# Extended Line Emission in the BCG of Abell 2390

Leo Y. Alcorn,1 H.K.C Yee,1 Laurent Drissen, 2,3 Carter Rhea, 3,4 Suresh
Sivanandam, 1,5 Julie Hlavacek-Larrondo, 3,4 Bau-Ching Hsieh, 6 Lihwai Lin, 6
Yen-Ting Lin, 6 Qing Liu, 1,5 Adam Muzzin, 7 Allison Noble 8 and Irene Pintos-
Castro9
1David A. Dunlap Department of Astronomy & Astrophysics, University of
Toronto, 50 St. George St., Toronto, ON M5S 3H4, Canada
2Département de Physique, de génie physique et d’optique, Université Laval,
Québec, QC, G1V 0A6, Canada
3Centre de Recherche en Astrophysique du Québec (CRAQ)
4Département de Physique, Université de Montréal, Succ. Centre-Ville,
Montréal, Québec, H3C 3J7, Canada
5Dunlap Institute of Astronomy and Astrophysics, University of Toronto, 50 St.
George St, Toronto, ON, Canada
6Institute of Astronomy and Astrophysics, Academia Sinica, No. 1, Section 4,
Roosevelt Road, Taipei 10617, Taiwan
7Department of Physics and Astronomy, York University, 4700 Keele Street,
Toronto, Ontario, ON MJ3 1P3, Canada
8School of Earth and Space Exploration, Arizona State University, Tempe, AZ,
85287, USA
9Centro de Estudios de Física del Cosmos de Aragón, Pl. San Juan, 1, 44001
Teruel, Spain E-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We report CFHT/SITELLE imaging Fourier Transform Spectrograph observations of
the Brightest Cluster Galaxy (BCG) of galaxy cluster Abell 2390 at $z=0.228$.
The BCG displays a prominent cone of emission in H$\alpha$, H$\beta$, [Nii],
and [Oii] to the North-West with PA = 42o, 4.4″ in length (15.9 kpc), which is
associated with elongated and asymmetric Chandra soft X-ray emission. The
H$\alpha$ flux map also contains a “hook” of H$\alpha$ and [Nii] emission
resulting in a broadened northern edge to the cone. Using SITELLE/LUCI
software we extract emission line flux, velocity, velocity dispersion, and
continuum maps, and utilize them to derive flux ratio maps to determine
ionization mechanisms and dynamical information in the BCG’s emission line
region. The Baldwin-Phillips-Terlevich diagnostics on the BCG cone indicate a
composite ionization origin of photoionization due to star formation and
shock. Strong LINER-like emission is seen in the nuclear region which hosts an
AGN. As Abell 2390 is a cool-core cluster, we suggest that the cooling flow is
falling onto the central BCG and interacting with the central AGN. The AGN
produces jets that inflate “bubbles” of plasma in the ICM, as is often
observed in local galaxy clusters. Furthermore, combining signs of AGN
activities from radio, optical emission line and X-ray data over a large range
of physical scale, we find evidence for three possible episodes of AGN
activity in different epochs associated with the Abell 2390 BCG.

###### keywords:

galaxies: clusters: Abell 2390 – galaxies: elliptical and lenticular, cD –
galaxies: individual: J21536+1741

††pubyear: 2022††pagerange: Extended Line Emission in the BCG of Abell
2390–Extended Line Emission in the BCG of Abell 2390

##  1 Introduction

The Brightest Cluster Galaxies (BCGs) of galaxy clusters are a unique
population of galaxies, with marked differences from typical cluster galaxies
(e.g. Lin et al., 2010). They are the brightest and most massive galaxies
observed in our universe, and are located near the galaxy cluster central
potential (Oegerle & Hoessel, 1991; Lin & Mohr, 2004; Lauer et al., 2014). BCG
dark matter and stellar halos transition smoothly into the dark matter halo of
the entire cluster, and into the diffuse intra-cluster light (see Montes,
2022, and references therein). The majority of BCGs are slow-rotating
elliptical galaxies (Fisher et al., 1995; Tran et al., 2008; Brough et al.,
2011; Jimmy et al., 2013), but in contrast to typical slow-rotating
ellipticals in clusters, they are more likely to have strong optical nebular
emission (McNamara & O’Connell, 1993; Crawford et al., 1999; Fogarty et al.,
2015; Hamer et al., 2016) in certain environmental conditions such as cool-
core clusters (Heckman et al., 1989; Cavagnolo et al., 2008; McDonald et al.,
2010; Tremblay et al., 2015; Hogan et al., 2017; Calzadilla et al., 2022).

In cool-core clusters, or, clusters which have a cooling flow, the density of
the density of the intra-cluster medium (ICM) is inversely proportional to the
cooling time, so cooling flows occur when the radiative cooling time of the
cluster gas is significantly shorter than the age of the cluster. The density
of the ICM at the centers of clusters increases because cool ICM gas is
compressed by the ICM outside the cluster center. In order to maintain the
hydrostatic equilibrium of the cluster, hot ICM gas begins to flow inward,
toward the peak of the cluster dark matter halo and the BCG. This flow of hot
gas in toward the center of the cluster is the "cooling flow" in cool-core
clusters (Fabian, 1994; Hudson et al., 2010; Blanton et al., 2010).

The BCGs of cool-core clusters are associated with peaks in X-ray emission and
the previously mentioned optical nebular emission (Crawford et al., 1999;
McNamara & Nulsen, 2007; Cavagnolo et al., 2010; Hudson et al., 2010; Fabian,
2012). They are also often hosts of radio-loud active galactic nuclei (AGN),
which create radio lobes and jets (McNamara et al., 2000; Egami et al., 2006;
Lin & Mohr, 2007; Ea et al., 2010). When the in-falling gas interacts with the
supermassive black hole (SMBH) in a BCG, this can transition the SMBH to its
active (kinetic) phase (Fabian, 2012; Hamer et al., 2016), although in
contrast accretion can also result in a radiative phase (Churazov et al.,
2005; Russell et al., 2013; Hlavacek-Larrondo et al., 2013). The jet activity
of a SMBH (AGN feedback) can force pristine gas out of the galaxy and quench
star formation (negative feedback), or compress the gas and cause a starburst
(McNamara et al., 2006; Ea et al., 2010). The AGN of cool-core BCGs have been
observed to inflate X-ray cavities from rising buoyant bubbles of plasma
fuelled by the jets, which entrain cool gas from the galaxy behind them (e.g.
Pope et al., 2010; David et al., 2011; Fabian et al., 2016; McNamara et al.,
2016; Su et al., 2017; Tremblay et al., 2018; Duan & Guo, 2018; Russell et
al., 2019; Chen et al., 2019; Smith & Donohoe, 2021; Zhang et al., 2022),
drawing them outward from the BCG, creating X-ray profiles which deviate from
circular symmetry of the X-ray surface brightness profile and optical emission
line nebulae that extend out from the BCG. Bubbles and entrained gas can be
disrupted via cooling instabilities, merger activity, and gas sloshing, which
create bent or horseshoe-shaped emission-line nebulae observed in
H$\alpha$(Fabian et al., 2003; Ueda et al., 2018). Examples of this process in
the local universe can be seen in NGC1275 of the Perseus cluster (Churazov et
al., 2000; Fabian et al., 2003; Aharonia et al., 2018), and NGC4696 of the
Centaurus cluster (Fabian et al., 2005), among many others.

Abell 2390 (hereafter, A2390) is a massive cluster at $z=0.228$, with
$M_{200}\sim 1.3\times 10^{15}M_{\odot}$ and $\sigma_{v,cluster}\sim 1100$
$km/s$ (Carlberg et al., 1996). It hosts a BCG with a prominent extended
nebular emission line region and an asymmetric X-ray profile. A2390111The
cluster center is located at a Right Ascension of 21:53:36 and a Declination
of +17:41:43 in the J2000 epoch (Haines et al., 2013) was observed as part of
the CNOC1 cluster survey on CFHT using the MOS multi-object spectrograph (Yee
et al., 1996a; Yee et al., 1996b; Abraham et al., 1996), providing a catalog
of redshifts, photometry, spectral feature measurements and morphological
indices for 371 cluster galaxies. The cluster contains several prominent
strong gravitational lensing arcs, making it a popular target for high-
redshift astronomy in addition to previously discussed dense evolved cluster
studies (e.g. Pierre et al., 1996; Bézecourt & Soucail, 1997; Frye &
Broadhurst, 1998; Pelló et al., 1999; Balogh & Morris, 2000; Li et al., 2009;
Stroe et al., 2017). X-ray data of A2390 indicate the cluster has a cool core
and anisotropic X-ray morphology (Allen et al., 2001; Hlavacek-Larrondo &
Fabian, 2011; Sonkamble et al., 2014), and it is a dynamically relaxed, non-
merging cluster with a strong radio source (Abraham et al., 1996; Augusto et
al., 2006; Egami et al., 2006).

The BCG of A2390 (named J21536+1741 in Smail et al. (2002) and radio source
center located at Right Ascension 21:53:36.82670 Declination +17:41:43.7260 in
Patnaik et al. (1992)) was observed in multi-wavelength studies (e.g.
Hutchings & Balogh, 2000; Egami et al., 2006; Augusto et al., 2006). The BCG
contains several interesting and unusual properties – an active nucleus with a
flat spectral index compact radio source in the core, and a strong H$\alpha$,
H$\beta$, [Nii], and Ly$\alpha$ emission line extended region with position
angle 42.7o northwest of the nuclear region, referred to as a "cone". This
interplay between the BCG, the SMBH, the surrounding hot ICM, and the cooling
gas make this galaxy and its cone an ideal laboratory for studying the extreme
physics of AGN jets, the most massive galaxies in the universe, and their
effect on the cluster environment, as well as the effect of cold gas accretion
onto a central galaxy AGN from the cluster’s cooling gas flow.

In this paper we present nebular emission line spectral imaging of the central
cD-type BCG of A2390. The data are obtained in three band-limiting filters
designed for the detection of $z\sim 0.25$ diagnostic optical nebular emission
lines (e.g. H$\alpha$, [Nii], H$\beta$, [Oiii], [Oii]). This project is part
of an ongoing $z\sim 0.25$ cluster survey reported in Liu et al. (2021) using
SITELLE on the Canada-France-Hawaii Telescope (CFHT), a Fourier Transform
Integral Field Spectrograph (Drissen et al., 2019). The $11^{\prime}\times
11^{\prime}$ SITELLE field of view makes it ideal for studying galaxy clusters
at low to intermediate redshifts, which can extend out to tens of arcminutes
on the sky. By combining the imaging emission-line data from SITELLE with
multi-wavelength data from radio to X-ray, we present a possible AGN activity
scenario that connects these data.

This paper is structured as follows: Section 2 discusses our CFHT/SITELLE data
and the processing of these data cubes. Observing conditions and data are
reported in Section 2.1, internal data processing and galaxy cataloguing are
the subjects of Section 2.2, and the emission line characterization software
LUCI and its use in this project is introduced in Section 2.3. In Section 3 we
describe existing multi-wavelength data in Chandra X-ray, HST optical, Spitzer
infrared, and multi-instrument, multi-band radio interferometry of the BCG
that are used to support our work. We present the results from our SITELLE
data in Section 4, and discuss their analysis and the possible interpretations
along with the multi-wavelength data in Section 5. Our data, methods, and
findings are summarized in Section 6. Throughout this work, we assume a
$\Lambda$CDM cosmology, with $\Omega_{m}=0.3$, $\Omega_{\Lambda}=0.7$, and
H0=70 km/s/Mpc. At the redshift of A2390, one arc second is equivalent to 3.65
kpc.

Table 1: SITELLE Observation Details of A2390 Proposal ID | Filter | Wavelength Range (nm) | Seeing (″) | Nsteps |  $\Delta\lambda$ (Å) | R | Emission Lines (Å) | Exposure (s)
---|---|---|---|---|---|---|---|---
16BC03 | C1 | 385-490 | 1.3 | 304 | 5.8 | 600 | [Oii]3727 (blended) | 12,160
19BT07 | C2 | 562-625 | 1.2 | 297 | 3.6 | 1300 | H$\beta$, [Oiii]4959,5007 | 13,365
17AD94 | C4 | 798-823 | 1.3 | 124 | 5.2 | 1080 | H$\alpha$, [Nii]6548,6583 | 11,408

##  2 SITELLE Observations of A2390

###  2.1 Observations and Data

SITELLE was originally intended for the study of emission line objects such as
local nebulae, supernova remnants, and the HII regions of nearby galaxies as
well as star-formation and emission line galaxies in galaxy clusters (Drissen
et al., 2019; Rousseau-Nepton et al., 2019). The wide 11’$\times$11’ field of
view of SITELLE/CFHT makes it an ideal instrument for studying nearby galaxy
clusters (see Figure 1). By observing rich clusters such as A2390 with
SITELLE, one can obtain imaging spectroscopy of an unbiased, luminosity-
weighted sample of cluster galaxies including the cD BCG, identified as
Obj1757C in the catalog generated in Liu et al. (2021). The proper name for
Obj1757C is J21536+1741 (from SIMBAD), but we use the catalog name for the BCG
for brevity.

Figure 1: SITELLE deep image of the central pointing of A2390 ($z=0.228$).
This image shows a single SITELLE pointing $11^{\prime}\times 11^{\prime}$ in
size. The inset and zoomed image is the location of the brightest cluster
galaxy (BCG), Object 1757C ($z=0.231$) from the catalog Liu et al. (2021). The
inset is a $10\arcsec\times 10\arcsec$ cutout under a different stretch to
emphasize the nuclear region in contrast to the cone region of the BCG

. Several bright foreground stars show saturation streaks.

We obtained SITELLE imaging Fourier Transform Spectrograph (iFTS) data cubes
in the C1, C2, and C4 band-limiting filters, which at the redshift of the
cluster will detect the [Oii] doublet from $z=0.03-0.31$ (albeit blended),
[Oiii] and H$\beta$ from $z=0.15-0.25$, and H$\alpha$ and [Nii] from
$z=0.21-0.25$, respectively. These filters cover the primary optical
diagnostic nebular emission lines at the redshift of Obj1757C. Images from
SITELLE have a spatial scale of $0.322\arcsec$/pix, which at the cluster
redshift of $z\sim 0.228$ is an angular scale of 3.6 kpc/$\arcsec$, or 1.17
kpc per pixel. These data are available on the Canadian Astronomical Data
Centre website at the observation IDs222https://www.cadc-ccda.hia-iha.nrc-
cnrc.gc.ca/en/ listed in Table 1. C1 was observed on August 27th, 2016 (PI:
Loubser) as part of the SITELLE commissioning dataset. C2 and C4 were observed
on September 27th, 2019 (PI: Hsieh) and July 6th, 2017 (PI: Yee),
respectively. Exposure times (including overhead between wavenumber steps),
resolution, seeing, and filter band widths are listed in Table 1.

The initial data products are data cubes, with a spectrum at each spatial
element (Figure 2). Each spectrum is made up of multiple “channels", or
wavelength steps. The data are reduced and calibrated through the ORB software
(Martin et al., 2016)333https://github.com/thomasorb/orbs. The C4 data is
processed through a software
pipeline444https://github.com/NGC4676/SITELLE_ELG_finder described in Liu et
al. (2021), which identifies H$\alpha$-emitting galaxies and removes sky
contamination.

###  2.2 Emission-line Galaxy Identification and Cataloguing

Figure 2: Example spectra at two single pixels in different locations of the
BCG. The “Nucleus" (left) column contains spectra in C4, C2, and C1 of the BCG
at the location of the H$\alpha$ centroid. The “Cone" (right) column contains
spectra from the same pixel in the cone at 3.4$\arcsec$ from the H$\alpha$
centroid. In each panel, the spectrum is shown in black, and flux units are in
erg s-1 cm-2 Å-1. Emission line locations are marked in red and pink. We only
plot spectral regions within the usable band-limiting filter channels.

Initial measurements of Obj1757C are generated from the cataloging algorithm
described in Liu et al. (2021). The software pipeline identifies emission line
objects in SITELLE data cubes by fitting emission line templates for the
H$\alpha$ and [Nii]6548,6583Å doublet, the H$\beta$ and [Oiii]4959,5007Å
doublet, and a single undefined emission line over the spectral coverage
(channels). The Liu et al. (2021) catalogs contain the centroid, ellipticity,
radius, and position angle (PA) of the H$\alpha$ emission-line region derived
from the specific spectral channels that contain the H$\alpha$ line feature.

We summarize the processing steps of Liu et al. (2021) here, which resulted in
the catalog used for this work. The sky background for each channel is
evaluated via a $128\times 128$ pix2 box using its mode, and this value is
subtracted from the channel. A low-pass filter is applied to the data cube to
minimize fringing, which is produced by the combination of the interferometric
nature of SITELLE and the presence of some bright sky lines. A stacked image
is created by summing along a restricted set of wavenumber channels which have
good transmission, and the stack is used to detect sources with the photutils
Python package. Emission line galaxies are identified by subtracting smoothly-
varied continua, and then matched to emission line templates using a cross-
correlation technique (see Section 3.6 of Liu et al. 2021 for details). The
cross-correlation measures the best-fit $z_{spec}$ and emission line
identities of each measured galaxy.

###  2.3 Spatially Resolved Emission Line Mapping

Figure 3: Flux maps of emission lines derived from LUCI. The flux (given by
the colorbar) is in units of erg s-1 cm-2 Å-1. Only three colorbars are shown
because flux maps from each filter use the same colormap (i.e. H$\alpha$ and
[Nii] from C4, [Oiii] and H$\beta$ from C2, and [Oii] from C1). Top row from
left: The flux maps derived from the C4 SITELLE filter, which provides
coverage of the redshifted H$\alpha$ and [Nii] emission lines. These maps show
weaker [Nii] emission in the cone and hook and bright emission in the core.
Bottom row: The farthest left three panels are flux maps derived from the C2
filter, covering the redshifted [Oiii] and H$\beta$ emission lines. The
farthest right panel is the [Oii] emission line doublet from the C1 filter. We
see very strong [Oii] emission compared to the [Oiii] and H$\beta$ lines,
indicating low ionization. Figure 4: Continuum maps of C4, C2, and C1 SITELLE
filters derived from LUCI, with all emission line flux removed. The flux is in
units of erg s-1 cm-2 Å-1, same as in Figure 3. The C4 filter (rest-frame
6482Å to 6686Å) displays only a mild elliptical shape of $\epsilon=0.29$ and
no cone. The continuum of the C2 filter (rest-frame 4565Å to 5077Å) is more
elliptical than C4 and has no cone. The C1 filter which covers rest-frame
3128Å to 3980Å is effectively the rest U-band continuum, and is elongated in a
direction consistent with the emission line cone.

Using LUCI555https://crhea93.github.io/LUCI/index.html (Rhea et al., 2021b,
a), a set of programs specifically designed to analyze iFTS spectral imaging
data such as those from SITELLE, we can extract maps of the redshifted
emission line flux. These steps are performed for each observed filter of
Obj1757C (C1, C2, and C4). From the Liu et al. (2021) catalog, we input the
redshift and spatial position of the estimated H$\alpha$ emission profile
centroid, and fit emission line profiles and continuum values over individual
pixels covering a square region 4$\times$ the galaxy’s H$\alpha$ equivalent
radius666From the Python package photutils (Bradley et al., 2020), the radius
of a circle with an area equal to the source segment on the segmentation map.
on each side. Pixels in this region are fit independently by LUCI. We subtract
background sky levels from an empty circular aperture near the galaxy 5.9″ in
radius, which in the case of Obj1757C was located $18\arcsec$ away southwest
from the H$\alpha$ centroid.

LUCI fits the velocity-shifted position of the emission line, which provides a
velocity measurement relative to the input object redshift derived from an
integrated spectral fit. LUCI measures the width, or broadening, of the
emission line via a SincGauss function, which is the convolution of a sinc
function with a Gaussian function (for broadening the sinc function) and
describes the instrumental line shape for extended objects as observed by
SITELLE (Martin et al., 2016). The extracted flux maps are fit over all of the
spectral channels to find the location of an emission line, and therefore any
detected emission will be included in the map despite a velocity shift or
kinematic broadening. Additionally, LUCI measures the continuum of the galaxy
spectrum simultaneously with the line flux in the measured pixel, therefore
providing a continuum-subtracted emission line flux. Continuum values are
obtained by fitting a flat continuum with zero slope, simultaneously with the
SincGuass function of the emission line, to the spectrum at each pixel. The
filters C1, C2, and C4 are all narrow band-limiting filters of under 110 nm;
visual inspection confirms a flat continuum over most pixels after background
subtraction.

Initial guesses for line broadening due to velocity dispersion and line shift
due to velocity are provided from a training set created in a similar method
to that described in Rhea et al. (2021c), which we briefly summarize here.
Reference spectra for C1, C2, and C4 are created for each filter over the
wavelength coverage of each filter. We redshift the emission lines to the
redshift value of Obj1757C ($z=0.231$, determined from the integrated spectrum
of the galaxy described in Liu et al. (2021) and Section 2.2). We use line
ratios for [Oii]3726,3729Å, H$\beta$, [Oiii]4959,5007Å, [Nii]6548,6583Å, and
H$\alpha$ taken from the Mexican Million Model database replicating HII
regions (Morisset et al., 2015). A set of 50,000 test spectra are generated
with velocity shifts $v=$ -1000km s-1 to 1000km s-1, ionized gas velocity
dispersion $\sigma_{g}$$=0-300$km s-1, and signal-to-noise ratio SNR $=2-30$.
An artificial neural network is trained on these data, separating them into a
training and test set to determine the ability of the network to recover input
values. When a spectrum is inputted to LUCI, the network predicts the initial
guess for the velocity shift and $\sigma_{g}$ broadening values of the
emission lines. The fit is run through an MCMC algorithm with priors clustered
around these initial guesses of velocity, $\sigma_{g}$, continuum levels, and
amplitude of the emission lines to determine errors on the fit at each pixel.

Emission lines observed in each filter are fit simultaneously and are tied at
a fixed velocity for each emission line complex. H$\alpha$ and the [Nii]
doublet emission lines are allowed to vary in the line broadening (as well as
the [Oiii] doublet and H$\beta$) but the fit does not produce significantly
different results if each emission line is fixed to the same broadening value
for all emission lines. The [Oii] doublet is fit as a single line because it
is not resolved at the SITELLE C1 filter resolution. The [Oiii] doublet,
[Oiii]4959Å and [Oiii]5007Å, are held to a constrained flux ratio of 1:2.98.
Additionally, [Nii]6548Å and [Nii]6583Å are constrained to a fixed flux ratio
of 1:3 (Dojčinović et al., 2022).

The results of the LUCI software on our SITELLE data include maps of the
emission line fluxes seen in Figure 3 and discussed in Section 4.1 (later used
to derive flux ratio maps described in Sections 4.2 and 5.1), continuum
emission maps seen in Figure 4, and velocity and broadening maps discussed in
Section 4.3. Fractional uncertainty levels in each of the flux maps and
continuum maps is low in the nuclear region (0.05-0.1 fractional error) and
increases outward in the cone and toward the hook (0.1-0.15 in H$\alpha$ and
[Nii], 0.15-0.35 in [Oiii], 0.1-0.2 in H$\beta$, and 0.5-0.2 in continuum).

##  3 Multi-wavelength Data on A2390

###  3.1 Chandra X-Ray Imaging

Figure 5: Chandra X-ray imaging of A2390. The top row are cutouts of the
Chandra images in a $10\arcsec\times 10\arcsec$ square. The H$\alpha$ centroid
location is marked with a red ’X’. The H$\alpha$ images with Chandra X-ray
contours are shown in the bottom row. Excess in the soft channels (left
panels) appear to be located in the same space as the H$\alpha$ cone,
indicating cooler gas temperatures. The hard channels (right) are brightest at
the galaxy center, the location of the AGN.

Chandra X-ray imaging of A2390 was obtained by Allen et al. (2002) in hard
(2-7 keV), medium (1.2-2 keV), and soft (0.5-1.2 keV) X-ray channels. The
Chandra observation (OBS ID: 4193) obtained from the archive consisted of a 96
ks ACIS pointing of the cluster center. The exposure-corrected flux images
were generated using the archive pipeline processed event file with CIAO v4.14
tools and CALDB v4.9.6 calibration files. The pipeline processed Level 2 event
file was used in conjunction with _dmcopy_ to extract the events and form
images in different energy ranges. The images were further binned 2x2 pixels
to increase the signal-to-noise ratio.

The hard X-rays peak at the BCG nucleus (Figure 5), and are associated with
the AGN in the galaxy’s center. Diffuse soft emission is asymmetric and
elongated along the extended emission line region (X-ray PA=46.1o from West)
and the H$\alpha$ cone (H$\alpha$ PA=42.7o from West), similar to many
H$\alpha$ filaments observed in BCGs which host buoyantly rising bubbles
(Fabian et al., 2003; McDonald et al., 2010; McNamara et al., 2016; Calzadilla
et al., 2022). This elongated region is associated with a cooler temperature
along the NW region of the ICM, in the same location as the H$\alpha$ emission
line cone (see Figure 5).

These data from the Chandra X-ray Observatory were also utilized by Sonkamble
et al. (2014), whose analysis identified five X-ray deficient cavities which
appear around the BCG within 30″. There is an edge to the 0.5-7 keV diffuse
X-ray gas along the NW direction at 68″, or 246 kpc, associated with cold
fronts (Sonkamble et al., 2014). Intriguingly, the NW X-ray edge is possibly
associated with the NW elongated region of H$\alpha$ flux and the elongated
soft X-ray emission. Buoyant gaseous bubbles are associated with the
appearance of X-ray cavities like the ones seen in this analysis.

###  3.2 HST Optical and Spitzer IR Data

Figure 6: Left: The HST/WFPC2/F555W image of the A2390 BCG. A two arcsecond
bar is shown as a scale. Striking and complex substructure not resolved in the
SITELLE flux maps is seen, including an extended region to the southeast and a
gap, most likely a dust lane. H$\alpha$ flux contours are overlaid in red.
Center: The same image as left, but shifted upward and with structure
directions overlaid. We include the narrow HST blue lane extending from the
southeast side of the nucleus to the northwest (blue line), and the H$\alpha$
cone (red) and hook (green) as detected by SITELLE. The HST extended
regions/blue lane emanating from the nucleus and the H$\alpha$ cone have
position angles misaligned by $\sim 4^{o}$. The most prominent clumps in F555W
are along the H$\alpha$ cone. Right: The HST/ACS/F850LP image of the BCG,
which also displays a less prominent dust lane and some of the substructure
seen in F555W as one might expect due to less reddening.  Figure 7:
Spitzer/IRAC infrared imaging of A2390. Left: The 3.6µm emission with
H$\alpha$ flux map contours (Figure 3). The 3.6µm channel is associated with
the stellar population of the BCG and shows no extended structure. Center: The
8µm emission, which has an extension towards the NW. Both images are
normalized to the peak emission in each image. Right: Difference image of
3.6-8µm. Regions dominated by 8µmemission is represented by darker shades. The
excess 8µmemission region coincides with the H$\alpha$ cone which is shown by
the red contours.

Hutchings & Balogh (2000) were the first to report a notably extended emission
line region in the A2390 BCG. They combine their CFHT/OSIS narrow-band
H$\alpha$ imaging with HST/WFPC2 F555W, and HST/ACS F850LP images. We obtained
the same HST/WFPC2 F555W imaging (Fort, 1994) for our own analysis from MAST,
and performed no additional processing. The extended HST continuum region was
observed on both sides of the galaxy, but the southeastern side of the
extended region is not observed in our SITELLE H$\alpha$ imaging or Hutchings
& Balogh (2000) Ly$\alpha$ grism spectroscopy. Knots of structure are seen
close to the galaxy nucleus and along the emission line cone region in all
observations. Additionally, a possible dust lane within the central region of
the galaxy is observable at a $90^{\circ}$ offset from the cone in both F555W
and F850LP (see Figure 6).

Legacy Spitzer/IRAC IR imaging of A2390 in 3.6 (Channel 1) and 8 $\mu$m
(Channel 4) were obtained using MAST (Egami et al., 2006) (Figure 7). The BCG
of A2390 was the third IR-brightest BCG in the sample of Egami et al. (2006),
but it is not bright enough to be classified as a LIRG ($>10^{11}L_{\odot}$).
A bright IR flux is correlated with a shorter radiative cooling timescale,
which is common in cool-core clusters. Based on IR spectral energy
distribution signatures, the highly luminous IR emission was judged by the
authors to be more likely caused by star formation rather than AGN emission. 8
µm imaging would provide us polycyclic aromatic hydrocarbon (PAH) distribution
in the BCG, and 3.6 µm imaging provides IR imaging of the stellar component.
In the right panel of Figure 7, we created a difference image of the 3.6 and 8
µm filters (left and middle panels, respectively) to emphasize the excess of
the 8 µm to the northwest. We note that the 8 µm emission is more extended
than the 3.6 µm, and the extended region is in the direction of the BCG cone.

###  3.3 Radio Interferometry

Augusto et al. (2006) observed the BCG of A2390 (the radio source of the BCG
is called B2151+174 in this study) using multi-frequency, multi-epoch radio
interferometry from several arrays such as VLBA, the VLA, and MERLIN. These
data indicated that the BCG radio source has a complex structure, consisting
of an extended mini-halo, and a number of compact mini-jets, with the two most
prominent milli-arcsecond jets in the north-south orientation. The authors
also state that it has one of the flattest radio spectrums and most spatially
compact radio sources known at the time of observation. The 1.4 GHZ radio
power of the source is $\sim 10^{25.1}$ W Hz-1. The MERLIN 5.0 GHz map
indicates a size of 0.49″ ($\sim$1.5 kpc), making it a bright core medium-size
symmetric object.

Savini et al. (2018) obtained LOFAR radio imaging of A2390. They list the
diffuse emission from low-resolution images as 1100 kpc in size. The authors
also identified two large extended lobes on the order of $\sim$600 kiloparsecs
from the center region, separate from the giant radio halo.

##  4 Results

###  4.1 Emission Line and Continuum Structure

Hutchings & Balogh (2000) noted the extended structure of the H$\alpha$
emission line in their CFHT/OSIS narrow band images, which the authors
referred to as the “emission cone"; we will continue this terminology. We
reconfirm the existence of an extended emission line cone in the BCG via the
LUCI emission line maps (Figure 3). The cone is observed in H$\alpha$,
[Nii]6548,6583Å, H$\beta$, and [Oii]3727Å at a position angle of 42.7o (from
the West of the image) extending northwest of the nucleus of the galaxy. The
cone is not observed in [Oiii]4959,5007Å. The cone is observed in the C1
continuum but not in the C2 and C4 continuua (Figure 4). Additionally, the
SITELLE maps can detect a North-Eastern protrusion from the cone, which we
will refer to as the “hook". The hook is observed in the H$\alpha$ map as well
as [Nii]6583Å. However, it is not observed in [Oii]3727Å, [Oiii]4959,5007Å,
and H$\beta$, in any continuum image including SITELLE, HST/F555W, and
HST/F850LP imaging, or in the H$\alpha$ narrow-band imaging of Hutchings &
Balogh (2000). Intriguingly, the hook is also correlated with soft X-ray
structure observed with Chandra (Figure 5, left panels).

At 1.8″ from the galaxy nucleus, clumpy structure is seen in the cone in
H$\alpha$ and [Nii]. These clumps are not visible in the other emission line
maps. The clumps are located in the same region as those found in the
HST/F555W image (Figure 6).

The nuclear region displays a possible dust lane in the HST images. The F555W
imaging (Figure 6, left) has a gap visible between the center of the nucleus
and the cone structure. The F850LP imaging (Figure 6, right) contains the same
overall structure as F555W. The dimming between the two nuclear clumps is less
pronounced in F850LP, as one might expect with lower reddening from dust.
Therefore the gap is likely a dust lane, rather than an indicator of a major
merger close pair. This possible dust lane is not observed in any SITELLE
emission line flux map due to the resolution limits of ground-based non-AO
imaging.

While no south-eastern extended region is seen in the H$\alpha$ flux maps, the
resolution of HST provides a clear image of the extended region in WFC3/F555W
and ACS/850LP (Figure 6), referred to in Hutchings & Balogh (2000) as the
"blue lane". The HST blue lane on the NW side and H$\alpha$ cone are offset by
4o. The significance of this offset is unclear due to the lower spatial
resolution of ground-based SITELLE data compared to HST spatial resolution.

###  4.2 Ionization and BPT Profiles

The Baldwin-Phillips-Terlevich (BPT) diagram (Baldwin et al., 1981) is a
diagnostic figure that plots the relative flux ratios of emission lines to
determine the state of the plasma or ionized gas. The most widely used version
of the BPT diagram is the $\log([\hbox{{\rm N}\kern 1.00006pt{\sc
ii}}]/H\alpha)$ vs. $\log([\hbox{{\rm O}\kern 1.00006pt{\sc iii}}]/H\beta)$
ratio map, which can distinguish between photoionized vs shock ionized gas, as
well as between LINERs and Seyfert type AGN. This diagnostic method is very
helpful for a galaxy such as Obj1757C, which has an unknown ionization source
in the cone, and a known ionization source (a radio AGN) at the nucleus.

Figure 8: Top: The log ratio of [Nii]6584Å and H$\alpha$, a component of the
BPT ionization diagnostic. This flux ratio map was smoothed using a 3X3 square
filter to reduce noise in the low flux regions of the flux maps. The ratio
appears relatively flat, but with a distinct drop between the nuclear region
and the cone. This is consistent with other BCG measurements from Hamer et al.
(2016). Bottom: Log ratio of H$\beta$ and [Oiii]5007Å. This ratio is also a
component of the BPT diagnostic. This flux ratio map was smoothed using a 5X5
square filter to reduce noise in the cone, due to low flux values in the cone.
The [Oiii] line is faint compared to other optical diagnostic emission lines,
so it was not measurable over most of the galaxy. In both maps, the red
contours are those of the H$\alpha$ as seen in Figure 3. Figure 9: Top: Median
$\sigma_{g}$ derived from the H$\alpha$ emission line fitting as a function of
distance from the galaxy nucleus. The points are colored as a function of
distance R from galaxy center (yellow) to the edge of the cone and hook (dark
violet). These points are similarly colored for the middle and bottom panels.
Errorbars in grey are the measurement errors derived from bootstrapping 1000
times per aperture. Each aperture is 1.0″ in width along the cone and 2.3″ in
length. Middle: The median ratio of [Nii]6584Å and H$\alpha$, a component of
the BPT ionization diagnostic. Bottom: Median ratio of H$\beta$ and
[Oiii]5007Å as a function of distance from the galaxy center. This ratio is
also a component of the BPT diagnostic. The shaded region is the detection
limit of the flux ratio of a 2$\sigma$ detection of both emission lines. The
[Oiii] emission line is not detected in the cone, making the points in the
cone upper limits.

To create ionization and BPT diagram maps, we use the LUCI emission line flux
maps and flux error maps to create flux ratio maps (Figure 8). We then extract
overlapping rectangular apertures from the flux ratio maps along the direction
of the cone from the galaxy center (as determined from the light-weighted
H$\alpha$ ionized gas profile) to the edge of the cone (Figure 9, lower two
panels). Each aperture is 1.0″ in width along the cone and 2.3″ in length, and
overlaps with the previous and following apertures along the cone to smooth
the resulting profiles. We measure the median flux from the BCG nucleus to the
cone. Errorbars are derived bootstrapping the flux maps 1000 times.

Given that [Oiii]$5007\r{A}$ is not observed in the cone and is below the flux
detection limit of $\sim 4.26\times 10^{-17}$ erg cm-2 s-1 arcsec-2, the
$\log([\hbox{{\rm O}\kern 1.00006pt{\sc iii}}]/H\beta)$ flux ratio is an upper
limit. In contrast, the $\log([\hbox{{\rm N}\kern 1.00006pt{\sc
ii}}]/H\alpha)$ flux ratio is a secure detection until the edge of the cone.
Figures 9 and 11 show these flux ratio detections, using the same color scheme
along the cone from lightest at the AGN core to darker along the cone.
Additionally, the 2$\sigma$ detectable limits on the flux ratios are displayed
for the $\log([\hbox{{\rm O}\kern 1.00006pt{\sc iii}}]/H\beta)$ ratio, where
shaded gray regions are below SITELLE’s flux detection limit. The 2$\sigma$
detectable limits for $\log([\hbox{{\rm N}\kern 1.00006pt{\sc ii}}]/H\alpha)$
are below the lower limit of the figure. These flux ratio limits are
calculated by measuring the surface brightness of the H$\alpha$ and H$\beta$
emission line structures in the cone, and determining the minimum flux ratio
detectable at the SITELLE 2$\sigma$ flux detection limit.

###  4.3 Kinematics of the H$\alpha$ Ionized Gas

We derive values for H$\alpha$ velocity and $\sigma_{g}$, the velocity
dispersion of the ionized gas, from LUCI by measuring the shift of emission
lines from the integrated redshift of the emission line gas in Obj1757C
($z=0.231$), and from the broadening of the H$\alpha$ emission line (Figure
10). The velocity of the emission line gas has a 700km s-1 gradient from the
nucleus to the cone and hook (Figure 10, upper). The AGN region appears to be
blueshifted and the cone and hook are redshifted relative to the luminosity-
weighted integrated redshift of the galaxy’s emission line gas. No smooth
velocity fitting is used (e.g. fitting to an arctangent rotation curve), but
the velocity field is, nevertheless, remarkably ordered and smooth, indicating
no recent disturbances due to major mergers, or a major merger with axes of
rotation aligned.

Figure 10: Top: Velocity shift of the H$\alpha$ line derived by LUCI. The
velocity offset is with respect to the redshift of the integrated emission
line gas from the H$\alpha$ and [Nii] emission lines ($z=0.231$) as observed
in the integrated spectrum. Bottom: Gas velocity dispersion $\sigma_{g}$
derived from the H$\alpha$ line broadening. In both maps, the contours are the
H$\alpha$ flux contours seen in Figure 3.

The velocity dispersion profile peaks at the nucleus of the galaxy at 233 km
s-1, and decreases to $\sim 130$ km s-1 in the cone and hook (Figure 10,
lower). The higher velocity dispersion region is associated with an AGN, but
the lower $\sigma_{g}$ region could be associated with rising buoyant bubbles
of plasma blown by the AGN (Aharonia et al., 2018; Zhang et al., 2022). These
bubbles have low $\sigma_{g}$ values because the motion of the gas is
primarily dominated by outward jet stream motion (see Section 5.2 for further
discussion of plasma bubbles). The clumpy substructure of the H$\alpha$ flux
map and the HST/F555W imaging (Figure 6) is also associated with a slight
flattening of the $\sigma_{g}$ values (Figure 10, top panel), before an
increasing decline in $\sigma_{g}$ to the edge of the cone and hook.
Alternatively, the velocity dispersion values of the cone could be due to
turbulence in the star forming regions, or due to slow shocks, or a
combination of these reasons.

##  5 Discussion

###  5.1 Star Formation or Shock Ionization in the Cone

The BPT diagnostics seen in Figures 11 and 12 indicate that the cone is in a
composite region; a combination of ionization state from photo-ionization and
shock heating. While emission lines can be the result of processes other than
stellar photoionization (e.g. Li, 2020), the clumpy structure of the cone and
hook in H$\alpha$ emission line flux and HST imaging is possibly of a star-
forming origin. The galaxy nucleus houses an AGN, which is located firmly in
the LINER region of the BPT diagram (Figure 12). In Spitzer/IRAC imaging of
the BCG, the 3.6µm emission morphology is concentrated in the central AGN
region (Figure 7, left). In contrast, the 8µm morphology is more extended to
the northwest, into the same region as the H$\alpha$ cone (Figure 7, center),
and could be associated with PAH emission from the star forming region
(Sivanandam et al., 2014). The soft X-ray emission (Section 3.1) is also
extended in the same orientation as the H$\alpha$ cone and the 8µm emission
(Figure 5, left), and could be similarly associated with star-formation.

Figure 11: BPT diagram with points color coded as in Figure 9, indicating
distance from the galaxy center (yellow) to the edge of the cone and hook
(dark violet). Errorbars are 2$\sigma$ range of values in each aperture as in
Figure 9. The red solid line marks the edge of the star-formation
photoionization region, derived in Kauffmann et al. (2003). The blue solid
line is the edge of the composite ionization region from Kewley et al. (2001).
The green solid line is the dividing line between LINER and Seyfert galaxies
empirically derived by Cid Fernandes et al. (2010). The nuclear regions are
located firmly in the LINER region, indicating shock-dominated photoionization
with a soft ionizing field. However, the furthest points in the cone and hook
region (dark violet points) move toward the composite photoionization region,
indicating both shock and photo-ionization.

Similar star-formation activity in BCGs with AGN activity has been observed in
the local universe up to $z\sim 0.5$ (e.g. Crawford et al., 1999; McNamara et
al., 2006; Von Der Linden et al., 2007; Fogarty et al., 2015; Hamer et al.,
2016; Ciocan et al., 2021; Maier et al., 2022). Our observations are
consistent with BPT diagnostic measurements of BCGs in Hamer et al. (2016),
from VLT/VMOS IFU data of 73 galaxy clusters. Note that the A2390 BCG was
observed in that study, but its H$\alpha$ morphology was classified as a
simple elliptical with centrally concentrated core, which is inconsistent with
our own conclusion. Instead, our observations of Obj1757C are more consistent
with BCGs classified as “Plumes" in Hamer et al. (2016), where the H$\alpha$
emission shows a clear extent in one preferential direction not shared by the
continuum. These objects also showed LINER-like emission in the nuclear
regions, and composite emission in the plumes, consistent with our own
measurements of Obj1757C (Figure 12).

H$\alpha$, while commonly used as an indicator of star-formation (Kennicutt,
1997), can also be produced from other processes such as shock heating. Based
on the high H$\alpha$ emission, the clumpy structure observed in both HST
imaging and the SITELLE H$\alpha$ maps, and the possible presence of PAH from
IRAC 8um imaging, star formation could be occurring in the cone, but further
data are needed to confirm the presence of star-formation. While the source of
ionization in the cone of Obj1757C is firmly in the composite region of the
BPT diagram, it is not shock dominated, because H$\alpha$ $\sigma_{g}$ values
are low ($\sim 100-200$km s-1, see Section 4.3), which is inconsistent with
$\sigma_{g}$ values for fast shocks (Fabian, 2012; Hamer et al., 2016;
Aharonia et al., 2018). Slow shocks could be powering the ionization, but
other processes such as the interplay between hot plasma and cold gas could
fuel the ionization seen in the cone. The soft X-ray plume seen in Figure 5
could be an indicator of this process (i.e. “turbulent diffusive reconnection"
Fabian et al., 2011).

To confirm if star-formation is occurring in the cone, high-resolution
observations of cold gas (CO features or H2) such as from ALMA would be
enlightening (Russell et al., 2017). Obj1757C has been observed with ALMA, but
the authors do not have access to these data currently. Additionally, high-
resolution H$\alpha$ imaging or IFU data would be useful, to determine whether
the H$\alpha$ flux is coincident with the HST clumpy structure seen in Figure
6.

Figure 12: BPT map with pixels color coded by location on the BPT diagram
(Figure 11). The H$\alpha$ and [Nii] flux ratio map was smoothed using a 3X3
square filter, and the [Oiii] and H$\beta$ flux ratio map was smoothed using a
5X5 square filter as shown in Figure 8 to reduce noise. Regions without
colored pixels were either outside the H$\alpha$ contours or contained an
unusable flux for flux ratio analysis. The galaxy nucleus is dominated by
LINER-like emission, powered by its AGN. The cone region is dominated by
composite emission.

###  5.2 Kinetic Mode AGN and ICM Bubbles

Obj1757C displays characteristics quite similar to the ICM “bubbles" (e.g.
Fabian et al., 2003; Salomé et al., 2006; McDonald et al., 2010; Pope et al.,
2010; Fabian, 2012; Tremblay et al., 2015; McNamara et al., 2016; Chen et al.,
2019). Fabian et al. (2003) note that filamentary structures in the central
BCGs of galaxy clusters are associated with H$\alpha$ emission and soft X-ray
emission, both of which are consistent with what is seen in Obj1757C.

Additionally, A2390 is a cool-core cluster, and these types of clusters quite
commonly show bubble activity (e.g. Fabian, 2012). Cool-core clusters have a
feeding mechanism for powering an AGN jet mode. Pristine cold gas from the
cooling flow falls onto the BCG and accretes onto the AGN. As the AGN is
fueled by this cooling flow, its jets inflate bubbles of plasma on either side
of the galaxy nucleus. The mechanism for how bubbles rise and maintain their
structure is not fully understood. Once the bubbles have formed, one possible
scenario includes that they separate from the jet and rise buoyantly through
the hot ICM (Churazov et al., 2000; McNamara et al., 2000). Models where the
bubble is allowed to rise as a rigid body (e.g. due to a magnetic draping
layer as proposed in Dursi & Pfrommer, 2008) show that detachment of the
bubble from the jet is due to eddies of thin gaseous structures, similar to
the H$\alpha$ filamentary structures seen in observations of the Perseus
cluster (Fabian et al., 2003). The filamentary structures form from gas
condensing along the path of a buoyantly rising bubble (Pope et al., 2010;
Gaspari et al., 2012; Sharma et al., 2012; Li & Bryan, 2014a, b; Voit et al.,
2017), or are lifted by the bubble itself (Churazov et al., 2001; Revaz et
al., 2008). The structures, after detaching, form a stream of gas falling back
toward the cluster center as the bubble rises (Gaspari et al., 2012; McNamara
et al., 2016; Zhang et al., 2022). In cool-core clusters, the bubble is later
disrupted by cold fronts (Fabian et al., 2022), which limits the number of
bubbles observed in the outer ICM regions of cool-core clusters.

###  5.3 Signatures of AGN Precession?

Figure 13: SITELLE deep image of the central pointing of A2390 ($z=0.228$) on
which sources from radio,optical and x-ray observations are marked. Larger-
scale radio lobes observed in Savini et al. (2018) by LOFAR are seen in dark
red, extending out past $\sim$300 kpc. The sizes and locations of the red
circles represent the sizes and locations of the radio lobe contours in Savini
et al. (2018). A region of flat spectral index radio emission not marked as a
lobe is indicated by the violet arrow. These arrows mark the locations and
fullest extent of the measured radio regions. Locations of five X-ray cavities
measured in Sonkamble et al. (2014) are indicated with cyan crosses. The size
of the crosses is not indicative of the size of the cavities. The inset is a
$7\arcsec\times 7\arcsec$ cutout in which the milliarcsecond-scale north-south
radio jets observed in Augusto et al. (2006) are marked in orange. Note that
the arrows are indicated for direction only and do not represent the size of
the jet, which is smaller than is feasible to depict at this scale. The
H$\alpha$ contours are shown in red. The radio lobes and jets radiate from the
radio center of the BCG, and the H$\alpha$ jet radiates from the H$\alpha$
centroid measured as in Section 2.2. Note that the position angle of the LOFAR
flat spectral index radio emission region is close to those of the H$\alpha$
cone and one of the X-ray cavities.

Multiple episodes of AGN activity is suggested by multi-wavelength data in the
literature and our SITELLE H$\alpha$ observations. The evidence of multiple
episodes is summarized and illustrated in Figure 13. Augusto et al. (2006)
first suggested that there may be multiple episodes of precessed AGN activity
in the A2390 BCG. They analyzed the (multi-wavelength, spatially resolved)
radio morphology of the core region of the A2390 BCG using radio
interferometry. The authors find their MERLIN 5.0 GHz map indicates a compact
size of 0.49″($\sim$1.5 kpc) with a very young milliarcsecond-scale two-jet
North-South structure in VLBI measurements. We stress that the orange arrows
in Figure 13 do not trace the extent of the north-south jets, but are in fact
much larger than the milliarcsecond scale of the jets and are enlarged to
emphasize the orientation with respect to other episodes of AGN activity.
Augusto et al. (2006) point out that the jets have a $\sim 45^{o}$
misalignment from the HST blue lane that extends from the South-east to North-
west as seen by Hutchings & Balogh (2000) (see Figure 6). The authors suggest
an interpretation of the misalignment as a result of a previous episode of AGN
activity in a different orientation. Furthermore, the northern radio jet
exhibits a twisted structure, similar to the hook structure of the H$\alpha$
cone (Figure 3), which bends to the North-east, suggesting there may be a
precession of the radio jet.

Savini et al. (2018) analyze the large-scale radio structure of A2390 using
LOFAR, noting an extended double lobe of $\sim 600$ kpc. We refer to these
lobes as the east-west lobes, which are shown schematically in Figure 13 as
dark red arrows, with dark red circles indicating the location and extent of
the widest section of the lobes. The radio lobes of A2390 are larger than any
other in their sample of 9 clusters, because usually the ICM prevents lobes
this size from forming in clusters (Wing & Blanton, 2011; Savini et al.,
2018). Structures with similar radio lobe scale include the BCG of cluster
MS0735.6+7421 ($\sim$600 kpc McNamara et al., 2009; Vantyghem et al., 2014)
and the BCG of the Ophiuchus cluster ($\sim$570 kpc Giacintucci et al., 2020).
While H$\alpha$ is observed in these galaxies, in contrast to A2390 there is
no extended nebular line structure associated with the BCG itself in either
cluster (McNamara et al., 2009; McDonald et al., 2010; Durret et al., 2015).

Savini et al. (2018) argue that the lobes observed are remnants of an older
AGN active phase, because the radio source at the BCG core contains a
relatively flat spectrum, whereas the lobes have a steeper spectrum and appear
to be fading. They suggest a possible scenario consistent with that proposed
by Augusto et al. (2006), is that there have been two epochs of AGN and
outflow activities. The older AGN activity is marked by the large, fading
east-west lobes, and the newer AGN episode would be associated with the small-
scale north-south jets. However, we note that Savini et al. (2018) did not
consider the extended optical emission, both continuum and emission line,
found in the HST image with a PA in between those of the radio jets and the
radio lobes.

Savini et al. (2018) also includes a spectral index map of A2390 (their Figure
13). There is a region of flat spectral index sources to the northwest of the
BCG, which was not commented upon by the authors. This region’s direction and
greatest extent is marked by a violet arrow in Figure 13. This region is
located at a similar orientation to the H$\alpha$ cone (see Figure 13),
although is extended to $\sim 1.5$ arcminutes ($\sim$275 kpc). The flat radio
spectrum region, along with the H$\alpha$ cone, can be interpreted as being
associated with the same AGN activity.

The X-ray analysis of Sonkamble et al. (2014) confirms the presence of AGN
feedback activity associated with the formation of buoyantly-rising bubbles
from the soft X-ray spectra. Additionally, the multiple X-ray cavities at
varying orientations indicates that the direction of the feedback is changing
with time. There are four major X-ray cavities in the north, south, east, and
west of the BCG, but Sonkamble et al. (2014) also observes a smaller cavity to
the northwest of the BCG. This cavity is located at a similar orientation to
the H$\alpha$ cone observed by SITELLE and the flat spectral index radio
region measured by LOFAR (see Figure 13).

Combining all these data, we propose a possible scenario of a third episode of
AGN activity associated with the H$\alpha$ cone, with an age between those of
the episodes that gave rise to the large radio lobes and the milli-arcsecond
jets. The emission line cone is possibly associated with an intermediate age
episode of AGN emission that is lifting cooler ionized and molecular gas into
the ICM, forming these X-ray deficient cavities. An alternative explanation
could be that the H$\alpha$ cone and the X-ray cavities and radio regions
associated with it could be an example of one of these bubbles that has
reached its maximum rising height, and the gas it entrained behind it has
fallen back toward the center of the cluster (Fabian et al., 2003). As it
falls, it forms a nebular filamentary structure with star formation present.

All lines of evidence including radio and X-ray data point to Obj1757C
harboring an extraordinary multi-episode active AGN, likely a kinetic-mode
AGN, receiving a cold gas stream which is feeding the central SMBH. The
kinetic mode in cool-core clusters is associated with the formation of the
bubbles seen in the Perseus cluster (Fabian et al., 2003, 2011), but the
ongoing question is whether this bubble formation happens in bursts or is a
constant process that occasionally results in a detached bubble (Fabian,
2012).

##  6 Conclusions

We present CFHT/SITELLE observations of the BCG (named Obj1757C) of the Abell
2390 galaxy cluster located at $z=0.228$. Obj1757C displays a complex
morphology, most notably an extended region of emission in HST/F555W,
HST/F850LP, Spitzer/IRAC 8µm, Chandra 0.5-1.2 keV and 1.2-2 keV, and now
H$\alpha$ emission line imaging, referred to as the “cone". The origin of the
cone was unclear due to multiple possible processes that could produce this
emission line morphology.

To investigate the origin of the extended region, we create emission line flux
maps of the blended [Oii] doublet (from the C1 filter), the [Oiii] doublet and
H$\beta$ (C2), and the [Nii] doublet and H$\alpha$ (C4). The BCG emission line
flux maps, continuum maps, and kinematic properties were extracted from LUCI
software, which performs a Bayesian analysis of each pixel of a SITELLE IFU
data cube, and extracts physical properties by fitting a SincGauss function to
each emission line.

Emission line ratio maps are derived from our emission line flux maps,
allowing us to perform a spatially resolved BPT analysis of the source of the
ionizing radiation. Emission line ratios show that the nucleus is dominated by
LINER emission from the AGN, and that the cone is in a composite region of the
BPT diagram that is fuelled by both photo-ionization and shock heating.
However, low values for $\sigma_{g}$ show that the cone is not shock
dominated, so fast shocks cannot be powering the ionization mechanism. Slower
shocks and the interplay between hot plasma and cold gas, and star formation
regions could fuel the ionization as an alternative explanation.

Our results indicate that Obj1757C is consistent with being a kinetic-mode AGN
that is forming H$\alpha$ filaments due to the inflation, detachment, and
rising of buoyant plasma-filled bubbles. The kinematic maps indicate that
Obj1757C has a velocity gradient of 700 km s-1 between the nucleus and end of
the extended emission region, a high $\sigma_{g}$ nuclear region ($\sim 240$
km s-1), and a low $\sigma_{g}$ region in the cone ($\sim 120$ km s-1).

Our SITELLE H$\alpha$ observations, in combination with radio interferometry
observations (Augusto et al., 2006; Savini et al., 2018) and Chandra X-ray
imaging (Sonkamble et al., 2014), point to at least three epochs of AGN
outflow activity. A very young milli-arcsecond scale North-South jet system is
identified in Augusto et al. (2006). Two large-scale ($\sim$600 kpc) fading
radio lobes to the East-West in Figure 13 are remnants of an older episode of
activity as discussed in Savini et al. (2018). There is a region of flat
spectral index radio sources that is located at a similar orientation but at a
much greater extent to the H$\alpha$ cone, and marks another episode of AGN
activity. X-ray cavities associated with each of these episodes are observed
in Sonkamble et al. (2014). We propose that these data point to three episodes
of AGN activity, with the H$\alpha$ cone associated with an intermediate-age
epoch of AGN activity.

Further information is needed to constrain the source of the ionizing
radiation in the cone of Obj1757C. The source of ionization will provide us
information on the physical processes of kinetic mode AGN in galaxy clusters,
and indicate how metal-rich gases are spread through the ICM from the cluster
BCG. High spatial resolution H$\alpha$ imaging or IFU data and high-resolution
IR imaging from JWST, could provide information on the structure of the cone
and clumpy regions seen in the HST imaging, and possible star formation or
shock activity causing the ionization of the gas. It would also be useful for
determining the smaller-scale structure of the H$\alpha$ cone, providing
information on whether the cone is moving outward from the BCG or is a
filament falling back onto the BCG. Additionally, deeper, higher resolution
radio interferometry of the flat-spectrum radio sources in A2390 would be
useful for determining the properties of the intermediate-age episode of AGN
activity.

## Acknowledgements

We thank the anonymous referee for their assistance with this article. The
research of LYA is supported by an NSERC Discovery grant to HKCY. JHL thanks
the NSERC Discovery grant program, the Discovery Accelerator Supplements
program and the Canada Research Chair program. Based on observations obtained
at the Canada-France-Hawaii Telescope (CFHT) which is operated from the summit
of Maunakea by the National Research Council of Canada, the Institut National
des Sciences de l’Univers of the Centre National de la Recherche Scientifique
of France, and the University of Hawaii. The observations at the Canada-
France-Hawaii Telescope were performed with care and respect from the summit
of Maunakea which is a significant cultural and historic site, and we thank
the indigenous and resident Hawaiian population for being our gracious hosts.
Based on observations obtained with SITELLE, a joint project between
Université Laval, ABB-Bomem, Université de Montréal and the CFHT with funding
support from the Canada Foundation for Innovation (CFI), the National Sciences
and Engineering Research Council of Canada (NSERC), Fond de Recheche du Québec
- Nature et Technologies (FRQNT) and CFHT. This research made use of
Photutils, an Astropy package for detection and photometry of astronomical
sources (Bradley et al. 2020).

## Data Availability

The data underlying this article are available on the Canadian Astronomical
Data Centre website, at https://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/. The
datasets can be downloaded using the proposal IDs listed in Table 1.

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