# Narrow-line Seyfert 1 galaxies with absorbed jets – insights from radio
spectral index maps

Emilia Järvelä 1,∗, Marco Berton 2,3 and Luca Crepaldi 4

###### Abstract

## 1

Narrow-line Seyfert 1 (NLS1) galaxies are active galactic nuclei (AGN)
believed to be in the early stages of their evolution. A fraction of them have
been found to host relativistic jets. Due to the lack of large-scale diffuse
radio emission they are believed to be experiencing one of their first
activity cycles, and can offer us an opportunity to study the early evolution
of more powerful AGN, such as radio galaxies and flat-spectrum radio quasars.
Recently, a group of intriguing jetted NLS1s was discovered: based on high
radio frequency data they host relativistic jets, but in the JVLA observations
they all showed steep radio spectra at least up to 9.0 GHz, indicating very
strong absorption at these frequencies. In this paper we study a subset of
these sources in detail by employing spatially resolved radio spectral index
maps at central frequencies of 1.6, 5.2, and 9.0 GHz. With spectral index maps
we can disentangle the different radio emission components over the radio-
emitting region, and get insights into the production mechanisms of radio
emission. In addition, we study their host galaxies in relation to the radio
emission to investigate if the host can provide us additional information
regarding the origin of the radio emission, or the launching mechanism of the
jets. It is fascinating how different the sources studied are, and certainly
more, especially wide frequency-range, and high-resolution observations will
be needed to understand their history and current properties, such as the
reason behind the extraordinary radio spectra.

##  2 Keywords:

narrow-line Seyfert 1 galaxies, active galactic nuclei, radio emission, host
galaxies, absorbed jets

##  3 Introduction

Narrow-line Seyfert 1 (NLS1) galaxies are a class of young active galactic
nuclei (AGN) that are extremely interesting from the point of view of AGN
evolution (Mathur, 2000). In their optical spectrum the permitted lines are of
comparable width to their forbidden lines, and they are defined based on this
property: by definition, the full-width at half maximum of the broad H$\beta$,
FWHM(H$\beta$) $<$ 2000 km s-1(Osterbrock and Pogge, 1985). In addition the
flux ratio [O III] / H$\beta<$ 3 is required to ensure that we have a direct
view of the broad-line region (BLR, Osterbrock and Pogge, 1985). NLS1s also
often show strong Fe II multiplets (Goodrich, 1989), but this feature is not
part of the official classification criteria (Cracco et al., 2016).

The narrow FWHM of broad H$\beta$ is usually interpreted to be caused by low
rotational velocity around a low- or intermediate mass supermassive black hole
(generally $<10^{8}$ M⊙, Peterson, 2011), in comparison to, for example,
broad-line Seyfert 1 (BLS1) galaxies. Since the bolometric luminosities of
NLS1s are comparable to those of BLS1s it implies that NLS1s are accreting
close to or even above the Eddington limit (Boroson and Green, 1992). An
alternative explanation is that the narrow FWHM(H$\beta$) is caused by a
flattened BLR seen face-on. In this case the permitted lines would appear
narrow due to the lack of Doppler broadening, and the foreshortening of the
velocity vector. In this scenario, the real central black holes masses can be
considerably higher than what is derived from the BLR emission line
properties, and NLS1s would be similar to BLS1s and other broad-line AGN
(Decarli et al., 2008). However, inclination-independent observational
properties of NLS1s, such as their host galaxy morphologies (e.g., Krongold et
al., 2001; Deo et al., 2006; Antón et al., 2008; Kotilainen et al., 2016;
Järvelä et al., 2018; Berton et al., 2019; Olguín-Iglesias et al., 2020;
Hamilton et al., 2021, but see D’Ammando et al., 2017, 2018), and their
different large-scale environments compared to BLS1s (Järvelä et al., 2017)
indicate that the black hole masses should be genuinely low. Detailed studies
of some individuals, using, for example, reverberation mapping, support this
hypothesis (Du et al., 2014; Wang et al., 2016; Du et al., 2018; Berton et
al., 2021), but more such studies are required to draw any definite
conclusions. Assuming that the black hole masses in NLS1s are low, they may be
at an early stage of AGN life cycle, and will eventually grow into fully
developed broad-line AGN (Mathur, 2000; Sulentic et al., 2000; Fraix-Burnet et
al., 2017).

In AGN a host of different effluxes, launched by the central engine, are seen.
These include collimated powerful relativistic jet, lower power non-
relativistic jets, and non-collimated outflows and winds. A wide variety of
AGN can launch low-power jets and outflows, but traditionally the most
powerful relativistic jets were exclusively associated with the most massive
supermassive black holes that reside in huge elliptical galaxies (Laor, 2000).
This idea is turning out to be outdated as more observational evidence is
gathered. For example, some NLS1s exhibit blazar-like properties, such as flat
radio spectra111Flat spectrum has $\alpha>$ -0.5, when
$S_{\nu}\propto\nu^{\alpha}$ at frequency $\nu$., dominant radio emission,
high brightness temperature, and prominent variability (Komossa et al., 2006;
Yuan et al., 2008). Indeed, the detection of the first NLS1 at gamma-rays,
proving that it hosts relativistic jets, did not come as a surprise (Abdo et
al., 2009). So far $\sim$20 NLS1s have been detected in gamma-rays, showing
that, just like blazars and radio galaxies, NLS1s can harbour powerful
relativistic jets (Romano et al., 2018; Järvelä et al., 2020; Rakshit et al.,
2021). These NLS1s might be the progenitors of more powerful jetted AGN, like
flat-spectrum radio quasars (Foschini et al., 2015; Foschini, 2017, 2020).

However, only a fraction of NLS1s are blazar-like. In fact, most of them are
very faint or not even detected in the radio band. It was thought that the
radio emission in these sources would be mostly produces by star formation, or
possibly by a less powerful form of AGN activity, such as low-power jets or
outflows. Thus the discovery of strongly variable, almost Jy-level, emission
at 37 GHz from some of these sources came as a surprise (Lähteenmäki et al.,
2018). The properties of this emission can be explained only by the presence
of relativistic jets. These sources became even more intriguing when they were
observed with the Karl G. Jansky Very Large Array (JVLA) at 1.6, 5.2, and 9.0
GHz, and were found to show steep spectral indices at least up to 9.0 GHz,
with no signs of jets, or even AGN activity (B20). All the sources showed mJy-
or sub-mJy-level flux densities, that could be explained by star formation
alone, and also their spectral indices were in agreement with this scenario.
To explaing the Jy-level detections at 37 GHz it seems evident that the
spectrum needs to turn inverted above 9.0 GHz, but based on the JVLA
observations it remained unclear whether these were young AGN with a semi-
stationary peak at very high frequencies, like peaked-spectrum sources, or AGN
with extreme variability. A connection between NLS1s and peaked-spectrum
sources has already been made before (Oshlack et al., 2001; Gallo et al.,
2006; Komossa et al., 2006; Caccianiga et al., 2014; Berton et al., 2016,
2017; Foschini, 2017; Caccianiga et al., 2017; Yao and Komossa, 2021; O’Dea
and Saikia, 2021). In this scenario the inverted spectrum would be caused by
either synchrotron self-absorption or free-free absorption.

In this paper we aim to study the peculiar sources of Berton et al. (2020b,
from now on B20) in more detail. Our goal is to better characterise the radio
emission at 1.6, 5.2, and 9.0 GHz to understand its origin, and investigate if
any signs of the jets are detectable. We achieve this by producing and
analysing the spatially resolved spectral index maps of these sources.
Spectral index maps are superior to traditional spectral indices since they
provide a spatially resolved view over the whole radio-emitting region, and
enable us to identify different radio components, such as, optically thick and
thin regions, and the radio emission production mechanisms. In addition to the
radio data, we use any data and studies available in the literature to draw a
more complete picture of these sources. We especially focus on their host
galaxies in connection to the radio emission, as it can also help us
understand the origin of the radio emission, and possibly the launching
mechanism of the jets in these sources, as jetted NLS1s have found to often
reside in interacting galaxies (Antón et al., 2008; Järvelä et al., 2018). The
paper is organised as follows: in Sect. 4 we briefly overview the sample, in
Sect. 5 we summarise the data reduction and the production of the radio and
spectral index maps, in Sect. 6 we present the results for individual sources,
in Sect. 7 we discuss our results and their implications, and finally, in
Sect.8 we summarise this study. Throughout the paper, we use the standard
$\Lambda$CDM cosmology, with $H_{0}$ = 70 km s-1 Mpc-1, and $\Omega_{\Lambda}$
= 0.73 (Komatsu et al., 2011). For spectral indices we adopt the convention of
$S_{\nu}\propto\nu^{\alpha}$ at frequency $\nu$.

##  4 Sample

The sample includes seven NLS1s with extraordinary properties. These sources
were either not detected at all or detected at very low flux densities in
previous radio surveys, including Faint Images of the Radio Sky at Twenty-
Centimeters (FIRST) (Becker et al., 1995) and National Radio Astronomy
Observatory (NRAO) Very Large Array (VLA) Sky Survey (NVSS) (Condon et al.,
1998). However, they are included in the Metsähovi Radio Observatory NLS1
monitoring programme (Lähteenmäki et al., 2017), and were detected at 37 GHz
at flux densities that strongly suggest that these sources host relativistic
jets (Lähteenmäki et al., 2018). Furthermore, the radio emission proved to be
highly variable, conclusively excluding any alternative mechanisms, for
example, star formation processes, as the origin of the radio emission. These
sources were observed with the JVLA in A-configuration in L-, C-, and X-bands,
or, at 1.6, 5.2 and 9.0 GHz, respectively (B20). Interestingly, all of them
show steep radio spectra between 1.6 and 9.0 GHz as shown in Fig. 1. To
explain the high flux densities seen at 37 GHz extreme flux density
variability, a form of absorption, or a combination of them is needed. Since
the required increase in the flux density would be two to three orders of
magnitude, absorption seems a more plausible explanation. It is still unclear
whether the main absorption mechanism is synchrotron self-absorption or free-
free absorption. The JVLA data of these sources were studied in Berton et al.
(2020a), but a more detailed analysis of the radio data, complemented by data
at other wavelengths, might help us shed light on their nature.

##  5 Data reduction and analysis

The observations and calibration are described in detail in Berton et al.
(2020b). However, for the analyses in this paper we used CASA version 5.6.2-3
due to its enhanced capabilities, for example, in producing spectral index
maps.

###  5.1 Spectral index maps

The spectral index, or $\alpha$ maps were produced using the procedure
described in Järvelä et al. (2021, submitted). We summarise the process in the
next Sections.

####  5.1.1 Cleaning

We cleaned the sources using the multi-term multi-frequency synthesis, mtmfs,
algorithm in CASA. It allows simultaneous fitting of a spectral index over the
whole band-width and as a function of the position within the image using a
simple power-law. The algorith performs the fit by modelling the spectrum of
each flux component (pixel) by a Taylor series expansion about the reference
frequency, $\nu_{0}$. In theory the number of Taylor terms is not limited, but
in practise the quality of the data limits it. The number of resulting maps is
equal to the number of Taylor terms. The first map, TT0, corresponds to the
specific intensities at $\nu_{0}$ and is equal to the normal radio map. The
second map, TT1, is defined so that $\alpha$ = TT1 / TT0. The third term, TT2,
describes the spectral curvature and is defined so that
$\rm\beta=TT2/TT0-\alpha(\alpha-1)/2$. We chose to fit our sources with two
Taylor terms to maximise the quality of the maps since the sources are faint,
and the signal-to-noise (S/N) of the data is not very high.

We produced the $\alpha$ maps in all bands to study possible changes in the
spectral index within the frequency range of our observations. We were able to
produce the maps for three sources, as the remaining four sources were either
non-detections or did not have high enough S/N to produce reliable maps. The
central frequencies of the $\alpha$ maps are 1.6, 5.2, and 9.0 GHz, and their
band-widths are 1, 2, and 2 GHz, respectively. In addition to the $\alpha$
map, the mtmfs also produces a $\Delta\alpha$ error map that describes the
empirical error estimate based on the errors of the TT0 and TT1 residual
images. These maps require some post-processing steps, described below.

####  5.1.2 Wide-band primary beam correction and 5$\sigma$ cut-off

The size of the primary beam depends on frequency and imposes its own spectral
index onto the Taylor-coefficient images and the $\alpha$ map. We corrected
this with the CASA task widebandpbcor, which computes a set of primary beams
at given frequencies, calculates the Taylor-coefficient images representing
the primary beam spectrum, performs the primary beam correction of the Taylor-
coefficient images, and finally computes the primary beam corrected maps using
the corrected Taylor-coefficient images.

This cannot take into account small variations during a single observations,
such as the slightly changing shapes of the telescopes, and the primary beam
rotation on the sky while tracking a source. However, we assume these errors
to be insignificant. In addition, the primary beam errors increase with
distance from the pointing centre, but according to Bhatnagar et al. (2013)
the effects are minimal up to the half-power beam width (HPBW). Our sources
are at all frequencies considerably smaller than the corresponding HPBW so
this should not cause additional errors in the maps.

Furthermore, we masked pixels with a S/N $<5\sigma$, where $\sigma$ is the rms
of the corresponding TT0 image of the source. In general, close to the edges
of the emitting region the S/N is low, and the maps consistenly show extreme
or clearly erroneous values. With this threshold we can remove unwanted noise
from the $\alpha$ and $\Delta\alpha$ maps.

####  5.1.3 $\Delta\alpha$ cut-off

After the primary beam correction the $\alpha$ maps can still show
considerable variations, especially near the edges. Correspondingly, the
$\Delta\alpha$ map shows very high errors in these regions, suggesting that
the data quality might not have been good enough to accurately fit the
spectral index. We cut off all the data that has $\Delta\alpha>$ 1\. Even
after this some erroneous values may remain, but we did not want to lose any
information due to too high a threshold.

####  5.1.4 Smoothing of the $\alpha$ and $\Delta\alpha$ maps

The Taylor coefficient maps are convolved with the clean beam, but as the
$\alpha$ and $\Delta\alpha$ maps are derived from them using mathematical
operations their final resolution is not the same as that of the Taylor
coefficient maps. We thus smoothed the $\alpha$ and $\Delta\alpha$ maps using
the parameters of the clean beam of each source at each frequency. This
considerably decreases the small-scale variations of the $\alpha$ maps and the
errors of the $\Delta\alpha$ maps, and produces $\alpha$ and $\Delta\alpha$
maps with a resolution similar to the normal radio maps of each source.

####  5.1.5 Measurements

We list the rms of the maps and the peak and the integrated flux densities in
Table 2. The rms was measured from an empty sky region off-source. We obtained
the peak flux density by fitting a 2D Gaussian to the data, and the integrated
flux density by summing up all the emission within the 3$\sigma$ contour. The
peak flux density error is given by CASA when fitting a 2D Gaussian to the
data, and the errors of the integrated flux densities were estimated with rms
per beam $\times$ the square root of the area of emission expressed in beams.
In addition, for each source we calculated an average spectral index over the
whole region that the $\alpha$ map covers weighted with the surface
brightness, and in the core in a region with a radius of 2 px. The total and
core spectral indices are listed in Table 3.

###  5.2 Traditional spectral indices

To confirm the results of the $\alpha$ maps we also derived the traditional
spectral indices of the three sources with existing $\alpha$ maps. We achieved
this by dividing the observed band into two (spectral windows 0 to 7, and 8 to
15), cleaning these data sets separately, and smoothing the radio map with the
higher resolution with the clean beam of the lower resolution map. We did this
for each band, resulting in three in-band spectral indices for each source. We
calculated the errors of the traditional spectral indices using standard error
propagation, which results in rather high errors when using logarithmic
values, and probably somewhat overestimates the final error. Since we are
especially interested in whether any signs of a jet can be seen, we estimated
the spectral indices using the peak flux densities, and they thus reflect the
spectral index of the core. The results are shown in Table 4.

##  6 Results

Individual sources are discussed below. We focus on sources for which we were
able to produce the $\alpha$ maps. In addition to the radio data, we gathered
any information and data available in the literature. We also obtained the
$i$-band host galaxy images and the colour images of the sources from the
Panoramic Survey Telescope and Rapid Response System (Pan-STARRS).

###  6.1 J1228+5017

J1228+5017 is an NLS1 at $z$ = 0.262. It did not have any prior radio
detections before the detections at Metsähovi at 37 GHz. Its integrated JVLA
flux densities are very modest: (0.97 $\pm$ 0.04) mJy, (0.29 $\pm$ 0.01) mJy,
and (0.19 $\pm$ 0.01) mJy, at 1.6, 5.2, and 9.0 GHz, respectively, indicating
a steep spectrum up to 9.0 GHz (Fig. 1).

Its 1.6 GHz radio map (Fig. 2a) shows an elongated structure toward west, with
a maximum extent of 18.1 kpc. The 5.2 and 9.0 GHz maps(Figs. 2c and e) do not
show signs of this structure, suggesting a steep spectrum. The 1.6 GHz
$\alpha$ map (Fig. 2a) confirms this since the extended emission shows steep
spectral indices, with values around -0.5 to -0.7, consistent with optically
thin radio emission. Interestingly, the core seems quite flat, and indeed, its
weighted spectral index is -0.38 $\pm$ 0.01. This result is supported by the
traditional spectral index of -0.19 $\pm$ 0.19 at 1.6 GHz. At 5.2 GHz both
methods give a steep spectral index, around -1.0. At 9.0 GHz the spectral
index seems to flatten again, the core spectral index is -0.43 $\pm$ 0.03, and
the traditional spectral index is -0.46 $\pm$ 0.91. However, taking into
account the faintness of the source, we cannot be certain how reliable
especially the 9.0 GHz spectral indices are, since a faint source might be
barely detected in the highest frequency spectral windows, which can affect
the fit, and a point-like source might suffer from the edge effects. Both of
these are true for J1228+5017, so some caution should be exercised when
intepreting the results. J1228+5017 was also detected in the Low Frequency
Array (LOFAR) Two-metre Sky Survey (LoTSS) at 144 MHz with an integrated flux
density of (2.3 $\pm$ 0.1) mJy. This gives a 144 MHz to 1.6 GHz spectral index
of -0.36 $\pm$ 0.04, consistent with the flat index seen at 1.6 GHz. It seems
like the radio spectrum of J1228+5017 is flat at low frequencies, steepens
above 1.6 GHz, might flatten again when reaching 9.0 GHz, and, to take into
account the Metsähovi detections, probably turns inverted at some point above
9.0 GHz. Radio spectra with similar properties has been seen in some peaked-
spectrum sources (e.g., Baum et al., 1990; Hancock et al., 2010; Callingham et
al., 2017). This feature is explained as restarted activity manifesting as a
peaked spectrum at higher frequencies, superimposed on a spectrum resulting
from a period (or periods) of earlier activity and peaking at lower
frequencies. Intermittent activity could be a plausible explanation since
according to theoretical models young sources with high accretion rates are
more prone to this kind of behaviour (Czerny et al., 2009). However, more
observations over a wider frequency range, and also monitoring observations,
are needed to confirm the shape of the spectrum, and the temporal evolution of
this source.

The host galaxy of J1228+5017 was found to be a late-type galaxy (Björklund et
al. in prep.). Also the host shows an elongated structure toward west and the
1.6 GHz radio emission traces this structure (Fig. 5a). This might be pure
chance, or indicate that the extended radio emission actually originates from
the host galaxy, in which case it would be produced by star formation related
processes. However, the Pan-STARRS $i$-$r$-$g$ colour image (Fig. 5b) shows
that the extended part is clearly red, unlike what would be expected in
presence of strong star formation. The colour difference is so drastic that it
is actually unclear if the blue and the red part belong to the same galaxy, if
they are a pair of merging galaxies, or if the red source is a foreground or a
background galaxy. The redshift of the red region needs to be confirmed with
optical spectroscopy. It would also be interesting to measure the redshift of
the galaxy on the west side of J1228+5017 as it might be a companion galaxy.
Also the origin and the morphology of the radio emission needs to be studied
in detail to understand what actually is the origin of the extended radio
emission.

###  6.2 J1522+3934

J1522+3934 is a nearby NLS1 at $z$ = 0.077. It was detected in FIRST with a
flux density of 2.52 mJy, which is comparable to its optical flux density,
implying that no enhanced activity was expected from the AGN. However, so far
its maximum flux density observed at 37 GHz has been 1.43 Jy, unquestionably
confirming the presence of relativistic jets (Lähteenmäki et al., 2018). Its
integrated JVLA flux densities are (3.18 $\pm$ 0.07) mJy, (0.40 $\pm$ 0.02)
mJy, and (0.35 $\pm$ 0.02) mJy at 1.6, 5.2, and 9.0 GHz, respectively,
consistent with a steep spectral index (Fig. 1).

At 1.6 GHz it shows considerable symmetric extended emission in east/south-
east - west/north-west direction (Fig. 3a). The extended emission is barely
detectable at 5.2 and 9.0 GHz (Figs. 3c and e), indicating that its spectral
index is quite steep. The total spectral index at 1.6 GHz is -0.80 and the
core spectral index is -0.71. The $\alpha$ map (Fig. 3a) shows regions of
slightly steeper indices, but also the errors in these regions are higher
(Fig. 3b). Nevertheless, the core does not seem to be as steep as the extended
emission regions. The traditionally estimated peak spectral index (-0.48 $\pm$
0.30) is flatter than the core spectral index, but the results are in
agreement within the error limits.

At 5.2 GHz the source shows only a slightly extended morphology toward east
(Fig. 3c). Interestingly, at this frequency, the source seems to be flat: the
total spectral index is -0.42, and the core spectral index is -0.47. The
traditional spectral index is in agreement with $\alpha$ = -0.55 $\pm$ 0.45.
The 9.0 GHz map does not show considerable structures either, just a faint
region of diffuse emission on the east side of the nucleus (Fig. 3e). Also at
this frequency the source seems flat: the total spectral index is 0.49, and
the core spectral index is 0.35. The source is so faint at 9.0 GHz that these
values are probably not totally reliable as edge and low S/N effects start to
affect. However, the traditional spectral index also indicates a flat spectrum
with $\alpha$ = -0.46 $\pm$ 1.08, but also in this case the result is affected
by the poor S/N, resulting in huge errors. However, the 5.2 and 9.0 GHz
behaviour together can be seen as a hint of the existence of the relativistic
jets in the nucleus of the source.

The end-to-end extent of the emitting region at 1.6 GHz is 19.4 kpc, and when
overlaid with its host galaxy $i$-band image from Pan-STARRS (Fig. 5c) it is
evident that the radio emission is perpendicular to the disk of the host. The
host was studied in detail in Järvelä et al. (2018) who found that it is a
barred disk-like galaxy with a pseudo-bulge, and merging with another,
possibly a dwarf, galaxy. The projected distance between the nuclei is 2.2 kpc
and the radio emission is centred at the NLS1. The NLS1 nucleus seems to be
strongly obscured in $i$-band compared to the near-infrared $Ks$-band (see
Fig. 13 in Järvelä et al., 2018). Indeed, a prominent dust lane, probably
obscuring the NLS1 nucleus, can be seen in the Pan-STARRS $g$ and $z$ -band
colour image (Fig. 5d).

Taking into account the approximately 90 degree angle between the position
angles of the host disk and the radio emission it seems improbable that star
formation could be responsible for the radio emission. Also B20, using the
strength of the mid-infrared emission as a proxy for the star formation,
reached the conclusion that star formation alone is not enough to account for
the 1.6 GHz radio emission. A more plausible explanation is that the AGN is
responsible for the extended emission. Indeed, based on the 37 GHz
observations we know that this source host relativistic jets, and taking into
account the maximum flux density levels detected, exceeding 1 Jy, it would
seem that beaming is needed, in which case we would be seeing the jet at a
small angle. Also the flat spectral indices observed in the core support this.
A small viewing angle is not consistent with the symmetrical extended
morphology we see at 1.6 GHz, which implies a considerably larger inclination.
One option is that the jets have changed direction, possibly by precession, or
alternatively, they have turned off and restarted with a different viewing
angle (Hernández-García et al., 2017). There is no evidence of the S-shaped
morphology usually seen in jet precession cases (Ekers et al., 1978; Parma et
al., 1985). In fact, the morphology does not seem perturbed at all.
Furthermore, the radio spectrum that remains steep at least up to 9.0 GHz, and
seems to turn very inverted above that, supports the intermittency scenario as
such extreme spectral indices are usually seen in kinematically young,
absorbed jets, such as in peaked sources (PS, e.g., O’Dea and Saikia, 2021).
J1522+3934 is not a traditional peaked source since they usually exhibit steep
core emission at kpc scales. Instead, J1522+3934 seems to be more core-
dominated, adding further proof to the scenario in which it is seen at a small
angle. Czerny et al. (2009) hypothesise that in young sources with high
accretion rates, such as NLS1s, radiation pressure instabilities of the
accretion disk can result in intermittent activity. They estimate that the
activity periods last 103-104 yr and are separated by periods of 104-106 yr.
In this scenario the extended emission would be a relic of the past activity
period, and not replenished anymore if the jet direction has changed. However,
it does not show extraordinarily steep spectral indices, as seen in the case
of Mrk 783 (Congiu et al., 2017, 2020), which might indicate that it is not
very old. This is not unheard of since such sources have been observed before
(e.g., Dennett-Thorpe et al., 2002).

Re-orientation of the jets, aligned with the spin axis of the black hole, can
happen when some incident perturbs the dynamics of the very innermost region
of an AGN, resulting in a change of the spin axis. Such an event can be, for
example, a black hole - black hole merger, or an accretion period of matter
with an angular momentum axis different to the current system axis, which can
re-orient the accretion disk, and finally lead to a realignment of the black
hole spin axis (Dennett-Thorpe et al., 2002; Gopal-Krishna et al., 2012).
These events are often related to galaxy-galaxy mergers or interaction, which
could be the culprit also in case of J1522+3934 since it resides in a merging
system. More observations are certainly needed to investigate the properties
of this intriguing source in more detail.

###  6.3 J1641+3454

J1641+3454 is an NLS1 at a redshift of 0.164. It was detected in FIRST with an
integrated flux density of 2.69 mJy, so the radio emission slightly dominates
over the optical emission. However, 37 GHz detections at Metsähovi, and later
a gamma-ray detection by Fermi (Lähteenmäki et al., 2017) confirmed the
presence of most likely relativistics jets in this source. The JVLA
measurements do not reflect this at all since its integrated flux densities
are (2.34 $\pm$ 0.06) mJy, (0.69 $\pm$ 0.02) mJy, and (0.35 $\pm$ 0.01) mJy,
at 1.6, 5.2, and 9.0 GHz, respectively (Fig. 1). Its 1.6 GHz radio map (Fig.
4a) shows diffuse extended emission around the centre. At 5.2 GHz the emission
resolves to north and southward extended parts, surrounded by diffuse-looking
patches (Fig. 4c). At 9.0 GHz there are no traces of the extended emission
(Fig. 4e). The $\alpha$ maps ((Fig. 4a, c, e) as well as the traditional
spectral indices agree that its radio emission has a steep spectral index at
all observed frequencies, consistent with optically thin emission, or even
steeper.

Estimating the star formation produced radio emission using mid-infrared
measurements B20 found that it is somewhat higher than what star formation
should produce. There are no clear signs of the nuclear activity, but the
source is slightly elongated in the north-south direction at 5.2 and 9.0 GHz
maps, which could be due to the nuclear activity. On the other hand, the
patchy emission around the core at 5.2 GHz resembles diffuse morphology often
associated with star formation. In NLS1s high-level AGN activity, as well as
nuclear star formation, can, and often do co-exist (Sani et al., 2010;
Caccianiga et al., 2015, Järvelä et al. 2021, submitted). The host (Figs.4e
and f) was found to be a barred late-type galaxy with a pseudo-bulge (Olguín-
Iglesias et al., 2020), so the presence of star formation could be expected.
It is interesting that unlike J1228+5017 and J1522+3923, J1641+3454 does not
show any signs of flattening even at 9.0 GHz. More observations are required
before we can determine whether it is similar to peaked-spectrum sources with
a semi-stationary spectrum peaking at high frequency, a source with extreme
variability, or possibly a combination of the two.

##  7 Discussion

###  7.1 Origin of the radio emission

Based on B20 the radio emission of these sources could be almost entirely
explained with the star formation processes in their host galaxies. However,
the spectral index maps revealed details, such as flat cores, not seen in the
normal radio maps, and were able to give us a better view of the different
mechanisms in play. Also the examination of the host galaxies in relation to
the radio maps proved fruitful, especially in case of J1522+3934. Based on
these new analyses it seems like J1641+3454 is the only source whose radio
emission could be explained by star formation only. Both, J1228+5017 and
J1522+3934 show a flat core, that can be consider as a certain sign of AGN
activity in form of a jet or at least a jet base, at least at some frequency.
It cannot be determined whether all the emission in J1228+5017 has an AGN
origin, but it seems to be the predominant source. Interestingly, based on the
90 degree mismatch between the host and the extended radio emission in
J1522+3934, and its flat core, it seems safe to assume that the AGN is the
origin of almost all radio emission seen in this source.

These results are somewhat surprising, taking into account the faintess of the
emission, and their steep spectral indices measured in B20. This highlights
the necessity to study AGN as individuals, as the general properties of these
sources before the 37 GHz ad JVLA observations pointed to very different
classification compared to what we have learned based on more detailed
studies. These results also demostrate that spatially resolved spectral index
maps can be utilised to examine the sources beyond what can be achieved with
normal radio maps.

###  7.2 Intermittent activity

Two of the three sources show possible signs of intermittent activity. As
discussed before in Sects. 6.1 and 6.2, this is not unexpected behaviour in
young sources, accreting at high rates (Czerny et al., 2009). Examples of
extreme realignments or precession, that can result in similar-looking
features, are seen among many classes of AGN (Dennett-Thorpe et al., 2002;
Hernández-García et al., 2017; Congiu et al., 2020). Naturally, these sources
show a host of different properties, depending on the duration of their
earlier and current activity periods, and if and how the jet direction has
changed. In radio band these sources can be identified by peculiar radio
morphologies and very steep spectral indices of the emission of the past
activity cycles, or by spectra showing unusual shapes.

In case of J1522+3934 the telltale sign is the discrepancy between the large-
scale radio moprhology and the fact that the currect radio emission seems to
be beamed. Since no signs of precession are seen we assume the jets have been
turned off during the re-orientation, or it has happened very rapidly. The
extended emission does not show signs of aging since its spectral index does
not deviate significantly from that of optically thin jet emission. This
indicates that the re-orientation must have happened quite a short time ago.
Also the peaked-spectrum source -like radio spectrum of J1522+3934 suggests
that it is currently a kinematically young source. It can be speculated that
the close galaxy merger J1522+3934 resides in might be the culprit behind the
re-alignement, probably by means of an accretion episode of matter with a
drastically different angular momentum vector than the earlier system axis.
The case of J1228+5017 is not so clear, but the supposedly double-humped shape
of its radio spectrum indicates that there might be some large-scale relic
emission present at lower frequencies. Both these sources are extremely
interesting, and, if confirmed, bring a valuable addition to the impressive
group of AGN with intermittent activity.

###  7.3 Host galaxies

It has been found in earlier studies that NLS1s with relativistic jets seems
to reside in interacting host galaxies more often than their non-jetted
counterparts (e.g., Antón et al., 2008; Järvelä et al., 2017; Crenshaw et al.,
2003; Ohta et al., 2007; Orban de Xivry et al., 2011). However, we remark here
that all the studied non-jetted NLS1s are at very low redshifts (e.g.,
Crenshaw et al., 2003; Orban de Xivry et al., 2011; Ohta et al., 2007) ,
whereas the jetted NLS1s tend to lie at higher redshifts (e.g., Romano et al.,
2018), so the role of the cosmic evolution is unclear. Either way,
observations of the hosts of non-jetted NLS1s at high redshifts are necessary
to make any solid conclusions. It should be also noted that the claim of their
non-jettedness is usually based on their low-frequency radio properties, and,
for example, this paper demonstrates why that is not an advisable way to
classify sources, let alone draw any conclusions about their properties.

All this kept in mind, our results do not lean either way. J1522+3934 is a
late-type galaxy clearly in a merger, and it has also been confirmed with
spectroscopy (Järvelä et al., 2017). Taking into account the small projected
separation of the nuclei it seems to be an evolved merger, and the galaxies
can be expected to affect, for example, the gas dynamics of each other. Like
mentioned before, this can be the cause behind the jet re-oriention in
J1522+3934. On the other hand, J1641+3454 was carefully studied in optical
band in Olguín-Iglesias et al. (2020), and they found it to be a pristine
late-type galaxy, with no signs of interaction. Though minor mergers can go
unnoticed when studying only the optical morphology, and require more
sensivite methods, for example, spatially resolved spectroscopy to detect them
(Longinotti et al., 2018). Nothing definite can be said about the status of
J1228+5017 before further observations to determine if the sources optically
close to it are at the same redshift. It is noteworthy that all these sources
host powerful jets, but reside in late-type galaxies, directly contradicting
the traditional jet paradigm (Laor, 2000). So much evidence has gathered
against it after it was formulated that we should finally let go of it, and
see the host galaxy as well as the black hole mass as one factor affecting the
evolution and properties of AGN, but definitely not the determining one
(Foschini, 2020).

###  7.4 Absorption or extreme variability?

There is a stark dissonance between the low- and high-frequency properties of
these sources: at least up to 9.0 GHz they show generally steep radio spectra,
and are very faint, at mJy or $\mu$Jy levels, on the other hand, at 37 GHz
their average flux densities are of the order of $\sim$500 mJy, exceeding 1
Jy. To explain these properties some form of absorption, extreme variability,
or both need to be present. It seems unlikely that only variability could
explain this behaviour. For example, if J1522+3934 had a flat spectrum from
9.0 to 37 GHz, its flux density would have to 4000-fold to explain the peak
flux densities seen at 37 GHz. The beam size difference between the JVLA and
Metsähovi can account for some of the discrepancy but hardly this much.
Furthermore, also the JVLA observations cover the whole host galaxy, so the
source of the variable radio emission would need to be outside the host, or
very extended, both of which seem very improbable.

It seems inevitable that the spectrum needs to turn inverted above 9.0 GHz,
for which a form of absorption is needed. With our current data it remains
unclear whether the spectral indices are extreme enough to require free-free
absorption (FFA, with a theoretical maximum spectral index of 4), or if
synchrotron self-absorption would be enough (SSA, with a maximum spectral
index of $\sim$2.5 in a simplified case). Either way, these sources resemble
peaked-spectrum sources, such as compact steep-spectrum and gigahertz-peaked
sources, that are known to be AGN with kinematically young jets (O’Dea and
Saikia, 2021), that are absorbed either via FFA or SSA. However, some
properties set them apart from the general peaked-spectrum source population:
flat core spectral indices in kpc-scale observations, with the exception of
J1641+3454, and the high-frequency variability. Generally peaked-spectrum
sources show very moderate variability, and flat cores are detected in them
only with very high resolution observations. Since our sources seem to be more
core-dominated and show signs of beaming, like the 37 GHz radio flares, an
alternative explanation might be needed. To avoid unphysical flux density
increase an absorbed component is likely present above 9.0 GHz, but it cannot
account for the high-amplitude, rapid flares, thus it seems plausible that the
jet-base and possibly the innermost parts of the jet are not absorbed. Also
the flat core spectra supports this scenario. Such a geometry can be achieved
if we assume that the high-frequency peaked component is caused by FFA in a
limited space, for example, in shocked ambient clouds in front of the jet
head, and we see the jet at an optimal angle where beaming is still effective,
but the angle is large enough so that the core/inner jet emission does not get
absorbed by the FFA screen. Another mechanism that produces flat, $\alpha\sim$
-0.1, radio emission in galaxies is the free-free emission of ionised gas. In
principle, if the jet is absorbed by FFA, we have ample ionised gas in the jet
- interstellar medium shock front, that can produce free-free emission.
However, according to studies this emission is swamped by the synchtroton
emission in the shock, and not prominent (Contini and Viegas-Aldrovandi,
1990). However, at this point our knowledge of these sources is so limited
that we can only guess what their real nature is. In any case, they are
extraordinary sources with distinctive properties, and should definitely be
studied in detail in the future.

##  8 Summary and conclusions

In this paper we studied a sample of NLS1 galaxies exhibiting strongly
variable emission at 37 GHz in single dish observations, but showing steep
spectral indices up to 9.0 GHz in JVLA observations, indicating strong
absorption and/or extreme variability. Our aim was to study their 1.6, 5.2,
and 9.0 GHz radio emission in more detail, to investigate the origin of their
radio emission, and check if any signs of the jet activity are detectable. Our
main tool were the spatially resolved spectral index maps of each source at
all observed frequencies. We were able to produce these maps for three
sources, J1228+5017, J1522+3934, and J1641+3454. All of these sources proved
to be quite different in their properties. The only common factor seems to be
that they are hosted by late-type galaxies, albeit J1522+3934 is in a merger,
and J1228+5017 might be interacting as well. J1228+5017 and J1522+3934 also
show signs of the AGN activity in form of flat core spectral indices, and in
J1522+3934 the AGN seems to be the predominant source of radio emission. Both
these NLS1s show some indication of past activity periods and re-started
activity, and, furthermore, the jet direction of J1522+3934 might have
changed. J1641+3454 does not show any signs of the jets as its total and core
spectral indices remain steep up to 9.0 GHz, and its radio emission could be
explained with star formation processes. However, a past gamma-ray detection
strongly implies that the jets are present, but they must be heavily absorbed
below 9.0 GHz.

NLS1s are complicated sources, where often an interplay between the AGN, the
various phenomena related to it, and the host galaxy can be seen. As such
they, however, offer us an unprecedented view of the early stages of the
evolution of powerful AGN, like flat-spectrum radio quasars, without the need
to go to high redshifts. They can offer us clues about the circumstances
required to trigger the jets, as well as the long-term behaviour of jetted
systems, including intermittent activity. For now our knowledge of NLS1s is
still very limited, and even more so with the peculiar individuals
investigated in this paper. Further observations, especially simultaneous
observations of the whole radio spectrum, more observations at high radio
frequencies, high-resolution radio imaging, and spatially resolved
spectroscopy will be needed to understand the emission and absorption
mechanisms in play in these sources.

## Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any
commercial or financial relationships that could be construed as a potential
conflict of interest.

## Author Contributions

Conceptualisation, analysis and writing, E.J.; analysis and writing, M.B.;
analysis and writing L.C. All authors have read and agreed to the published
version of this manuscript.

## Funding

This research received no external funding.

## Acknowledgments

E.J. is a current ESA research fellow. The authors thank E.C. for his help in
the data visualisation. This paper is based (in part) on results obtained with
LOFAR equipment. LOFAR (van Haarlem et al., 2013) is the Low Frequency Array
designed and constructed by ASTRON. The National Radio Astronomy Observatory
is a facility of the National Science Foundation operated under cooperative
agreement by Associated Universities, Inc. The Pan-STARRS1 Surveys (PS1) and
the PS1 public science archive have been made possible through contributions
by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS
Project Office, the Max-Planck Society and its participating institutes, the
Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute
for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham
University, the University of Edinburgh, the Queen’s University Belfast, the
Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory
Global Telescope Network Incorporated, the National Central University of
Taiwan, the Space Telescope Science Institute, the National Aeronautics and
Space Administration under Grant No. NNX08AR22G issued through the Planetary
Science Division of the NASA Science Mission Directorate, the National Science
Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand
University (ELTE), the Los Alamos National Laboratory, and the Gordon and
Betty Moore Foundation. 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 Participating
Institutions are The University of Chicago, Fermilab, the Institute for
Advanced Study, the Japan Participation Group, The Johns Hopkins University,
the Korean Scientist Group, Los Alamos National Laboratory, the Max-Planck-
Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics
(MPA), New Mexico State University, University of Pittsburgh, University of
Portsmouth, Princeton University, the United States Naval Observatory, and the
University of Washington. 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. This research has made use of
the SIMBAD database, operated at CDS, Strasbourg, France.

## Data Availability Statement

The datasets analysed for this study can be found in the NRAO Archive,
https://archive.nrao.edu/archive/advquery.jsp.

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## Figure captions

Table 1: Summary of the source properties, and archival average Metsähovi flux densities. Columns: (1) source name, (2) and (3) right ascension and declination, from the JVLA X or C band observations in case of a detection, otherwise from FIRST (marked after the source name with X, C, or F, respectively), (4) redshift, obtained from the Sloan Digital Sky Survey (Abazajian et al., 2009), (5) black hole mass, (6) scale at the redshift of the source, (7) Metsähovi 37 GHz average flux density. source | RA | Dec | $z$ | log $M_{\text{BH}}$ | scale | $S_{\text{MH, 37~{}GHz}}^{\text{ave}}$
---|---|---|---|---|---|---
| (hh mm ss.s) | (dd mm ss.s) |  | ($M_{\odot}$) | (kpc/arcsec) | (mJy)
J102906.69+555625.2F | 10 29 06.69 | +55 56 25.25 | 0.451 | 7.33 | 9.685 | 420
J122844.81+501751.2X | 12 28 44.82 | +50 17 51.24 | 0.262 | 6.84 | 5.657 | 410
J123220.11+495721.8C | 12 32 20.11 | +49 57 21.79 | 0.262 | 7.30 | 5.626 | 460
J150916.18+613716.7F | 15 09 16.17 | +61 37 16.80 | 0.201 | 6.66 | 4.313 | 670
J151020.06+554722.0X | 15 10 20.05 | +55 47 22.11 | 0.150 | 6.67 | 3.214 | 450
J152205.41+393441.3X | 15 22 05.50 | +39 34 40.46 | 0.077 | 5.97 | 1.650 | 590
J164100.10+345452.7X | 16 41 00.10 | +34 54 52.66 | 0.164 | 7.15 | 3.518 | 370
Table 2: Summary of the radio flux densities of the sources. Columns: (1) source name, (2) units, (3) JVLA flux densities and rms at 1.6 GHz, (4) JVLA flux densities and rms at 5.2 GHz, (5) JVLA flux densities and rms at 9.0 GHz, (6) LoTTS flux densities at 144 MHz. N/A means the source in not within the published LoTTS region. source |  | $S_{\text{1.6~{}GHz, JVLA}}$ | $S_{\text{5.2~{}GHz, JVLA}}$ | $S_{\text{9.0~{}GHz, JVLA}}$ | $S_{\text{144~{}MHz, LoTTS}}$
---|---|---|---|---|---
J1228+5017 | rms (mJy) | 0.020 | 0.008 | 0.007 |
| int (mJy) | 0.97 $\pm$ 0.04 | 0.29 $\pm$ 0.01 | 0.19 $\pm$ 0.01 | 2.3 $\pm$ 0.1
| peak (mJy beam-1) | 0.73 $\pm$ 0.02 | 0.24 $\pm$ 0.01 | 0.18 $\pm$ 0.01 | 2.1 $\pm$ 0.1
J1522+3934 | rms (mJy) | 0.017 | 0.010 | 0.007 |
| int (mJy) | 3.18 $\pm$ 0.07 | 0.40 $\pm$ 0.02 | 0.35 $\pm$ 0.02 | N/A
| peak (mJy beam-1) | 1.05 $\pm$ 0.05 | 0.33 $\pm$ 0.01 | 0.19 $\pm$ 0.01 |
J1641+3454 | rms (mJy) | 0.025 | 0.007 | 0.007 |
| int (mJy) | 2.34 $\pm$ 0.06 | 0.69 $\pm$ 0.02 | 0.35 $\pm$ 0.01 | N/A
| peak (mJy beam-1) | 1.82 $\pm$ 0.03 | 0.51 $\pm$ 0.01 | 0.28 $\pm$ 0.01 |
Table 3: Weighted total and core spectral indices measured from the $\alpha$ maps. Columns: (1) Source name, (2) are within which the spectral index was measured, (3) 1.6 GHz spectral index, (4) 5.2 GHz spectral index; (5) 9.0 GHz spectral index. source | area | $\alpha_{\text{1.6~{}GHz, JVLA}}$ | $\alpha_{\text{5.2~{}GHz, JVLA}}$ | $\alpha_{\text{9.0~{}GHz, JVLA}}$
---|---|---|---|---
J1228+5017 | total | -0.41 $\pm$ 0.01 | -1.00 $\pm$ 0.02 | -0.31 $\pm$ 0.01
| core | -0.38 $\pm$ 0.01 | -1.24 $\pm$ 0.06 | -0.43 $\pm$ 0.03
J1522+3934 | total | -0.80 $\pm$ 0.01 | -0.42 $\pm$ 0.01 | 0.49 $\pm$ 0.01
| core | -0.71 $\pm$ 0.01 | -0.47 $\pm$ 0.02 | 0.35 $\pm$ 0.01
J1641+3454 | total | -0.85 $\pm$ 0.01 | -0.61 $\pm$ 0.01 | -0.83 $\pm$ 0.01
| core | -0.99 $\pm$ 0.01 | -0.78 $\pm$ 0.01 | -1.16 $\pm$ 0.01
Table 4: Traditional spectral indices calculated using the peak flux densities. Columns: (1) Source name, (2) In-band spectral index with a central frequency of 1.6 GHz, band-width 1 GHz, (3) in-band spectral index with a central frequency of 5.2 GHz, band-width 2 GHz, (4) in-band spectral index with a central frequency of 9.0 GHz, band-width 2 GHz. source | $\alpha_{\text{1.6~{}GHz, JVLA}}$ | $\alpha_{\text{5.2~{}GHz, JVLA}}$ | $\alpha_{\text{9.0~{}GHz, JVLA}}$
---|---|---|---
J1228+5017 | -0.19 $\pm$ 0.19 | -1.11 $\pm$ 0.43 | -0.46 $\pm$ 0.91
J1522+3934 | -0.48 $\pm$ 0.30 | -0.55 $\pm$ 0.45 | -0.46 $\pm$ 1.08
J1641+3454 | -0.88 $\pm$ 0.13 | -0.94 $\pm$ 0.22 | -1.13 $\pm$ 0.52
Figure 1: Radio spectra of our sources from 144 MHz to 37 GHz. The JVLA data
for J1228+5017, J1522+3934, and J1641+3454 from this paper, for other sources
from B20. Simultaneous detections are connected with a solid line, archival
detections and upper limits with a dashed line. The colours and symbols
explained in the legend. Figure 2: a) 1.6 GHz $\alpha$ map, rms = 20$\mu$Jy
beam-1, beam size 9.56 $\times$ 7.41 kpc; b) 1.6 GHz $\Delta\alpha$ map; c)
5.2 GHz $\alpha$ map, rms = 7.5$\mu$Jy beam-1, beam size 2.83 $\times$ 2.31
kpc; d) 5.2 GHz $\Delta\alpha$ map; e) 9.0 GHz $\alpha$ map, rms = 6.5$\mu$Jy
beam-1, beam size 1.58 $\times$ 1.19 kpc f) 9.0 GHz $\Delta\alpha$ map.
Contour levels at -3, 3 $\times$ 2n, $n\in$ [0, 3] in all figures. The scale
is 5.657 kpc/arcsec. Figure 3: a) 1.6 GHz $\alpha$ map, rms = 17$\mu$Jy
beam-1, beam size 2.26 $\times$ 2.13 kpc; b) 1.6 GHz $\Delta\alpha$ map; c)
5.2 GHz $\alpha$ map, rms = 10$\mu$Jy beam-1, beam size 0.73 $\times$ 0.66
kpc; d) 5.2 GHz $\Delta\alpha$ map; e) 9.0 GHz $\alpha$ map, rms = 7$\mu$Jy
beam-1, beam size 0.41 $\times$ 0.38 kpc f) 9.0 GHz $\Delta\alpha$ map.
Contour levels at -3, 3 $\times$ 2n, $n\in$ [0, 4] in a) and b), and $n\in$
[0, 3] in c)-f). The scale is 1.650 kpc/arcsec. Figure 4: a) 1.6 GHz $\alpha$
map, rms = 25$\mu$Jy beam-1, beam size 4.96 $\times$ 4.64 kpc; b) 1.6 GHz
$\Delta\alpha$ map; c) 5.2 GHz $\alpha$ map, rms = 7$\mu$Jy beam-1, beam size
1.48 $\times$ 1.44 kpc; d) 5.2 GHz $\Delta\alpha$ map; e) 9.0 GHz $\alpha$
map, rms = 7$\mu$Jy beam-1, beam size 0.84 $\times$ 0.81 kpc f) 9.0 GHz
$\Delta\alpha$ map. Contour levels at -3, 3 $\times$ 2n, $n\in$ [0, 4] in
a)-d), and $n\in$ [0, 3] in e) and f). The scale is 3.518 kpc/arcsec. Figure
5: a) J1228+5017 1.6 GHz radio map overlaid with the $i$-band Pan-STARRS
image; b) J1228+5017 Pan-STARRS $i$-$r$-$g$ colour image; c) J1522+3934 1.6
GHz radio map overlaid with the $i$-band Pan-STARRS image; d) J1522+3934 Pan-
STARRS $z$-$g$ colour image; e) J1641+3454 5.2 GHz radio map overlaid with the
$i$-band Pan-STARRS image; f) J1641+3454 Pan-STARRS $z$-$g$ colour image.