of the ionic abundances of O+/H+ and O++/H+ uses the standard
equations of the classic Te method, that is the same formulae (3) and (5) from
Izotov et al. (2006).

As it became clear later, the good approximation of O/H, derived by this
method, to O/H values, obtained by the direct method, has the limited
application. This is because it was fitted from a sample of Hii-regions with a
rather limited range of the excitation parameter $O_{\mathrm{3}2}$ (the ratio
of line fluxes of [Oiii]$\lambda$5007 and [Oii]$\lambda$3727). This parameter
is a proxy of the physical parameter $U$ \- so-called the ionisation
parameter, characterising the flux of ionising photons per unit area of the
illuminated gas cloud surrounding the central source. While the original
method of Izotov & Thuan (2007) works well for the intermediate values of
$O_{\mathrm{3}2}$ ($\sim$ 2–6), it leads to the significant systematical bias
in the Te and O/H estimates at both, small and large values of
$O_{\mathrm{3}2}$. The whole range of $O_{\mathrm{3}2}$ in known Hii regions
spreads from $\lesssim$0.3 to $\sim$50.

This effect was examined in Pustilnik et al. (2021) on the large sample of
Hii-regions with O/H derived via the direct method. The sample covers the
broad range of parameter $O_{\mathrm{3}2}$ and O/H. The modified formula was
suggested to estimate Te, which includes the dependence of Te on the both
parameters, $R_{\mathrm{2}3}$ and $O_{\mathrm{3}2}$. The respective method is
called the modified semi-empirical (mse). The mse method has the moderate
internal scatter in log(O/H) of 0.09 dex, while allows one to eliminate the
systematic bias of the original method of Izotov & Thuan (2007) for the
extreme values of parameter $O_{\mathrm{3}2}$.

This is crucial for our project, since the typical Hii-regions in our void
galaxy sample, in which the auroral line [Oiii]$\lambda$4363 was too noisy or
undetected, have rather low excitation parameter, of $O_{\mathrm{3}2}\lesssim$
1.0. The O/H estimates derived by this method are marked as (mse) in Column 9
of Table 3.

Recently Izotov et al. (2019) suggested a new empirical method, the best
suited for Hii-regions with ’metallicity’ 12+log(O/H) $\lesssim$ 7.4 dex. This
uses only the relative fluxes of strong Oxygen lines with respect of H$\beta$.
Namely, their Equation (1) reads as:

$12+\log({\rm O/H})=0.950\times\log(R_{\mathrm{2}3}-0.08\times
O_{\mathrm{3}2})+6.805$

This relation, calibrated on the large number of Hii-regions with O/H derived
via the direct (Te) method, empirically accounts for the large scatter in the
ionisation parameter log(U) (from about –4 to –1) in various Hii-regions and,
thus, reduces the relatively large internal rms scatter of the other empirical
methods based on the strong oxygen lines, down to only $\sim$0.04 dex. The
latter value of the rms scatter was obtained in Pustilnik et al. (2021) on the
sample of $\sim$70 Hii-regions with the range of 7.0 $\lesssim$ 12+$\log$(O/H)
$\lesssim$ 7.5 dex. However, its applicability is limited only to the range of
12+$\log$(O/H) $\lesssim$ 7.5. The above relation was slightly modified by
Izotov et al. (2021) to account Hii regions with the highest observed values
of O32.

Among our galaxies there are 16 objects falling to this category. For them, we
use this method. For derived this way value of O/H, we attach index (s) in
Column 9 of Table 3, and denote them hereafter as O/H(s).

As shown in Pustilnik et al. (2021), there is a tiny offset in the zero-point
of 12+log(O/H)(s) relative to that of 12+log(O/H)(Te), of $\sim$0.01 dex. In
Table 3, we present therefore the values of 12+log(O/H)(s) with the
subtraction of 0.01 dex from the values obtained with the formula from Izotov
et al. (2019). This allows us to compare them directly with the estimates of
12+log(O/H)(Te) for other galaxies.

The contribution of errors in the strong line fluxes to the related
uncertainty of 12+log(O/H)(s), $\sigma_{\rm log(O/H)}$, is rather modest. For
the high S-to-N spectra with $\sigma_{\rm R23}$/$R_{\mathrm{2}3}<$0.02, the
total $\sigma_{\rm log(O/H)}$ is close to the internal error of the method,
namely, $\sigma_{\rm log(O/H)}\sim$0.04 dex. For the lowest S-to-N our spectra
of $\sigma_{\rm R23}$/$R_{\mathrm{2}3}\sim$0.14, the error of $\log$(O/H)(s)
increases to 0.12 dex. Intermediate S-to-N of $R_{\mathrm{2}3}$ results in the
typical $\sigma_{\rm log(O/H)}$ = 0.05 – 0.06 dex.

Table 3: Observed nearby void dwarfs and new O/H data No. | Name | J2000 Coord | Vorig | VSALT | D$\dagger$ | Bt | $M_{\rm B}$ | 12+$\log$(O/H) |  Notes
---|---|---|---|---|---|---|---|---|---
|  |  | km s-1 | km s-1 | Mpc | mag | mag |  $\pm$err. |
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10
1 | J0015+0104 | J001520.7+010437 | 2055$\pm$02 | 2141$\pm$15 | 28.8 | 18.31 | –14.32 | 7.14$\pm$0.04 d+s |  prototype XMP
2 | ESO294-010 | J002633.4–415120 | 113$\pm$04 | 84$\pm$20 |  $\dagger$2.0 | 15.56 | –10.90 | 7.79$\pm$0.12 mse |
3 | PGC004055 | J010822.0–381233 | 654$\pm$60 | 633$\pm$10 | 6.8 | 16.09 | –13.09 | 7.60$\pm$0.10 mse+d |
4 | UGC01085 | J013118.9+074716 | 652$\pm$02 | 680$\pm$07 | 10.0 | 16.75 | –13.44 | 7.87$\pm$0.11 mse |
5 | AGC124137∗ | J023137.0+093144 | 897$\pm$06 | 900$\pm$10 | 14.0 | 17.98 | –12.10 | 7.20$\pm$0.12 s+mse |  low S-to-N
6 | ESO199-007 | J025804.1–492256 | 630$\pm$05 | 623$\pm$15 |  $\dagger$6.0 | 16.50 | –12.49 | 7.33$\pm$0.06 s |  aver. on 2 knots
7 | PGC1166738∗ | J030646.9+002811 | 710$\pm$04 | 728$\pm$20 | 11.0 | 18.41 | –12.27 | 7.19$\pm$0.04 s |  aver. on 2 knots
8 | PGC013294∗ | J033556.8–451129 | 737$\pm$42 | 750$\pm$10 |  $\dagger$7.3 | 16.07 | –13.29 | 7.90$\pm$0.06 d |
9 | PGC712531 | J033903.0–304921 | 839$\pm$123 | 692$\pm$10 | 9.5 | 18.29 | –11.96 | … |  only faint H$\alpha$
10 | PGC681755 | J033955.0–330309 | 747$\pm$26 | 705$\pm$15 | 9.5 | 16.55 | –13.39 | 7.71$\pm$0.11: mse |  low O32 $\sim$0.25
11 | ESO359-024 | J041057.5–354951 | 847$\pm$05 | 815$\pm$30 | 10.9 | 15.43 | –14.80 | 7.44$\pm$0.13 d |
12 | PGC016389 | J045658.7–424802 | 662$\pm$06 | 710$\pm$12 |  $\dagger$7.0 | 14.46 | –14.80 | 7.20$\pm$0.05 s |  $\sim$7.0 in O32$\sim$0.2
13 | HIPASSJ0517-32 | J051721.6–324535 | 798$\pm$06 | 811$\pm$06 | 10.7 | 15.96 | –14.81 | 7.93$\pm$0.05 d |  aver. 2 knots
14 | ESO553-046 | J052705.8–204040 | 542$\pm$06 | 538$\pm$05 |  $\dagger$6.7 | 14.72 | –14.71 | 8.03$\pm$0.03 d |
15 | PGC138836 | J055735.2+072913 | 428$\pm$04 | 407$\pm$15 |  $\dagger$5.5 | 18.40 | –11.28 | 7.56$\pm$0.10 mse |
16 | PGC018431 | J060719.7–341216 | 774$\pm$13 | 816$\pm$07 |  $\dagger$9.6 | 15.66 | –14.34 | 7.65$\pm$0.06 d |  aver. 2 knots
17 | ESO308-022 | J063932.9–404317 | 822$\pm$04 | 888$\pm$11 |  $\dagger$9.4 | 16.22 | –14.03 | 7.34$\pm$0.05 s |
18 | PGC020125 | J070517.7–583108 | 564$\pm$04 | 489$\pm$05 |  $\dagger$5.3 | 14.95 | –14.16 | 7.37$\pm$0.05 s |  aver. 2 knots
19 | 6dFJ0935-1348 | J093521.6–134852 | 800$\pm$44 | 877$\pm$22 | 11.7 | 16.43 | –14.10 | 7.28$\pm$0.07 s |
20 | PGC029033 | J100138.2–081455 | 446$\pm$03 | 258$\pm$28 | 7.0 | 15.41 | –14.07 | 8.01$\pm$0.15 mse |  V(HI)-V(opt)$\sim$190
21 | PGC041667 | J123307.9–003158 | 740$\pm$09 | 737$\pm$37 | 10.2 | 16.50 | –13.63 | 7.84$\pm$0.14 mse |
22 | PGC091215 | J123655.0+013654 | 590$\pm$01 | 623$\pm$12 | 6.1 | 16.35 | –12.65 | 7.37$\pm$0.05 s |  aver. on 3 knots
23 | AGC227970 | J124601.4+042252 | 643$\pm$04 | 625$\pm$10 | 9.2 | 16.00 | –13.93 | 7.71$\pm$0.08 d |
24 | AGC225197 | J124942.1+052922 | 739$\pm$18 | 820$\pm$30 | 10.1 | 18.38 | –11.77 | … |  abs. spectrum
25 | UGC07983 | J124947.0+035033 | 694$\pm$02 | 743$\pm$20 | 10.1 | 16.11 | –14.08 | 8.03$\pm$0.14 mse |
26 | PGC1289726 | J124959.2+054916 | 618$\pm$01 | 600$\pm$10 | 9.1 | 16.76 | –13.17 | 7.68$\pm$0.11 mse |
27 | AGC227972 | J125024.0+045422 | 650$\pm$04 | 647$\pm$20 | 9.1 | 19.50 | –10.46 | … |  only H$\alpha$ in 2 knots
28 | AGC227973 | J125039.9+052052 | 675$\pm$04 | 695$\pm$15 | 9.1 | 19.50 | –10.42 | 7.07$\pm$0.04 s |  aver. on 2 measur.
29 | AGC226122 | J125215.4+042727 | 700$\pm$04 | 756$\pm$15 | 10.0 | 18.33 | –11.86 | … |  only H$\alpha$
30 | PGC1264260 | J125343.3+040914 | 761$\pm$01 | 810$\pm$12 | 10.0 | 17.45 | –12.69 | 7.58$\pm$0.08 d |
31 | UGC08055 | J125604.4+034846 | 616$\pm$02 | 622$\pm$18 | 6.7 | 15.94 | –13.34 | 7.79$\pm$0.11 mse |
32 | PGC044681∗ | J125956.6–192441 | 827$\pm$02 | 796$\pm$15 |  $\dagger$7.3 | 17.00 | –12.73 | 7.34$\pm$0.08 s |  check earlier data
33 | KKH86 | J135433.4+041440 | 286$\pm$02 | 298$\pm$30 |  $\dagger$2.6 | 17.08 | –10.14 | … |  only Bal. absorb.
34 | AGC716018∗ | J143048.7+070926 | 1365$\pm$04 | 1415$\pm$12 | 18.1 | 18.18 | –13.22 | 7.93$\pm$0.14 mse |
35 | AGC249197∗ | J144950.7+095630 | 1809$\pm$05 | 1822$\pm$10 | 24.2 | 18.69 | –13.35 | 7.39$\pm$0.06 s |
36 | HIPASSJ1738-57∗ | J173842.9–571525 | 858$\pm$08 | 815$\pm$13 | 7.3 | 16.62 | –13.12 | 7.72$\pm$0.11 mse |  7.77$\pm$0.18 (d)
37 | PGC408791 | J202608.8–552950 | 893$\pm$67 | 812$\pm$10 | 6.8 | 16.88 | –12.71 | 7.39$\pm$0.05 s |
38 | PGC129680 | J210804.8–471942 | 860$\pm$33 | 858$\pm$12 | 7.7 | 15.69 | –13.90 | 8.00$\pm$0.10 mse |  aver. 2 knots
39 | PGC1016598∗ | J213902.9–073443 | 1283$\pm$02 | 1305$\pm$12 | 15.4 | 18.63 | –12.43 | 7.23$\pm$0.04 s |  aver. 2 knots
40 | ESO289-020 | J222111.7–454035 | 912$\pm$67 | 892$\pm$06 | 8.8 | 15.62 | –14.17 | 7.59$\pm$0.05 mse |  aver. 3 knots
41 | ESO238-005∗ | J222230.5–482414 | 706$\pm$03 | 714$\pm$04 |  $\dagger$8.0 | 15.30 | –14.28 | 7.35$\pm$0.04 s |
42 | PGC1028063 | J224223.4–065010 | 899$\pm$06 | 861$\pm$06 | 10.7 | 16.02 | –14.28 | 7.79$\pm$0.09 d |
43 | ESO347-017 | J232656.2–372049 | 691$\pm$03 | 698$\pm$06 | 6.5 | 14.93 | –14.20 | 7.82$\pm$0.03 d |  aver. 3 knots
44 | PGC3192333 | J234133.8–353023 | 630$\pm$89 | 524$\pm$18 | 6.6 | 18.86 | –10.31 | 7.26$\pm$0.08 d |  7.18$\pm$0.05 (s)
45 | PGC680341 | J234147.5–330841 | 510$\pm$89 | 446$\pm$10 | 4.4 | 16.59 | –11.69 | 7.34$\pm$0.04 d |  7.29$\pm$0.04 (s)
46 | ESO348-009 | J234923.4–374622 | 647$\pm$02 | 587$\pm$15 | 6.1 | 14.81 | –14.18 | 7.42$\pm$0.05 s |
47 | ESO149-003 | J235202.8–523438 | 574$\pm$04 | 526$\pm$09 |  $\dagger$7.0 | 15.09 | –14.20 | 7.66$\pm$0.03 d |
Table 3 content is described in detail in Sect. 4.1. Here we give the brief
information. Col. 2: target name from NVG.
Col. 3: galaxy coordinates adopted from NVG. Col. 4: original radial velocity
with its error, in km s-1. Col. 5: SALT radial velocity
with its error; Columns 6, 7 and 8: the adopted distance, total blue magnitude
and absolute blue magnitude. Col. 9: derived O/H
as 12+$\log$(O/H) and its error, in dex, with indication of the method used:
(d) – the direct (Te) method; (mse) – the modified
semi-empirical method of Izotov & Thuan (2007), accounting for the large range
of excitation (Pustilnik et al., 2021);
(s) – the new empirical strong line O/H estimator of Izotov et al. (2019),
with subtracted 0.01 dex to account for a small offset
relative to O/H(Te). In Col. 10 we show brief notes with more detailed
information, when necessary, presented in Sec. 4.1.1, 4.1.2.
∗ \- from list of 60 XMP candidate in PEPK20; $\dagger$ marks velocity-
independent distances obtained with the TRGB method at HST.
Table 4: SALT results for objects with wrong velocities No. | Name | J2000 coord. | Vorig | VSALT | B-mag | MB | Notes
---|---|---|---|---|---|---|---
|  |  | km s-1 | km s-1 |  |  |
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8
1 | PGC3197756$\dagger$ | J001736.3–312618 | 749$\pm$123 | 2703$\pm$10 | 19.28 | –13.64 | doubtful, too narrow H$\alpha$
2 | PGC901638 | J002951.4–160954 | 724$\pm$59 | z = 0.092 | 17.74 | –20.30 | abs. spectrum
3 | PGC3207684 | J020140.1–291749 | 660$\pm$123 | z = 0.109 | 19.45 | –18.90 |
4 | PGC3210819 | J022308.8–295233 | 779$\pm$89 | z = 0.16274 | 18.85 | –20.30 |
5 | PGC504827$\dagger$ | J024024.4–471257 | 812$\pm$29 | 1828$\pm$15 | 19.24 | –12.68 | only faint H$\alpha$
6 | PGC645417$\dagger$ | J032733.9–354303 | 723$\pm$79 | 9942$\pm$20 | 16.84 | –18.86 | only H$\alpha$
7 | PGC016383 | J045701.2–424803 | 3443$\pm$61 | 2953$\pm$10 | 15.48 | –17.30 | on slit with PGC016389
8 | AGC208329 | J101531.9+033508 | 1018$\pm$04 | 1370$\pm$10 | 19.42 | –12.1: | 12+log(O/H)(mse)=7.82. see text
9 | [MU2012]J1355+04B | J135429.5+041237 | … | 4831$\pm$9 | 19.5: | –14.6: | 12+log(O/H)(mse)=7.56. see text
10 | PGC1069207$\dagger$ | J143011.3–033552 | 690$\pm$123 | 988$\pm$9 | 18.34 | –12.50 | New D = 12.2 Mpc
11 | PGC264615 | J161354.9–721446 | 383$\pm$11 | z = 0.0695 | 17.94 | –19.3 | LINER?
12 | PGC064718 | J202733.8–550525 | 831$\pm$52 | 10710$\pm$35 | 15.24 | –21.2 | ……
13 | PGC162688 | J204847.3–121654 | 843$\pm$45 | 5860$\pm$12 | 17.12 | –17.6: | ……
14 | PGC163318$\dagger$ | J213935.2–401653 | 1034$\pm$59 | z = 0.0645 | 17.25 | –19.95 | with close E-gal at z = 0.0630
15 | 6dFJ2226336-291728 | J222633.6–291728 | 1025$\pm$45 | z = 0.315 | 19.57 | –21.0: | AGN Sy1.5
16 | ABELL3888$\\_$14:[PSE2006]1816 | J223329.3–372731 | 1008$\pm$19 | –166$\pm$10 | 18.49 | … | star-like
17 | ABELL3888$\\_$12:[PSE2006]2246 | J223633.3–373005 | 760$\pm$13 | z = 0.388 | 18.80 | –22.3: | star-like
$\dagger$ grism PG2300, range $\sim$6060–6880 Å |

##  4 Results of spectral observations and O/H estimates

###  4.1 Void galaxies

The 1D SALT spectra of the observed void dwarfs are presented in the on-line
supplementary materials in Appendix B, Figs. B1–B4. The measured fluxes of
emission lines and their derivatives: EW(abs) – the adopted equivalent width
of Balmer absorption in the underlying stellar continuum, the extinction
coefficient C(H$\beta$), and the equivalent width of the H$\beta$ emission
line EW(H$\beta$) are presented in Tables C1-C14 of the on-line supplementary
materials in Appendix C. Some of the obtained spectra, show rather strong
Balmer absorptions in the UV. In these cases the underlying continuum and
Balmer absorptions were fitted by a model with the ULySS package
(http://ulyss.univ-lyon1.fr, Koleva et al., 2009). This model model spectrum
corrected the flux of H$\beta$ emission to a first approximation. In these
cases, the EW(abs) derived with the procedure from Izotov et al. (1994) at the
next step, appears the residual EW(abs), since this is mainly accounted for by
the ULySS fitting. As mentioned in Section 2, the absolute flux calibration at
SALT is rather uncertain. Therefore, we do not provide the absolute flux in
the emission H$\beta$.

The derived physical parameters — Te in the zones of emission of [Oiii] and
[Oii], the relative numbers of ions O+, O++ and the total abundance of Oxygen
relative to Hydrogen, O/H — are shown in the bottom of these tables. As
described in Section 3, Te(O++) is calculated with the direct method when the
faint auroral line [Oiii]$\lambda$4363 is detected. In the remaining cases it
is estimated with the modified semi-empirical method from Pustilnik et al.
(2021), based on the semi-empirical method of Izotov & Thuan (2007). As
explained in that section, Te(O+) was calculated via the relation (2), adopted
from Izotov et al. (1994).

Since the [Sii]$\lambda\lambda$6717,6731 was outside the range of SALT
spectra, the estimate of electron density Ne in the observed Hii regions was
impossible. We adopted for further calculations of O/H in the direct and mse
methods Ne = 10 cm-3 as a typical value for Hii regions in the low-mass late-
type galaxies.

For each galaxy, we present the derived parameter 12+$\log$(O/H). For majority
of the obtained spectra, this is derived either with the direct method (for
the twelve objects, where it is applicable), or with the empirical strong-line
method of Izotov et al. (2019) (for 15 the lowest O/H objects). For the
remaining 12 objects, with the higher values of O/H, we apply a modified semi-
empirical (mse) method, in which we account for the dependence of the
empirically derived Te on the excitation parameter $O_{\mathrm{3}2}$ (see
details in Pustilnik et al., 2021).

We note that 12+$\log$(O/H), derived with the mse method, needs a small
correction (up to 0.03 dex, depending on the value of O/H) in order to make
its zero-points consistent with that for the direct Te method. See Appendix in
Pustilnik et al. (2021) for details. This correction is already applied in
Tables C1–C15 and the respective value of O/H is indicated as (mse,c). It is
reasonable to add a caution on the applicability of mse method for the lowest
excitation Hii regions. As discussed in the Appendix of Pustilnik et al.
(2021), the fitting formulae for deriving Te via this method, are limited by
the lower values of parameter $O_{\mathrm{3}2}$. For the range of
12+$\log$(O/H) $>$ 7.5 dex, this lower limit of $O_{\mathrm{3}2}$ is
$\sim$0.4, while for the range of 12+$\log$(O/H) $<$ 7.5 dex, this limit is
$\sim$0.5. For several our galaxies with the lowest excitation, the used
formulae are slightly extrapolated outside these limits. For these objects,
the uncertainty of the derived value of 12+log(O/H) can be somewhat
underestimated.

Finally, where it is suitable, just to demonstrate the consistency of our
estimates with other popular methods, we provide the value of O/H, derived
with the empirical estimator of Pilyugin & Thuan (2005) (lower branch), which
is based only on the flux ratios of the strong Oxygen lines. Their formulae
also account for parameter of excitation $O_{\mathrm{3}2}$, which enters to
their parameter ’P’. Their formulae are obtained via the fitting of the
observational data for Hii regions with the direct value of 12+$\log$(O/H).
However, the applicability of their formulae is limited by the range of
parameter ’P’ of 0.55 and 0.97. This translates to the range of
$O_{\mathrm{3}2}$ of 0.92 to 24.2. Therefore, for our galaxies with the lowest
values of $O_{\mathrm{3}2}$, we do not show values of O/H (PT05) due to their
systematic shifts.

In Table 3 we summarise the adopted parameter 12+log(O/H) and provide some
other parameters of the studied galaxies. The content of the column is as
follows: Col. 1 – the number of galaxy, as it appears in Table 1; Col. 2 – the
galaxy name as adopted in the NVG catalog, which, in turn, is mainly from the
HyperLEDA database333http://leda.univ-lyon1.fr; Col. 3 – J2000 epoch
coordinates; Col. 4 – the original heliocentric velocity with the cited error,
in km s-1; Col. 5 – the heliocentric velocity with its error obtained from our
SALT spectra; Col. 6 – Distance in Mpc. For 13 galaxies this is measured with
the Tip of RGB (TRGB) method. For the remaining objects, the peculiar velocity
correction is used according to the velocity field from Tully et al. (2008) as
also adopted in the Nearby Void Galaxies catalog (PTM19); Col. 7 – an estimate
of the total $B$-band magnitude; Col. 8 – the absolute magnitude $M_{\rm B}$,
with the adopted MW extinction correction from Schlafly & Finkbeiner (2011);
Col. 9 – the value of 12+log(O/H), its 1-$\sigma$ uncertainty and the method
used; Col. 10 – notes for some of the program objects.

In total, we observed on both programs 47 objects belonging to the NVG (Nearby
Void Galaxy) sample. Also, we observed 15 more galaxies, adopted in the NVG
catalog to reside in the nearby voids. This was done based on their HyperLEDA
radial velocities. On results of our spectroscopy, they appeared to be not the
NVG objects. Two more galaxies were observed in order to check their radial
velocities. One was a potential companion of the void galaxy KKH86, while
another galaxy was in a visual contact with the void galaxy PGC16389. We
devote to these 17 objects Sect. 4.3.

Nine galaxies (of them, eight – new) were observed as a continuation of the
two previous papers to search for XMP dwarfs (Pustilnik et al., 2020b, 2021)
among the 60 preselected void XMP candidates (Pustilnik et al., 2020a).
Accounting for results on 46 already published objects from this program and
several remaining galaxies in the Northern hemisphere, observed at BTA, the
program is very close to its completion.

####  4.1.1 Comments on the most metal-poor galaxies

SDSS J0015+0104 = AGC103435. This galaxy, residing in the Eridanus void
(Pustilnik et al., 2013; Kniazev et al., 2018), appeared as an XMP object in
two papers, with the values of 12+log(O/H) from 7.07$\pm$0.06 to 7.03 dex
(Guseva et al., 2009; Izotov et al., 2019). This galaxy is one of the
prototype objects for the search for void XMP dwarfs. Therefore, it was
important to improve its O/H accuracy. The line [Oii]$\lambda$3727 was either
outside the available spectral range (Izotov et al., 2019), or the original
semi-empirical method was used (Izotov & Thuan, 2007) for too low excitation
parameter O32 of $\sim$0.5 (Guseva et al., 2009). Our spectrum is quite
similar to that of Guseva et al. (2009), with a higher S-to-N ratio, since we
have even marginally detected a faint [Oiii]$\lambda$4363\. The main
difference with their spectrum is our factor $\sim$1.2 larger relative fluxes
of [Oii]$\lambda$3727 and [Oiii]$\lambda$5007.

AGC124137 = J023137.0+093144. This new XMP dwarf was originally selected as a
candidate XMP object in Pustilnik et al. (2020a). The emission lines of the
only compact knot are faint and overlay on the underlying blue continuum with
the Balmer-line absorptions. In Figure B1 we show both, the original spectrum
(black) and the result of the subtraction of the SSP model spectrum (blue)
after the ULySS package application. While the S-to-N in the used lines for
the strong-line and mse methods is quite low, both methods give the consistent
O/H estimates. Since the uncertainty of the measured flux of H$\gamma$ is
large, we varied it in order to check its effect on the derived C(H$\beta$)
and EW(abs) and on the value of O/H. While the C(H$\beta$) varied from 0.32 up
to 0.66–0.69, the respective estimates of 12+log(O/H) (s) varied from 7.08 to
7.30–7.32 dex. The value of C(H$\beta$) related to the Milky Way extinction is
0.13. Therefore, we believe that C(H$\beta$) = 0.32, adopted in Table C2, is a
more reliable since is a more typical for this type of dwarfs. However, taking
into account the low S-to-N of the data, we adopt for this object in Table 3,
the value of O/H, which is an average between the two current extreme
estimates. The better quality data will be necessary to qualify confidently
this dwarf as an XMP void galaxy.

PGC1166738 = J030646.9+002811. This new XMP dwarf was originally selected as a
candidate XMP object in Pustilnik et al. (2020a). We show in Table 3 the value
of O/H as an average for two knots. For the knot with the extremely low value
of 12+log(O/H)(s) = 7.16$\pm$0.05 dex, we provide all data in Table C3. For
the second knot, we obtain the value of 12+log(O/H)(s) = 7.22$\pm$0.05 dex.

In the spectrum of this XMP knot, obtained on 2019.09.06, we detected unusual
transient emission lines (H$\beta$, H$\alpha$, and [Oiii$\lambda$5007]), which
differed in velocity and strength from those observed in the two later dates,
in December 2019. Since the available data are too limited, the nature of this
transient remains puzzling. Probably, new observations of this knot will
uncover a puzzle of this phenomenon.

PGC016389 = J045658.7–424802 = HIPASSJ0457–42. In this blue patchy elongated
oval we have on the slit three different Hii regions with varying S-to-N
emission lines and with substantial contribution of the underlying Balmer
absorptions. To correctly subtract this continuum, we modelled it with the
ULySS package as explained in Section 4.1. In Figure B1 (Appendix B,
supplementary material) we present the 1d spectrum of the most metal-poor
region (a). The respective line fluxes and derived physical parameters for
this region are presented in Table C4 (Appendix C, supplementary material).
The very low value of 12+log(O/H) (s) = 6.98$\pm$0.05 dex derived with the
method of Izotov et al. (2019) is practically insensitive to the adopted value
Te (with the range of 18.2 KK to 26.2 kK, estimated either via semi-empirical
method of Izotov & Thuan (2007), or with the modified method (mse) from
Pustilnik et al. (2021).

The main problem with this (as well as with 12+log(O/H) (mse)) estimate, is
the very low excitation of this region. The parameter
O${}_{\mathrm{3}2}\sim$0.2 is 2.5 times smaller than its lower boundary for
the sample of Hii regions with the direct O/H, which is used to derive the
empirical relation in Izotov et al. (2019). Therefore, we should treat the
derived very low value of O/H with a caution and rather as an indicative one.
In two other regions, ’b’ and ’c’, we derived 12+log(O/H) (s) of 7.20$\pm$0.05
dex and 7.31$\pm$0.09 dex, respectively. While for region ’c’, O32 is very
small, of $\sim$0.1, for region ’b’, O${}_{\mathrm{3}2}\sim$0.46. Having all
this information in hands, we currently adopt its 12+log(O/H) (s) =
7.20$\pm$0.05 dex. Since the galaxy shows multiple SF regions, we hope that
the follow-up high S-to-N spectroscopy will allow one to better determine the
range of metallicities in this dwarf.

AGC227973 = J125039.9+052052. This new faint XMP dwarf, with the value of
12+log(O/H) = 7.07 dex, appears to be one of the lowest metallicity LV dwarfs.
It is similar on many parameters to the other void XMP galaxy, the Leoncino
dwarf (AGC198691 = J0943+3326) (Hirschauer et al., 2016). They have similar
distances and atomic gas masses, very close absolute magnitudes and
metallicities and amplitude of Hi gas motions. The main difference is the a
much higher excitation of Hii region in AGC198691 that allows to determine its
metallicity via the direct method and to address the issue of the primordial
Helium (Aver et al., 2022).

PGC044681 = J125956.6–192441. For this dwarf, we already obtained the SALT
spectrum presented in Pustilnik et al. (2020b). For those rather noisy data
for the used emission lines, we found its 12+log(O/H)(s) = 7.22$\pm$0.09 dex,
with the upward correction by 0.02 dex relative to that paper, due to the
updated zero-point correction for Izotov et al. (2019) method, as described in
Sect. 3. Since that time, a much better quality images of this dwarf appeared
in the Legacy surveys and the Hubble Legacy Archive. We use them to select the
different slit position for the repeat observation. The new value of
12+log(O/H)(s) = 7.34$\pm$0.08 dex, is, from the one hand, marginally
consistent with the first measurement. On the other hand, since it probes the
other Hii region, it does not exclude the lower value for that region.

####  4.1.2 Comments on the other individual void galaxies

ESO308-022 = J063932.9–404317. The slit was positioned far from the main dwarf
galaxy body, on the outer SF region well seen at the HST image. Its radial
velocity differs by $\sim$60 km s-1 from that of the main galaxy derived from
its Hi emission (Huchtmeier et al., 2000). Since the width of Hi profile,
W${}_{\mathrm{2}0}\sim$ 70 km s-1, one can think that the studied here SF
region can belong to ESO308-022, or alternatively, is a smaller gas-rich
companion in which the current SF episode was triggered by the tidal
interaction with a more massive neighbour. Probably Hi mapping of this galaxy
will clear up the nature of this star-forming gas blob.

PGC029033 = J100138.2–081455. For this galaxy, we detected unusually large
difference between the Hi-line velocity of V(Hi) = 446$\pm$1.5 km s-1
(Huchtmeier et al., 2003) and our emission line velocity V(opt) = 258$\pm$28
km s-1 for the slit position crossing the W edge of galaxy body. The rotation
velocity of the galaxy is rather small as follows from the width of Hi profile
W0.5 = 49 km s-1 (Huchtmeier et al., 2003). While the optical morphology of
this dwarf does not look disturbed, this very large difference in radial
velocities of the atomic and ionised gas hints on its non-equilibrium state,
probably caused by a recent interaction or a minor merger. The gas metallicity
of this dwarf, despite bearing rather large uncertainties, also looks rather
enhanced in comparison to the similar luminosity void dwarfs. Also, it is not
clear which of the two velocities reflect the systemic velocity of the galaxy,
which is used for the distance determination. For the moment, we use the Hi-
based velocity. So, the more detailed study of Hi gas kinematics can shed
light on the recent processes in this object.

PGC1016598 = J213902.9–073443. In this galaxy the emission on the slit is
splitted onto two close ($\sim$7 pixels, or $\sim$3.5 arcsec, in between)
knots (W and E) with quite different flux ratios of
[Oiii]$\lambda\lambda$4959,5007 lines to H$\beta$. The half-widths of their
extent along the slit are close to the distance between the knots, so that
there is a substantial overlapping of their light if we simply extract
spectrum of each knot. To obtain the more reliable line strengths in the E and
W knots, we undertook a two-gaussian fitting along the slit for all lines of
interest. Despite the knots show quite different excitation parameter O32 (1.9
versus 0.44), their parameter 12+log(O/H) (s) is close within rather small
uncertainties (7.26$\pm$0.05 dex and 7.19$\pm$0.06 dex, respectively).
Therefore, we adopt their average value, 7.23$\pm$0.04 dex.

###  4.2 Improved velocities and distances for void galaxies

For several void objects from Table 3, the HyperLEDA radial velocities have
moderate to low accuracies (60 –120 km s-1, or for related distances,
$\sim$0.9–1.8 Mpc). For objects in the LV and its environs, this results in
the substantial uncertainties in both the relative positions to the nearby
neighbours and to their distance-dependent parameters. In Table 3 we give for
all objects the distances and related MB based on NVG (HyperLEDA) data. Here
we briefly summarise galaxies, for which we obtained a better accuracy radial
velocities.

PGC712531 = J033903.0–304921. Vhel changed from 839 to 692 km s-1 and the
related D – from 10.95 to 9.3 Mpc.

6dFJ0935216–134852. Vhel changed from 800 to 877 km s-1 and the related D –
from 11.7 to 12.7 Mpc.

PGC408791 = J202608.8–552950. Vhel changed from 893 to 812 km s-1 and the
related D – from 7.5 to 6.4 Mpc.

PGC3192333 = J234133.8–353023. Vhel changed from 630 to 524 km s-1 and the
related D – from 6.6 to 5.2 Mpc.

PGC680341 = J234147.5–330841. Vhel changed from 510 to 446 km s-1 and the
related D – from 4.4 to 3.5 Mpc.

###  4.3 Mistaken objects

In Table 4 we summarise the results of observations for 15 objects, which
appeared to have wrong radial velocities in HyperLEDA and/or in the original
papers. Due to these errors, they reside far from the distances adopted in the
PTM19, and, hence, should be excluded from the NVG sample. The two exceptions
in this Table are the galaxies [MU2012]J1355+04B and PGC016383. For the
former, the information on its radial velocity was absent. Makarov & Uklein
(2012) suggested that this is a fainter companion of the known LV dwarf KKH86.
For the latter, its known radial velocity placed the galaxy far outside the
distance of 25 Mpc, in which the NVG sample is picked-up. We obtained its
independent value just because positioned the long slit at the void galaxy
PGC016389 so that it also crossed PGC016383. See Sect. 4.3.1.

We show 1D spectra of all these objects in the on-line supplementary materials
in Appendix B, Fig. B5–B6 and present the related data in Table 4. The Table
includes the following information. Column 2 - the name of the object adopted
from HyperLEDA. Column 3 - its J2000 coordinates. Column 4 - the original
heliocentric velocity from HyperLEDA used to assign the object to the NVG
sample. Column 5 - the heliocentric velocity on the results of the SALT
spectroscopy. For seven very distant objects we give their redshifts instead
of radial velocity. Column 6 - B-band magnitude adopted from HyperLEDA. Column
7 - estimated absolute B-band magnitude based on parameters in Columns 5 and
6. Column 8 - brief notes. For several objects, we present below more detailed
comments.

####  4.3.1 Comments on individual mistaken objects

PGC016383=J045701.2–424803. This known background galaxy looks to be in
contact with the unrelated void galaxy PGC016389 from Table 3. The SALT slit
was positioned to cross both galaxies. We did not expect to find something
new. However, the E+A spectrum of PGC016383 shows the radial velocity of Vhel
= 2953 km s-1, about 500 km s-1 smaller than the adopted value in HyperLEDA.

AGC208329 and SDSSJ101531.9+033508. This blue irregular galaxy is identified
in ALFALFA (Haynes et al., 2018) with Hi-source at position J101528.4+033544
with the catalog velocity Vhel,HI = 1018$\pm$2 km s-1. The velocity determined
on two Hii-regions, V${}_{\rm hel,opt}\sim$ 1370 km s-1, drastically differs
from that Vhel,HI. Due to this inconsistency, we have checked the original
data of this Hi-source as presented in the ALFALFA database. Indeed, there is
a rather strong source at Vhel,HI = 1018 km s-1 (Fig. 1). In addition, there
is a faint, marginally-detected source with F(Hi)$\sim$0.2 Jy km s-1 at
V${}_{\rm hel,HI}\sim$ 1370 km s-1. We identify this Hi source with the star-
forming dwarf, for which we obtained the same radial velocity of the ionised
gas.

The optical galaxy has no alternative names in HyperLEDA. However, it is
identified with two SDSS objects: J101531.88+033508.4 – for its ’centre’, and
J101531.32+033508.9 – for the blue compact knot at the W edge. Both objects
were on the SALT slit. They have rather similar spectra and the close radial
velocities. This SDSS dwarf is seemingly associated with several dwarf
galaxies in the group of the Sa galaxy NGC3169 (J101414.8+032759) with MB =
-20.6 mag, at Vhel = 1232 km s-1. Therefore, this optical SDSS object should
be excluded from the NVG sample.

As for the nature of AGC208329 itself and an associated with it ’galaxy’, it
remains puzzling, since no other potential optical counterparts are visible in
the Legacy surveys colour images within the five-arcmin radius. One can
suggest that the optical counterpart of AGC208329 is a very low SB void dwarf
which falls close to light sight of a much brighter dwarf
SDSSJ101531.88+033508.4, for which we obtained optical spectrum. For the
typical ratio of M(Hi)/L${}_{\rm B}\sim$ 1, for AGC208329 with F(Hi)= 1.15 Jy
km s-1, one expects for its counterpart a galaxy with Btot = 16.9 mag, 2.5 mag
brighter than that of SDSSJ101531.88+033508.4. To be invisible, this
counterpart should be comparable on the total magnitude with this SDSS dwarf
(and hence, to be very gas-rich, with M(Hi)/LB $\sim$10) and to have a much
larger angular extent. Probably, the future very deep images of this region
will detect such an unusual LSB void dwarf.

Figure 1: ALFALFA Hi profile of AGC208329 at Vhel = 1018 km s-1 and a fainter
object at V${}_{\rm hel}\sim$ 1370 km s-1. The latter is close to that of the
optical velocity of SDSSJ101531.88+033508.4 measured in this work.

[MU2012]J1355+04B = J135429.5+041237. This blue irregular galaxy was suggested
by Makarov & Uklein (2012) as a counterpart for a nearby void dwarf KKH86 with
Vhel = 220 km s-1 (see Table 3). Our SALT spectrum shows this is a dwarf
(M${}_{\rm B}\sim$ –14 mag) emission-line galaxy at the D $\sim$ 66 Mpc. The
strong Oxygen lines in its Hii region allow one to estimate its Oxygen
abundance via the ’mse’ method described above. This gives the value of
12+log(O/H) = 7.56$\pm$11 dex.

PGC264615 = J161354.9–721446, not HIPASS J1614-72. This blue galaxy was
suggested as an optical counterpart for the faint Hi-source HIPASS J1614-72 by
Kilborn et al. (2002). It is separated from the Parks radio position
(J161422.45-721554.9, with an accuracy of $\sim$1.9 arcmin) by 2.4 arcmin. For
the Hi radial velocity of Vhel(Hi) = 383 km s-1, the distance to this object,
derived via the kinematical model of Tully et al. (2008), is only $\sim$2.5
Mpc. Hence, this Hi-source belongs to a few per cent of the nearest NVG sample
galaxies. Due to the poor coverage of this sky region by the imaging surveys,
its real optical counterpart is still waiting for identification. PGC264615
itself appears a distant SF galaxy with the redshift of $\sim$0.07. NED
(NASA/IPAC Extragalactic Database) gives a half-dozen galaxies with close
redshifts within the radius of 1 degree.

PGC163318=J212935.2–401653. The target galaxy looks like as a small disc with
a brighter central core. An off-set red ’star’ is situated at $\sim$5 arcsec
to NE. Both objects on the slit display rather similar spectra, with the
prominent absorption features slightly shifted one relative to the other. The
non-shifted strong absorption at the edge of the spectrum is the telluric O2
band at $\approx$$\lambda$6870 Å. The other prominent absorption doublet at
$\sim$$\lambda$6270 Å shows the shift and thus should be intrinsic for both
objects. The most probable identification of this doublet is NaI D1,D2
(5889.95 and 5895.92 Å) at the redshift of z = 0.0645. Then ’red’ star appears
to be an E-galaxy with the radial velocity $\sim$450 km s-1 lower than that of
the target object. The fainter absorption features in the spectra of the both
galaxies are well consistent on the wavelengths with the absorption lines
visible in spectra of K7-M0 dwarfs from templates in paper by Kesseli et al.
(2017). Probably these two galaxies comprise a group with a brighter E galaxy
at $\sim$30 arcsec to SW.

##  5 Discussion

Figure 2: Positions of 48 void dwarfs from NVG galaxies selected in PEPK20 as
candidate XMP galaxies. The solid red line is the reference relation derived
for a sample of the LV late-type galaxies by Berg et al. (2012). Two blue
dash-dotted lines at $\pm$0.15 dex show the rms scatter of the reference
sample. Magenta dash-dotted line runs at –0.30 dex (–2 rms) from the reference
relation. Nine prototype objects (red triangles) are known from the earlier
studies of XMP void dwarfs summarised in PEPK20. Five of them have O/H
determined with the direct method, while the rest four – with the strong-line
method of Izotov et al. (2019). The vertical dashed line at M${}_{\rm
B}\sim-14.0$ shows the range of O/H for DDO68 from Annibali et al. (2019).

As described in the Introduction, this work includes the observational results
on the ionised gas metallicity from two different projects: a) the search for
new XMP dwarfs among galaxies in the NVG sample, and b) the unbiased study of
all void galaxies in the LV, that is about 260 the nearest galaxies of the
whole NVG sample of 1354 objects. We summarise the respective results below.

###  5.1 Search for XMP dwarfs in the NVG sample

Altogether, it was selected 60 NVG sample galaxies with M${}_{\rm B}>-14.3$,
which on the other properties were similar to the prototype XMP void dwarfs
(Pustilnik et al., 2020a). For 46 of them, the spectral data were obtained
either with SALT, or with BTA. The related results were published in Pustilnik
et al. (2020b, 2021). In this paper, we present the results of observations
for 8 new objects and for one already published dwarf and briefly summarise
the preliminary statistics of such unusual dwarfs relative to the more common
void galaxy population.

Of the preselected 60 NVG XMP candidates, to date, total 54 are observed and
presented in Pustilnik et al. (2020b, 2021) and in this paper. Six more
objects remain in the BTA part (paper in preparation). Two of these 60
candidates appeared galaxies with wrong velocities (Pustilnik et al., 2020b).
One preselected galaxy appeared too Southern, so that it can not be observed
with SALT. Of the remaining 57 NVG objects, for 50 we were able to derive the
parameter 12+log(O/H). For 11 of these 50, the parameter 12+log(O/H) appears
extremely low, in the range of 6.98 to 7.21 dex. 23 more dwarfs of this
candidate XMP dwarf sample, have 12+log(O/H) in the range 7.23 – 7.39 dex,
that corresponds to the range of Z⊙/30 $\lesssim$ Z(gas) $<$ Z⊙/20. That is
$\sim$20 percent of that preselected candidates appear XMP galaxies with
Z(gas) $<$ Z⊙/30. The additional $\sim$40 percent of the preselected objects
appear very-metal poor, with Z⊙/30 $<$ Z(gas) $<$ Z⊙/20.

The total number of the nearby void XMP dwarfs comprises about 20, including
the prototype XMP BCG IZw18 and discovered several decades later, well-known
galaxies DDO68, with 12+log(O/H) in the range 6.98 – 7.3 dex (Pustilnik et
al., 2005; Izotov & Thuan, 2007; Annibali et al., 2019), J0926+3343 with
12+log(O/H) = 7.12 dex (Pustilnik et al., 2010), and Leoncino dwarf
(J0943+3326), with 12+log(O/H) = 7.07 dex (Aver et al., 2022).

In Figure 2, we show how the NVG galaxies, observed as the preselected XMP
candidates, sit on the diagram 12+log(O/H) versus MB. The relation between
these parameters was derived by Berg et al. (2012) for the reference sample of
the LV late-type galaxies with the well-known O/H and distances. We draw this
linear relation and two parallel lines showing the value of the rms scatter of
this sample ($\pm$0.15 dex) around the linear regression. The pink dot-dashed
line is drawn at the 2 rms distance (that is –0.3 dex) below the reference
linear relation. The galaxies with O/H below this line, deviate systematically
downwards from the reference relation.

We use O/H estimates derived with the three different methods, discussed
above, depending on the situation. When the auroral line [Oiii]$\lambda$4363
is detectable, we use the direct method (black symbols). Otherwise, we use the
strong-line method of Izotov et al. (2019) for the range of 12+log(O/H)
$\lesssim$7.5 dex (green symbols) and the mse (modified semi-empirical) method
for the higher O/H (blue symbols). Some systematics between the data, obtained
with the different methods of the O/H estimate, are basically due to the
selection effects.

The appearance of the faint auroral line [Oiii]$\lambda$4363 correlates with
the value of ionisation parameter log(U) or its proxy, the excitation
parameter O32, and metallicity. The former, in turn, are related with the
strength of SF ’burst’ and its age. Since in the unbiased sample of void
galaxies, the probability to catch a young and strong burst is rather small,
this translates to the low incidence of [Oiii]$\lambda$4363-line-detected
objects. Similarly, since we apply either the ’strong-line’ or ’mse’ methods
in the ranges of 12+log(O/H) $<$7.5 or $>$7.5 dex, the average values of O/H
for these two methods show, by definition, the substantial difference.

###  5.2 Intermediate results on metallicities of the LV void galaxies

The next steps on the spectroscopy of the NVG dwarfs are conducted in the
framework of the on-going project of the unbiased study of all void galaxies
in the LV. It also finds new very metal-poor objects albeit less frequently.
The smaller incidence of the newly-found XMP dwarfs in this project is a
natural consequence of the unbiased character of the study of the whole NVG
sample in the LV. This is also a reflection of the relative rarity of XMP
objects. In this paper, we add two more LV XMP dwarfs, with 12+log(O/H) = 7.07
and 7.20 dex, and 7 new dwarfs with 12+log(O/H) $\lesssim$ 7.39 dex. A couple
of new XMP dwarfs within the LV, which were not preselected as the mentioned
above XMP candidates, were also found at the Northern hemisphere with BTA
(paper in preparation).

The unbiased approach in the study of void objects allows one to address their
diversity, including the scatter of gas metallicity for the similar global
parameters, such as luminosity or stellar mass. To date, we collected data on
the gas metallicity in the LV void galaxies for about a hundred objects. They
include both, our own results from this paper, some of the LV objects appeared
in the other our published samples, as well as about two dozen objects with
known O/H, found in the literature. Despite this number comprises less than a
half of the whole LV void sample (the updated number of 243 objects), it is
useful to have a first look on the available data.

For discussion of the intermediate results on the LV sample, we take at the
moment only the data with the direct and strong-line O/H estimates. The mse
estimates of O/H from this and the previous papers will be included in the
next publication, along with the earlier data. The latter need a more careful
reanalysis, including those where we used the semi-empirical method of Izotov
& Thuan (2007).

The plot with the intermediate results for the LV void sample is presented in
Figure 3. The lines for the reference sample are the same as in Figure 2.

One of the features, which distinguishes this void galaxies sample from, e.g.,
the sample of Lynx-Cancer void, is its heterogeneity in the context of their
distances to the nearest massive galaxies. As described in PTM19, the NVG
galaxy sample was selected of objects falling within the empty spheres
comprising the voids themselves. About 20 per cent of this way selected
galaxies appeared to reside closer than two Mpc to the bordering massive
galaxies. They were conditionally named outer void galaxies, while the great
majority of galaxies ($\sim$80 per cent) with the DNN (nearby luminous
neighbour) distances larger than 2 Mpc were assigned to the inner void
galaxies.

Figure 3: Distribution in the diagram ’12+log(O/H) vs MB’ of the 90 LV void
galaxies with known O/H, derived either by the direct (Te), or by the ’strong-
line’ (Izotov et al., 2019) methods. The lines on the plot are the same as in
Figure 2. The data include results from this paper (25 objects) and from our
published papers Pustilnik et al. (2016, 2020b, 2021) (31 objects), as well as
12 unpublished BTA objects and 22 objects from the literature (Izotov et al.,
1994, 1997; Izotov & Thuan, 2007; Izotov et al., 2012; van Zee, 1997; van Zee
& Haynes, 2006; van Zee et al., 2006; Pustilnik et al., 2003; Kniazev et al.,
2005; Berg et al., 2012; Skillman et al., 2013).

As one can see in Figure 3, the scatter of the LV void galaxies (filled and
empty octagons with error bars) on log(O/H) at a fixed MB, appears
significantly larger than for the reference sample of Berg et al. (2012). For
the latter sample, the scatter is mostly limited by the blue dot-dashed
parallel lines ($\pm$1 rms scatter). While the sizeable part of void galaxies
falls within the log(O/H) range of the reference sample, about one third of
the void objects have a substantially reduced metallicity. In particular, in
Figure 4 one can see that 28 of 90 LV void galaxies have the log(O/H), reduced
by more than 0.30 dex (two rms) relative to their values expected from the
reference relation of Berg et al. (2012).

The majority of void objects with the substantially reduced metallicity in
Figure 3 belong to galaxies with very low O/H, namely with 12+log(O/H)
$\lesssim$7.4. Of them, the great majority have values of O/H derived with the
strong-line method of Izotov et al. (2019). In Figure 4 we show histograms of
the difference 12+log(O/H)(observed) – 12+log(O/H)(reference, MB), separately
for O/H obtained by the direct method (black) and O/H, derived with the
strong-line method of Izotov et al. (2019) (green). The mean difference for
O/H(Te) is –0.049 dex, with rms=0.175 dex. For O/H(s), the mean is –0.346 dex,
with rms=0.116 dex.

Figure 4: The same data as in Figure 3. The histograms show the differences of
measured 12+log(O/H) for the LV void galaxies and those expected from the
reference relation from Berg et al. (2012). Black hatched histogram is for
O/H(Te), while the green histogram is for O/H derived with the strong-line
method of Izotov et al. (2019).  Figure 5: The same data as in the previous
figure, but divided according to their distribution within voids. 19 ’outer’
void galaxies are shown in black. 71 ’inner’ void galaxies are coloured with
magenta. See a more detailed discussion in the text.

Figure 6: The same data as in Figure 5. The histograms show the differences of
measured 12+log(O/H) for the LV void galaxies and those expected from the
reference relation from Berg et al. (2012). left-hand panel: separate
histograms for ’inner’ (71 galaxies, black) and ’outer’ (19 galaxies, green)
subsamples for the case of the threshold DNN = 2.0 Mpc. right-hand panel: same
as the left-hand panel, but for the case of the threshold DNN = 1.7 Mpc. Black
histogram is for 78 ’inner’, while the green one – for 12 ’outer’ galaxies.
See text for more detail.

The effect of the reduced metallicity in void galaxies, already mentioned in
several our papers, and its elevated scatter for a given galaxy
luminosity/mass, can be due to the interplay between various factors. In
particular, the chemical evolution of void galaxies and their current
metallicity can depend on their local environment and clustering, and on the
faster or slower accretion of the unprocessed intergalactic gas. Since void
substructure resembles that of a mini-Universe (Gottlöber et al., 2003;
Aragon-Calvo & Szalay, 2013), void galaxies can be associated with more
massive hosts or to be well isolated. In general, the local environment is
expected to affect the secular evolution, especially of the smaller-mass
companions. To uncover these factors, it takes a more careful analysis of the
studied sample.

On the other hand, since we study the LV void galaxy sample without any
additional bias, we can, in particular, examine, whether the outer and inner
void galaxies show differences in their properties. For this, we show outer
and inner galaxies in the similar plot in Figure 5. While the statistics is
still rather limited, there is a hint that the ’outer’ void objects show a
smaller scatter in log(O/H) and follow closer the reference relation of Berg
et al. (2012). While the criterion of separation of outer void subsample by
the inequality of D${}_{\rm NN}<$ 2 Mpc, is somewhat arbitrary, it seems to
catch a transition of galaxy properties from the most rarefied regions to a
denser environment. Therefore, one can think that the large scatter of void
galaxies on log(O/H) in Figure 3 can be partly explained by the contribution
of the outer objects.

In Figure 6, we show the distribution of differences in log(O/H) for 90 LV
void galaxies, separately for the ’inner’ (black) and ’outer’ (green)
subsamples. The left-hand panel shows the histograms for the threshold DNN =
2.0 Mpc, as adopted in the original paper on the NVG sample. The mean for the
71 ’inner’ galaxies is –0.193 dex with rms=0.202 dex. For the 19 ’outer’
galaxies, the mean is –0.098 dex, with the rms=0.222 dex. While the scatter
for both groups is similar, the ’inner’ group has a substantially more reduced
O/H.

To check, how this effect depends on the adopted threshold for DNN, we divided
these 90 LV void galaxies into the ’inner’ and ’outer’ subsamples, taking the
threshold DNN = 1.7 Mpc. This means that the selected ’outer’ void galaxies
are in average somewhat closer to the bordering large galaxies. The histograms
in the right-hand panel show the difference of log(O/H) for this case. For the
78 ’inner’ galaxies, the mean is –0.191 dex with rms=0.204 dex, practically
the same as for the previous histogram. For the 12 ’outer’ galaxies, the mean
is –0.055 dex, with rms=0.208 dex. That is, void galaxies with the smaller DNN
show the tendency to have the smaller difference of O/H with the reference
relation, albeit the scatter is larger than 0.15 dex characteristic of the
reference sample of Berg et al. (2012). We notice that the statistics is still
limited, so the future data, incorporating the majority of the LV void
galaxies, hopefully will result in more firm conclusions.

###  5.3 New dTr galaxies in void environment

Several studied void galaxies appear on their properties similar to the
handful known voids dwarfs of transitional type (dTr) (Karachentsev et al.,
2014; Pustilnik et al., 2022a, and references therein). They show very little
current SF as evidenced either by non-detected H$\alpha$ emission, or a single
region with the faint H$\alpha$. At the same time, their Hi content does not
differ much from dIr galaxies of the same luminosity. The unbiased study of
the LV void dwarfs allows one to extend the sample of these rare objects and
to address the issue of their origin on the larger statistical data.

Below we enumerate the void galaxies, according to the new data, falling into
this category with the higher or lower confidence. Several of them are found
as Hi-sources in the blind ALFALFA survey (Haynes et al., 2018). A few similar
ALFALFA galaxies are found in the SHIELD project described by Cannon et al.
(2011); Hauberg et al. (2015); McQuinn et al. (2014).

PGC712531 = J033903.0-304921. This bluish dwarf shows only a faint H$\alpha$
in emission.

AGC225197 = EVCC1184 = J124942.11+052922. A blue low-surface brightness (LSB)
dwarf with the non-central blue compact object. The long slit was positioned
to this blue object. Spectrum of this galaxy in SDSS DR12 (Alam et al., 2015)
looks similar to our.

AGC227972 - J125024.0+045422. A bluish oval galaxy without prominent SF
regions.

PGC2807150 = KKH86 = AGC231980. This LSB dwarf is rather bright Hi source,
with F(Hi) (ALFALFA) = 0.8 Jy km s-1 and the ratio of M(Hi)/L${}_{\rm
B}\sim$0.75. At the same time, the on-line database of Karachentsev et al.
(2013) provides its H$\alpha$-net image with no tracers of emission. Our value
of its Vhel = 298$\pm$40 km s-1 comes from the wavelengths of the Balmer
absorption lines in the body.

###  5.4 Issue of mistaken redshifts in the NVG sample

As noticed in the Introduction, the great majority of the NVG sample galaxies
in PTM19 are taken from the HyperLEDA data base. On results of our spectral
observations of a hundred NVG galaxies (that is residing in the nearby voids),
presented in Pustilnik et al. (2020b, 2021) and in this work, we found that
$\sim$15 per cent of them have velocities different from the catalog
(HyperLEDA) values much larger than one expects from the cited uncertainties.
The great majority of them comes from objects in the Southern hemisphere. Of
these, the majority of objects with wrong velocities appear in the HyperLEDA
from the 2dFGRS galaxies with redshifts derived via absorption lines.

This allows us to formulate a caution for the use of the 2dFGRS redshift data
for statistical studies of galaxies, at least in the nearby Universe, where we
have checked many of their radial velocities.

##  6 Summary and Conclusions

In the previous papers (Pustilnik et al., 2020b, 2021) we presented results of
spectroscopy for 46 objects from the sample of 60 preselected candidate XMP
void objects from the fainter part ($M_{\rm B}\gtrsim-14.3$) of the Nearby
Void Galaxies (NVG) sample. Ten of them were found to be the XMP objects, that
is having 12+log(O/H) $\lesssim$ 7.21 dex. 13 more new void galaxies were
found somewhat less metal-poor, with 12+log(O/H) $\lesssim$ 7.35 dex.

In this work we observed 8 galaxies of the remaining 13 XMP candidates and
reobserved one XMP dwarf from Pustilnik et al. (2020b). Two new XMP dwarfs are
found of these eight objects. Two additional dwarf galaxies with the Oxygen
abundance of 12+log(O/H) $\lesssim$ 7.35 dex are found as well. Two known XMP
void dwarfs were reobserved to improve the accuracy of their metallicity.

The rest 52 observed galaxies are selected to reside in the LV. They represent
mostly a fainter part of the total subsample of the LV void galaxies. The
currently updated list comprises of 243 LV void objects. Of 39 objects with
the well detected strong lines, nine new void galaxies appeared to be very
metal-poor, with 12+log(O/H) $\lesssim$ 7.39 dex. Of them, two more XMP dwarfs
are found, with 12+log(O/H) = 7.07 and 7.20 dex.

Summarising the results, presented here and the related discussion, we draw
the following conclusions:

  1. 1.

The spectroscopy of 8 remaining ’candidate’ XMP dwarfs from the sample of 60
objects residing in the nearby voids, results in the discovery of two
additional XMP dwarfs, with 12+$\log$(O/H) $\sim$7.19 – 7.20 dex
($\sim$Z⊙/30).

  2. 2.

The almost completed program of search for void XMP dwarfs results in 11 new
objects with Zgas = Z⊙/50 – Z⊙/30, that comprises $\sim$20 per cent of the
selected candidates. In addition, 23 very low-metallicity dwarfs (Zgas = Z⊙/30
– Z⊙/20) are found, that is $\sim$40 per cent of the initial list. This
finding increases substantially the number of the known nearby very metal-poor
dwarfs and allows us to conduct a deeper study of their individual and group
properties and of their possible relation to Very Young Galaxies.

  3. 3.

The first results are presented for the on-going project of the studying a
subsample of 243 NVG galaxies residing in the Local Volume. Of 52 observed
objects only 37 appeared in real to reside in the nearby voids, while the rest
15 had the wrong radial velocities in HyperLEDA.

  4. 4.

The strong lines of Oxygen are detected and its abundance is estimated in 32
of these 37 NVG objects, with the total range of 12+log(O/H) of 7.07 dex to
$\sim$8.0 dex.

  5. 5.

The addition of the other available data on the LV void dwarfs allows us to
probe the relation between 12+log(O/H) and MB on a sample of $\sim$90 objects.
The general trend of the reduced metallicity for a given luminosity, already
known from our earlier studies of the nearby void galaxies, is also well seen
on the LV void galaxies. Besides, on this compilation of the LV void galaxies,
we find a large scatter in O/H for a fixed MB. Partly, the reason of this
effect can be the presence in the LV void sample of galaxies in the
’bordering’ regions of voids.

  6. 6.

The studied void galaxies show a wide range of the star-forming activity.
While the most typical dwarfs, dIrrs and late Spirals, show several SF
regions, we find four new dTr galaxies with the substantial neutral gas
reservoir and the absent or very subtle signs of the current/recent SF.

  7. 7.

Two Local Volume Hi objects, HIPASS J1614–72 and AGC208329 appear to have a
mistaken optical identification. The search for their alternative optical
counterparts did not reveal potential candidates. Since the full census of the
LV population has the important implications for comparison of the LV with
cosmological simulations, the understanding of the nature of these Hi objects
will take special efforts, including their Hi-mapping and the improvement of
their position accuracy.

## Acknowledgements

This work is based on observations obtained with the Southern African Large
Telescope (SALT), programs 2017-2-MLT-001, 2020-2-MLT-005, 2022-1-MLT-003 (PI:
Kniazev). The reported study was funded by Russian Science Foundation
according to the research project 22-22-00654. AYK acknowledges support from
the National Research Foundation (NRF) of South Africa. We thank the anonymous
reviewer for the useful comments and suggestions, which allowed us to improve
the content and clarity of the paper. The use of the HyperLEDA database is
greatly acknowledged. This research has made use of the NASA/IPAC
Extragalactic Database (NED) which is operated by the Jet Propulsion
Laboratory, California Institute of Technology, under contract with the
National Aeronautics and Space Administration. We also acknowledge the great
effort of the ALFALFA team which opened access to the nearby Universe gas-rich
dwarfs with low or moderate SFR and thus helped us to identify the majority of
very low metallicity galaxies of this study.

We also acknowledge the use of the SDSS and Legacy surveys databases. Funding
for the Sloan Digital Sky Survey (SDSS) has been provided by the Alfred P.
Sloan Foundation, the Participating Institutions, the National Aeronautics and
Space Administration, the National Science Foundation, the U.S. Department of
Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS Web
site is http://www.sdss.org/. The SDSS is managed by the Astrophysical
Research Consortium (ARC) for the Participating Institutions.

The DESI Legacy Imaging Surveys consist of three individual and complementary
projects: the Dark Energy Camera Legacy Survey (DECaLS), the Beijing-Arizona
Sky Survey (BASS), and the Mayall z-band Legacy Survey (MzLS). DECaLS, BASS
and MzLS together include data obtained, respectively, at the Blanco
telescope, Cerro Tololo Inter-American Observatory, NSF’s NOIRLab; the Bok
telescope, Steward Observatory, University of Arizona; and the Mayall
telescope, Kitt Peak National Observatory, NOIRLab. NOIRLab is operated by the
Association of Universities for Research in Astronomy (AURA) under a
cooperative agreement with the National Science Foundation. Pipeline
processing and analyses of the data were supported by NOIRLab and the Lawrence
Berkeley National Laboratory (LBNL). Legacy Surveys also uses data products
from the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), a
project of the Jet Propulsion Laboratory/California Institute of Technology,
funded by the National Aeronautics and Space Administration. Legacy Surveys
was supported by: the Director, Office of Science, Office of High Energy
Physics of the U.S. Department of Energy; the National Energy Research
Scientific Computing Center, a DOE Office of Science User Facility; the U.S.
National Science Foundation, Division of Astronomical Sciences; the National
Astronomical Observatories of China, the Chinese Academy of Sciences and the
Chinese National Natural Science Foundation. LBNL is managed by the Regents of
the University of California under contract to the U.S. Department of Energy.

## Data Availability

The data underlying this article are available in Appendices A, B and C, which
are available only in the on-line supplementary materials of the paper.

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