# Signatures of AGN feedback modes: A Green Bean Galaxy with
150 kpc jet-induced radio emission

Kelly N. Sanderson Grote Reber Fellow<EMAIL_ADDRESS>Department of
Astronomy, New Mexico State University, P.O.Box 30001, MSC 4500, Las Cruces,
NM, 88033, USA  National Radio Astronomy Observatory, Domenici Science
Operations Center, PO Box 0, Socorro, NM 87801, USA  Anna D. Kapińska National
Radio Astronomy Observatory, Domenici Science Operations Center, PO Box 0,
Socorro, NM 87801, USA  Moire K. M. Prescott Department of Astronomy, New
Mexico State University, P.O.Box 30001, MSC 4500, Las Cruces, NM, 88033, USA
Audrey F. Dijeau Department of Astronomy, New Mexico State University, P.O.Box
30001, MSC 4500, Las Cruces, NM, 88033, USA  Savannah R. Gramze Department of
Astronomy, University of Florida, Gainesville, FL 32611 USA  Jacqueline
Hernandez Space Telescope Science Institute, Baltimore, MD 21218, USA
Katherine T. Kauma Institute of Astronomy, University of Cambridge, Madingley
Road, Cambridge CB3 0HA, UK

(Received September 7, 2023; Revised May 2, 2024; Accepted May 23, 2024)

###### Abstract

Jetted Active Galactic Nuclei (AGN) hosting extended photoionized nebulae
provide us with a unique view of the timescales associated with AGN activity.
Here, we present a new Green Bean galaxy at $z=0.304458\pm 0.000007$ with
large scale jet-induced radio emission. The Spectral Energy Distributions
(SEDs) of the radio components show steep spectral indices ($\alpha=-0.85$ to
$-0.92$ for the extended regions, and $\alpha=-1.02$ for the faint radio
core), and spectral age modeling of the extended radio emission indicates that
the lobes are $>$6 Myrs old. It is unclear whether the jet is active, or
remnant with an off-time of 2-3 Myr. Several detached clouds lie around the
host galaxy up to 37.8 kpc away from the nucleus, and their ionization profile
indicates a decline ($\sim$2 dex) in the AGN ionizing photon production over
the past $\sim$0.15 Myr. Furthermore, we measure a blue shift for one of the
clouds that is spatially coincident with the path of the radio jet. The cloud
is likely illuminated by the photoionizing AGN, and potentially underwent an
interaction with the relativistic jet. Our multiwavelength analysis suggests
that RGB1 was in a phase of jet production prior to the radiatively efficient
accretion phase traced by the detached cloud emission. It is unclear whether
RGB1 transitioned into a low-excitation radio galaxy or an inactive galaxy
over the past $\sim$0.15 Myr, or whether the extended radio and optical
emission trace distinct accretion phases that occurred in succession.

AGN host galaxies (2017), Active galaxies (17), Radio galaxies(1343), Galaxy
mergers (608), Circumgalactic medium (1879), Extragalactic astronomy (506)

##  1 Introduction

The circumgalactic medium (CGM) forms the boundary between a galaxy and the
intergalactic medium (IGM). Super-massive black hole (SMBH) accretion and
active galactic nuclei (AGN) feedback play a role in galaxy evolution, and may
leave imprint on the CGM gas as well as environments around the galaxies. Two
main types (modes) of AGN feedback are recognized. The radiative mode feedback
caused by radiation-driven winds is typical of the most luminous AGN at high
redshift, while the kinetic mode feedback caused by radio jets transporting
energy and material into the host galaxy surroundings is typical of low
redshift jetted AGN (Silk & Rees, 1998; McNamara & Nulsen, 2012). Radiative
mode feedback is thought to accompany episodes of radiatively efficient high-
Eddington accretion, while kinetic mode feedback is associated with episodes
of radiatively inefficient low-Eddington accretion, and the production of
radio jets and associated structures. The duration of the timescales
associated with these modes of feedback are an important factor in determining
how the gas supply available to galaxies is regulated.

Information about the timescales associated with the duty cycle of AGN can be
easily extracted from studies of Type 2 AGN hosting extended gaseous nebulae
because our view of the host galaxy and its “extended emission line region”
(or EELR; Stockton & MacKenty, 1983) is not hidden by the bright and energetic
accretion disk. Light-travel times can provide lower limits on the duration of
AGN activity by telling us that the AGN must have been producing ionizing
photons for a time that is at least as long as the light travel time between
the central AGN and the maximum extent of the EELR. If the production of
ionizing photons in the central engine has declined more than 0.001-0.01 Myrs
ago, the sources appear as an ionization echo where the extended gas around
the AGN shows signatures of AGN activity but the nucleus of the host galaxy
lacks evidence of active accretion (Keel et al., 2012a; Oppenheimer & Schaye,
2013; Esparza-Arredondo et al., 2020). In these systems, we also obtain
constraints on how long ago the central source of ionizing photons switched
off (Lintott et al., 2009; Schawinski et al., 2010; Keel et al., 2012a).

Additional information about the timescales associated with AGN duty cycles is
revealed when the Type 2 AGN simultaneously host radio jets. The population of
accelerated charged particles that have been transported by the jets into the
circumgalactic and intergalactic media will begin to radiate away their
energy, with the higher energy charged particles radiating their energy away
faster than the lower energy ones. This aging of the synchrotron-emitting
radio plasma manifests as a curvature in the broadband radio spectrum of the
radio structures. Modeling this aging plasma using spectral aging analysis
(Kardashev, 1962; Jaffe & Perola, 1973; Tribble, 1993; Harwood et al., 2013)
allows us to infer the time since the plasma was last accelerated by the
magnetic field. In the case of active radio jets, this provides information on
the duration of the jet production phase. For ‘jetted’ AGN that have slowed
their accretion rate and entered a dormant phase of radio activity, this type
of modeling can also help constrain the time elapsed since the end of the jet
production phase. By using light-travel times, and in the case of jetted Type
2 AGN spectral aging analyses, we can infer the timescales over which AGN
activity varies in these sources.

Comprehensive knowledge of the AGN activity requires understanding the
relationship between the duty cycle and the host galaxy properties. One
interesting sample that shows evidence for AGN that have undergone recent
changes in their accretion state (ramped down) was found by Schirmer et al.
(2013). “Green Bean” (GB) galaxies are a class of nebulous objects, discovered
within the Sloan Digital Sky Survey (SDSS) DR8 out to z$\sim$0.6, that have
strong spatially extended [OIII]$\lambda$5007 emission which has been
redshifted into the r-band filter, causing them to appear elongated and bright
green in optical imaging (Schirmer et al., 2013). These objects have been
shown to emit enough Ly$\alpha$ emission to classify them as low-z Ly$\alpha$
nebulae (Schirmer et al., 2016) and have bluer continua compared to other Type
2 AGN (Prescott & Sanderson, 2019), consistent with recent central star
formation. So far only 17 of these objects have been identified in all of
SDSS, making them a rare occurrence ($4.4/{\rm Gpc}^{3}$), and supporting the
idea that these sources may be experiencing a short-lived phase in their
evolution.

Kapińska et al. (2017) serendipitously discovered a GB-like object with
extended radio emission in a search for radio galaxies with different
Fanaroff-Riley (FR; Fanaroff & Riley, 1974) classifications on opposite sides
of the nucleus (“HYbrid MOrphology Radio Source” or HyMoRS; Gopal-Krishna &
Wiita, 2000), using the Radio Galaxy Zoo (RGZ; Banfield et al., 2015). The
source, RGZ J123300.2+060325 (hereafter RGB1) was found to meet 11 out of 12
of the Schirmer et al. (2013) color cuts used to identify the mostly non-
jetted sample of GB galaxies, show no clear radio core in shallow FIRST
imaging (potentially consistent with being ramped-down), and have extended
radio morphology with a full projected linear size of $\sim 250\pm 30$kpc
(Kapińska et al., 2017). Although this source was not included in the original
Schirmer et al. (2013) GB sample (due to missing one colorcut) the source does
pass the updated GB galaxy color selection criteria of Schirmer et al. (2016).

In this paper, we use low-frequency, high resolution follow-up radio
observations from the Karl G. Jansky Very Large Array (VLA) to confirm the
association of the radio galaxy core with the spatially coincident optical
host galaxy, constrain the spectral age of the extended radio components, and
investigate its magnetic field in an attempt of disentangling its radio
morphology. Furthermore, we obtain new medium-resolution optical spectra using
the Apache Point Observatory (APO) 3.5m Dual Imaging Spectrograph (DIS)
spectrograph to investigate the kinematics of the host galaxy and its EELR,
and investigate the morphology of the [OIII]-emitting gas along the radio axis
of the source. We also obtain new low-resolution optical spectra from APO’s
Kitt Peak Ohio State Multi-Object Spectrograph (KOSMOS111KOSMOS was converted
for use at APO in 2021) to constrain the source redshift and the mechanisms
powering the EELR. Through these analyses we attempt to reconstruct the
evolutionary path of the galaxy over the past $\sim$10 Myr.

The paper is structured as follows. In Section 2, we present our observations,
and data reduction. The results are presented in Section 3. In Section 4, we
discuss these results and their implications for the accretion history of the
AGN in RGB1. We assume the standard $\Lambda$CDM cosmology, i.e.,
$\Omega_{m}=0.3$, $\Omega_{\Lambda}=0.7$, $h=0.7$. The angular scale at
$z=0.305$ is 4.51 kpc/′′.

##  2 Observations & Data Reduction

###  2.1 Radio Observations

The VLA radio observations were taken on 2019 July 1 in C(4-8 GHz) and X(8-12
GHz) bands, in the BnA configuration. A broadband setup, ultimately covering
the whole 8 GHz bandwidth across both bands was used. The target was observed
for 15.5 minutes in each band. We observed 3C286 as the flux density scale and
bandpass calibrator, and J1224+0330 as the gain calibrator. For polarization
calibration 3C286 was used as the polarization angle calibrator, and
J1407+2827 as the leakage calibrator. These radio observations provide us with
sub-arcsec angular resolution, and $\mu$Jy level image sensitivity.

In order to investigate the unusual radio morphology of RGB1, observations of
the southwestern radio structure of the target were also obtained with the
Very Long Baseline Array (VLBA), with the addition of a single VLA antenna
(VLBA+Y1) to provide short-spacing baselines, on 2022 Mar 25. A dual
polarization setup was used to observe the target source for 28 minutes in
C-band (4.1-4.5 GHz). We observed 4C39.25 as the fringe finder, J1231+0418 and
J1239+0443 as the phase-reference calibrators, and 1219+044 as an amplitude
check source. These long-baseline observations provide us with milliarcsec
angular resolution, and $\mu$Jy level image sensitivity.

####  2.1.1 VLA Total Intensity Calibration and Imaging

Calibration of the radio data was done with the Common Astronomy Software
Application (CASA). For the VLA-only data, we used its standard calibration
pipeline (CASA 6.2.1.7). The calibrated target data were then inspected by eye
to determine if any additional flagging of left over radio frequency
interference (RFI) was needed. In total 45.7% of the data were flagged.

Imaging of the source was done using CASA’s tclean task with a standard
gridder and a multi-scale multi-frequency synthesis (mtmfs) deconvolver. We
used two Taylor terms during deconvolution, corresponding to a power law, to
enable modeling of the spectral properties within the large bandwidths being
imaged. Due to the asymmetric nature of the BnA configuration point-spread-
function (PSF) for these observations, Briggs weighting with a robust
parameter of 1.0 was implemented to produce a more symmetrical PSF for
deconvolution. This weighting also favors the shorter baseline signals and
thus enhances the sensitivity to extended emission. A number of images, with
different bandwidths, were created, as follows. First, in order to study the
SED of the source across the C and X bands, we imaged both bands in 1 GHz
bandwidth chunks. We repeated this imaging of 1 GHz bandwidth chunks with an
added restriction to the $uv$ range of the data such that the largest angular
scale is consistent across the images. These $uv$-restricted images are only
used to extract the integrated radio SED, which ensures that we are not
introducing a systematic bias into our measured SED. The angular resolution
for each 1 GHz subimage was matched to the lowest angular resolution as given
by the 4–5 GHz subimage, $1.6\times 0.5$ arcsec2. The rms levels for the flat-
noise subimages vary from 12.4 $\mu$Jy/beam (4.5 GHz) to 24.0 $\mu$Jy/beam
(11.5 GHz). The rms increases with frequency except for 6.5 and 7.5 GHz, which
have rms = 12.1 and 11.9 $\mu$Jy/beam, respectively. The rms levels are the
same for the $uv$-restricted and unrestricted images.

Larger bandwidth images were required for the detection of the faint radio
core at higher frequencies; we created a 3 GHz bandwidth image at 10.5 GHz for
this purpose. The image centered at 10.5 GHz was smoothed to the resolution of
the 4.5 GHz image ($1.6\times 0.5$ arcsec2) and the rms levels in this image
was 11.0 $\mu$Jy/beam. All images were primary beam corrected to allow for the
correct interpretation of the extracted SED’s spectral properties.

####  2.1.2 VLA Polarization Calibration and Imaging

The CASA standard calibration pipeline by default applies parallactic angle
corrections without performing polarization calibration steps. Therefore, we
removed the correction and recomputed the data weights before our manual
calibration of the polarization products. The data were then further inspected
for RFI. The instrumental (multiband) delays between the cross-hands were
derived using 3C286, and the leakage terms were derived using J1407+2827 as
the unpolarized calibrator. The data were then flagged a final time to remove
residual RFI revealed by these calibration steps. In total, 47.5% of the full
4-12 GHz data was flagged. The resulting data were then imaged in full Stokes
(IQUV) across the full 4-12 GHz bandwidth, again with standard gridding and an
mtmfs deconvolver with two Taylor terms. Again, Briggs weighting with a robust
parameter of 1.0 was used. We also imaged the data in full Stokes in 1 GHz
bandwidth chunks in order to investigate how the polarized intensity and
polarization position angle of the observed emission vary with wavelength.

####  2.1.3 VLBA Total Intensity Calibration and Imaging

For the VLBA+Y1 radio data, we followed the steps outlined in Linford (2022).
Specifically, the data amplitude corrections were determined from
autocorrelations between stations, system temperature and gain curve
information were generated from the data, instrumental delays were determined
as a function of frequency for all time, and then a global fringe fit was
performed to determine delays as a function of frequency and time using 45s
solution intervals. Next, the shape of the bandpasses were corrected using the
fringe finder 4C39.25, and then final amplitude scaling was applied. During
the calibration process of the VLBA+Y1 data, 11.2% of the data was flagged.

The VLBA+Y1 data were imaged using CASA’s tclean task with a standard gridder
and an mtmfs deconvolver. We used two Taylor terms during deconvolution,
corresponding to a power law, to enable modeling of the spectral properties
within the large bandwidths being imaged. Natural weighting was used to favor
shorter baselines and thus enhance the sensitivity to extended emission. The
angular resolution and rms of the resulting image are 9.05 $\times$ 2.38
miliarcsec2 and 23.1 $\mu$Jy/beam, respectively.

####  2.1.4 Supplementary Radio Data

In our analysis we refer to images from the VLA Faint Images of the Radio Sky
at Twenty-Centimeters (FIRST; Becker et al., 1994) and VLA Sky Survey (VLASS;
Lacy et al., 2020) radio surveys. The FIRST survey has an angular resolution
of 5.4″and an rms level of 0.14 mJy/beam at 1.4 GHz. The VLASS survey is
ongoing, and at present we use its quick look images with angular resolutions
of $2.5\times 2.1$ arcsec2 and $2.5\times 2.0$ arcsec2 for epochs one and two,
respectively. Stacking images from these epochs resulted in a noise level of
0.11 mJy/beam at 3 GHz. We also include low-frequency observations of RGB1
from the LOFAR Virgo Cluster survey (Edler et al., 2023), which contains RGB1
in the background of the targeted cluster, and auxiliary low-frequency low-
resolution measurements from various radio sky surveys. The LOFAR Virgo
cluster survey has an angular resolution of $9.0\times 5.0$ arcsec2 and median
rms level of 0.28 mJy/beam at 120–168 MHz. The low resolution surveys covering
the location of RGB1 span radio frequencies only up to 1.4 GHz and are
incompatible with our high resolution data due to the latter resolving out
diffuse emission on angular scales $>10-15$ arcsec. Therefore these data are
treated separately (Table 1).

Table 1: Total radio flux density measurements from low resolution radio
surveys. The specific surveys are: GLEAM – Galactic and Extragalactic All-sky
MWA (Murchison Widefiled Array) survey, VICTORIA – Virgo Cluster multi-
telescope observations in radio of interacting galaxies and AGN, TGSS – TIFR
(Tata Institute of Fundamental Research) GMRT (Giant Metrewave Radio
Telescope) Sky Survey, RACS – Rapid ASKAP (Australian Square Kilometre Array
Pathfinder) Continuum Survey, NVSS – NRAO VLA Sky Survey. The measurements
were taken from the surveys’ catalogs unless stated otherwise.

Frequency | Sν [mJy] | Notes and references
---|---|---
88 MHz | $252\pm 56$ | GLEAMa, Hurley-Walker et al. (2017)
118 MHz | $260\pm 30$ | GLEAMa, Hurley-Walker et al. (2017)
144 MHz | $221\pm 44$ | VICTORIAb, Edler et al. (2023)
150 MHz | $155\pm 13$ | TGSSc, Intema et al. (2017)
154 MHz | $194\pm 22$ | GLEAMa, Hurley-Walker et al. (2017)
200 MHz | $189\pm 26$ | GLEAMa, Hurley-Walker et al. (2017)
887 MHz | $65.8\pm 5.3$ | RACS, Hale et al. (2021)
1.37 GHz | $48.1\pm 0.8$ | RACS, Duchesne et al. (2023)
1.40 GHz | $42.3\pm 1.7$ | NVSS, Condon et al. (1998)

footnotetext: (a) Measured from 30 MHz bandwidth image. (b) Measured from the
image. (c) Lower flux density from the TGSS ARD1 (Alternative Data Release 1)
catalog as compared to VICTORIA and GLEAM may be due to its known calibration
issues.

###  2.2 Optical Spectroscopic Observations

Table 2: APO Observations of RGB1 Instrument | # Exps | PA | Slit size | Spectral Resolution | Exposure Time | Seeing | Airmass | UT Date
---|---|---|---|---|---|---|---|---
DIS | 4 | 24.2∘ | 1.5″ | R$\sim$3200-3900 | 600 | 2.45″ | 2.02 | 03/06/2021
KOSMOS | 5 | 22.5∘ | 1.2″ | R$\sim$2300 | 1200 | 1.67″ | 1.47 | 04/21/2023

Spectroscopic observation of RGB1 were conducted at APO on the 3.5m telescope
using the DIS long-slit spectrograph on the night of UT 2021 March 6, and
using the KOSMOS long-slit spectrograph on the night of UT 2023 April 21. The
dual imaging feature of DIS allows simultaneous observations of the blue
(2570–6230 Å) and red (5190–9810 Å) wavelength ranges, and allows for a user-
specified central wavelength in each range. We use DIS to obtain medium
resolution (R$\sim$3500) spectra of RGB1 centered on the expected locations of
the [O III] doublet and the [O II] doublet on the red and blue sides,
respectively. KOSMOS allows for a user-specified combination of slit size,
slit location, and disperser (red or blue) to control the wavelength range and
spectral resolution. We use KOSMOS to target other emission lines of interest
on the red side of the RGB1 spectrum (H$\alpha$, [N
II]$\lambda\lambda$6548,6583, [S II]$\lambda\lambda$6716,6730), while still
retaining moderate spectral resolution (R$\sim$2000).

The DIS observations were obtained with a position angle aligned to the radio
jet axis of the source (24.2∘), as inferred from the alignment of the two
hotspots. The pixel scales for DIS are 0.42″/pixel for the blue side and
0.40″/pixel for the red side. The DIS observations were taken with central
wavelengths 4798Å and 6600Å respectively, as well as a 1.5” slit, resulting in
a resolution element of 1.5-1.7 Å as determined by a bright skyline near the
central wavelengths. The resolving power of these DIS observations was
R$\sim$3200-3900. Four exposures of 600s were obtained with this setup. In
these DIS observations we identified evidence of a detached cloud to the South
of RGB1 along the direction to the southern radio structure. This motivated us
to shift the KOSMOS position angle to align with the radio core and the
brightest part of the southern radio structure. The KOSMOS observations were
obtained with a position angle of (22.5∘). The pixel scale for KOSMOS is
0.258″/pixel. The central wavelength of the KOSMOS spectrum, which was taken
with a 1.18 ″ slit, was 7280 Å, resulting in a resolution element of 3.15 Å as
determined by a bright skyline near the central wavelengths, and a resolving
power of R$\sim$2300\. Five exposures of 1200s were obtained with this setup.
Details of these observations, including seeing, position angles, and
airmasses are listed in Table 2.

Figure 1: Top: Low resolution optical APO KOSMOS spectrum for the core of RGB1
extracted from a 8.8″ wide aperture. Bottom: Low resolution optical APO KOSMOS
spectrum for the detatched cloud in RGB1’s spectrum, extracted from a 5.0″
wide aperture. In both spectra, the KOSMOS long-slit is aligned with the radio
core and the brightest part of the southern radio component. The [OIII] line
is the brightest feature in these spectra.

Figure 2: Top: Medium resolution optical APO DIS spectrum for the core of RGB1
extracted from a 8.8″ wide aperture. Bottom: Medium resolution optical APO DIS
spectrum for the detatched cloud in RGB1’s spectrum, extracted from a 5.0″
wide aperture. In both spectra, the DIS long-slit is aligned with the radio
jet axis of the source as determined from the location of the NE and SW
hotspots. The [OIII] doublet line profiles of the host galaxy (top panel)
suggests multiple kinematic components are present, and the [OIII] doublet
line profiles of the cloud appear narrow, symmetric, and slightly blue shifted
with respect to the central wavelengths of the emission lines in the host
galaxy spectrum.

####  2.2.1 Data Reduction

The reduction of the APO observations were carried out using Pyvista
222https://github.com/holtzmanjon/pyvista, a python-based reduction software
written specifically for the standard reduction of various data products
produced at APO. For each set of observations, individual exposures were first
bias-subtracted, flat-fielded, and then median-combined into a 2-D spectrum. A
2-D wavelength solution determines the wavelength solutions in each row of the
master images, where rows are different spatial locations along the slit. The
2-D wavelength solution was found using HeNeAr calibration lamp exposures
taken at the beginning of the night. The resulting rms accuracy for both the
blue and the red channels in the 2-D wavelength solution for DIS medium-
resolution spectra was 0.02 Å, and the resulting rms accuracy of the 2-D
wavelength solution for the KOSMOS observations was 0.07 Å. The 2-D wavelength
solutions were then used to correct for deviations in wavelength solution
along the spatial axis of the target 2-D spectra by interpolating the spectra
on a row-by-row basis to the wavelengths at the location of the target trace.
This ensures that the wavelengths are constant along the spatial direction and
do not drift with distance from the target trace due to e.g. flexure issues.
For observations of the red side of RGB1’s KOSMOS spectrum, the final
wavelength solution at the location of the target trace was corrected using
the observed optical skylines, with a resulting rms accuracy of 0.15 Å. Due to
the lack of standard star observations on the night of UT 2021 March 6, we
omit flux calibrating our medium-resolution DIS data. The KOSMOS data were
flux calibrated using standard star observations of Feige98 and corrected for
Galactic extinction using the E(B-V) values from the dust maps of Schlegel et
al. (1998), the standard extinction curve from Fitzpatrick (1999), and RV=3.1.

The sizes of the slits in both data sets (1.2-1.5″; Table 2) are narrower than
the width of RGB1 on the sky, and narrower than the seeing during the
observations, which means that some of the source’s light will be missed. This
loss of light should not affect the measured line central wavelengths, line
widths, or ratios between different lines, however, this does mean that our
measured fluxes are only lower limits to the total source fluxes. We do not
perform slit-loss corrections to account for these losses.

Both 2-D target spectra revealed that the extended emission ([O
II]$\lambda$3726,3728, [Ne III]$\lambda$3868, [O III]$\lambda$5007,4959,
H$\alpha$) in RGB1 is fractured along the radio axis, with the host galaxy
component and associated continuum, and a detached cloud to the south-west of
the host galaxy. One-dimensional spectra were extracted from the 2-D spectra
as follows. First, we extracted a spectrum for the host galaxy alone using a
8.8″ wide aperture. Then, we extracted a spectrum for the detached cloud alone
using a 5.0″ wide aperture. In both of the rest-frame KOSMOS and DIS spectra,
shown in Figures 1 and 2, respectively, we also include labels indicating key
emission lines. An inset displaying the blue channel of the DIS spectrum,
highlighting the [O II]$\lambda$3726,3728 emission line detection in the core,
is also included. The inset to the right side of the [O
III]$\lambda\lambda$4959,5007 doublet in each panel in Figures 1 and 2
displays a zoom-in on the [O III] doublet, highlighting the line profiles of
the [O III] doublet.

####  2.2.2 Fitting Emission Lines

Table 3: Observed emission line properties from the APO KOSMOS observations of
RGB1 separated by component. The host galaxy measurements were taken using a
8.8″ wide aperture centered on the spatial centroid of the continuum. The
cloud measurements were taken using a 5.0″ wide aperture centered on the
spatial centroid of the [O III]$\lambda$5007 emission.

Component | Line | Flux | Equivalent | Rest frame $\lambda$
---|---|---|---|---
|  |  | width |
|  | 10-15[erg s-1cm-2] | [Å] | [Å]
Host Galaxy | He II$\lambda$4685 | 0.304 $\pm$ 0.051 | -10.396 $\pm$ 2.051 | 4684.764
| H$\beta$ | 0.572 $\pm$ 0.074 | -18.597 $\pm$ 3.432 | 4861.162
| [O III]$\lambda$4959 | 1.837 $\pm$ 0.055 | -57.688 $\pm$ 4.055 | 4958.831
| [O III]$\lambda$5007 | 5.469 $\pm$ 0.054 | -211.088 $\pm$ 14.293 | 5006.877
| [N II]$\lambda$6548 | 0.205 $\pm$ 0.038 | -14.669 $\pm$ 6.645 | 6550.61
| H$\alpha$ | 1.511 $\pm$ 0.048 | -109.806 $\pm$ 22.489 | 6565.379
| [N II]$\lambda$6583 | 0.615 $\pm$ 0.114 | -47.52 $\pm$ 26.369 | 6586.02
| [S II]$\lambda$6716 | 0.224 $\pm$ 0.033 | -24.971 $\pm$ 4.582 | 6718.152
| [S II]$\lambda$6730 | 0.077 $\pm$ 0.019 | -10.063 $\pm$ 2.742 | 6732.522
Cloud | He II$\lambda$4685 | 0.051 $\pm$ 0.031 | – | 4687.934
| H$\beta$ | 0.108 $\pm$ 0.027 | – | 4860.685
| [O III]$\lambda$4959 | 0.208 $\pm$ 0.022 | – | 4958.991
| [O III]$\lambda$5007 | 0.733 $\pm$ 0.025 | – | 5006.816
| H$\alpha$ | 0.159 $\pm$ 0.011 | – | 6565.391

Table 4: Observed emission line location and velocity dispersion from the
medium-resolution APO DIS observations of RGB1 separated by component. The
host galaxy measurements were taken using a 8.8″wide aperture centered on the
spatial centroid of the continuum. The cloud measurements were taken using a
5.0″wide aperture centered on the spatial centroid of the [O
III]$\lambda\lambda$4959,5007 emission.

We derive flux estimates for the emission lines ( He II$\lambda$4685,
H$\beta$, [O III]$\lambda\lambda$4959,5007, H$\alpha$+[N
II]$\lambda\lambda$6548,6584, and [S II]$\lambda\lambda$6717,6731) seen in the
KOSMOS observations (Figure 1) by fitting a Gaussian to each line. The [O
III]$\lambda\lambda$4959,5007 doublet in the KOSMOS host galaxy spectrum
(Figure 1; Top panel) is used to constrain the redshift of the host galaxy to
be 0.304458 $\pm$ 0.000007, which is consistent to within the SDSS photometric
estimate error ($z=0.269\pm 0.058$). This redshift is used to determine the
central wavelength of the other emission lines for the host galaxy and the
detached cloud in Table 2.2.2.

For individual lines that are well separated in the KOSMOS data (He
II$\lambda$4685, [O III]$\lambda$4959, [O III]$\lambda$5007, and H$\beta$) we
fit each line with a single Gaussian profile. For the H$\alpha$+[N
II]$\lambda\lambda$6548,6584 complex all three lines were fit simultaneously,
requiring that each individual line in the [N II] doublet has the same width,
and that the [N II]$\lambda\lambda$6548,6584 has the theoretical flux ratio
3.0 (Storey & Zeippen, 2000; Dojčinović et al., 2023). For the [S
II]$\lambda\lambda$6717,6731 doublet, both lines were fit simultaneously,
requiring that each individual line in the [S II] doublet has the same width.

We use these fits to measure the flux in each line, and we summarize these
measurements in Table 2.2.2, along with the equivalent width of each line. For
the cloud, which does not have a continuum detected above the noise in the
spectra, we do not report the equivalent width. The signal in the [S
II]$\lambda$6730 line is relatively weak and we note that the [S
II]$\lambda\lambda$6716,6730 doublet may have been affected by an over-
subtracted skyline.

The [O III]$\lambda\lambda$4959,5007 doublet in the DIS host galaxy spectrum
(Figure 2; Top panel) gives a redshift of z=0.305184 $\pm$ 0.000011, which is
used to determine the central wavelength corrected for the systemic redshift
of [O III] and line dispersion of the other emission lines for the host galaxy
and the detached cloud in Table 2.2.2. For each emission line ([O
II]$\lambda\lambda$3726,3728, H$\beta$, [O III]$\lambda\lambda$4959,5007) in
the DIS spectra shown in Figure 2, we fit a single Gaussian to the line to
determine the observed central wavelength and line dispersion ($\sigma_{v}$).
We then correct the line dispersion for instrumental broadening, and correct
the observed central wavelength and the corrected line dispersion for the
systemic redshift of the source. The resulting best-fit parameters are shown
in Table 2.2.2.

####  2.2.3 Supplementary Optical Data

In our analysis, we refer to existing Hubble Space Telescope (HST) broadband
imaging covering the location of RGB1 as part of the Gems of the Galaxy Zoo
(Zoo Gems) project gap-filler HST program (Keel et al., 2022). Observations of
the RGB1 were taken with a total exposure time of 674s in the Wide Field
Camera (WFC) F625W filter (center = 6323.81 Å, FWHM = 1389.28 Å), covering the
[O III]$\lambda\lambda 4959,5007$+H$\beta$ line complex at the redshift of
RGB1. The WFC/F625W image was made science ready by the ACS pipeline, which
performs a drizzle combination of individual exposures, and corrects for
charge transfer efficiency (CTE) issues, which could otherwise lead to
underestimating the pixel image counts. We use this imaging to aid in our
understanding of the EELR around RGB1, the morphology of the host galaxy, and
the evolution of the central AGN. We do not attempt to separate out the
continuum emission from the image, but we do comment on how the included
continuum light affects our interpretations, where necessary.

##  3 Results

###  3.1  Radio Properties and SEDs

In Figure 3, contours from the subimage containing the highest signal-to-noise
detection of the radio source (S/N$\sim$ 65 at a central frequency of 4.5 GHz)
are overlaid on the HST F625W broadband optical imaging of the host galaxy
(Keel et al., 2022). These contours reveal three distinct radio components
associated with this source: a northeast hotspot with diffuse lobe emission (
NE region, resolved), a southwest compact component with diffuse emission
trailing away from the radio core ( SW region, resolved), and an unresolved
compact core coincident with the optical host galaxy, RGB1. The location of
the detected radio core unambiguously confirms the association of the radio
galaxy with the GB host galaxy, originally suggested by Kapińska et al.
(2017). Assuming that the two compact components are the terminal hotspots of
the jets, the size of the radio galaxy is 33.3 arcsec (150 kpc) in projection
(see Section 4.5 for further discussion on the radio morphology). The high
resolution VLA data are not sensitive to angular scales larger than 10–15
arcsec in C band, hence we use LOFAR and NRAO VLA Sky Survey (NVSS; Condon et
al., 1998) to measure total flux density of the radio source, which translates
to radio luminosity densities of $5.2(\pm 1.1)\times 10^{24}$ W/Hz sr at 144
MHz, and $1.0(\pm 0.05)\times 10^{24}$ W/Hz sr at 1.4 GHz.

Figure 3: Radio and optical morphology. HST F625W broadband optical image of
RGB1 with overlaid radio contours extracted from our 4.5 GHz radio image. The
color scale of the optical images is Log normalized between 0 and 1. The radio
contour levels are drawn down to 4$\sigma$ (50.8, 87.9, 151.9, 262.6, and
453.8 $\mu$Jy per beam). The beam size of the radio contours is $1.6\times
0.5$ arcsec2, and is shown in the bottom left corner (dashed black) of each
image. A scale bar indicating 5″ is shown in the bottom left corner. The radio
core of the emission is coincident with the optical host galaxy, confirming
the association between the extended radio emission and the Green Bean Galaxy
candidate host.  Table 5:  Radio flux density measurements (Sν) centered on
the compact components of the NE region, SW region, and radio core of RGB1.
All measurements were taken with the CASA’s imfit() task, fitting a single
Gaussian. The $uv$-range of each subimage was matched to that at the highest
frequency of 12 GHz to avoid potential bias of emission being resolved out.
The subimage bandwidth is 1 GHz unless stated otherwise

.

††footnotetext: (a) Measurement based on wideband (3 GHz) image with center
frequency 10.5 GHz.

Figure 4:  Radio SEDs of the compact components of the NE region (left), SW
region (middle), and Core (right). Our data presented in this paper are drawn
as circles (orange). All three components were each best fitted by a simple
power law with a reference frequency at 8.0 GHz. For the Core, FIRST and VLASS
3$\sigma$ upper limits are drawn as a diamond (dark green) and a square (light
green), respectively. The upper limits on FIRST and VLASS measurements were
not included in the fitting.

The radio core is clearly detected in the 1 GHz bandwidth images created from
our new higher sensitivity VLA observations up to 8.5 GHz, however, detection
of the core at higher frequencies requires larger bandwidth images (Section
2.1.1). The integrated radio SED between 4.5 GHz and 11.5 GHz for each radio
component are presented in Table 3.1 and Figure 4. The components are fitted
by a single 2D Gaussian profile to the NE and SW regions, and the core. We do
not attempt to measure the low signal-to-noise diffuse emission in the SW
region. The NE region measurements are resolved, and therefore we assume that
these integrated flux densities include all of the hotspot emission, as well
as some of the surrounding lobe emission.

The spectral index, $\alpha$, is defined as $S_{\nu}\propto\nu^{\alpha}$,
where $S_{\nu}$ is the source flux density at a frequency $\nu$. We find that
the compact radio components were each best fitted by a simple power-law
between 4.5 GHz and 11.5 GHz with $\alpha^{11.5}_{4.5}=-0.85\pm 0.04$
($\chi^{2}=5.08$) for the NE region, $\alpha^{11.5}_{4.5}=-0.92\pm 0.04$
($\chi^{2}=4.50$) for the SW region, and $\alpha^{10.5}_{4.5}=-1.02\pm 0.09$
($\chi^{2}=0.26$) for the core, with reference frequency of 8.0 GHz.

The radio core is undetected in the FIRST (dark green) and VLASS (light green)
surveys with $3\sigma$ upper limits of 0.43 mJy/beam and 0.34 mJy/beam,
respectively. While the upper limits on the FIRST and VLASS measurements were
not included in the fitting, for reference we plot these data points along our
high resolution VLA data (Figure 4). We also show the total low-frequency flux
densities up to 1.4 GHz in the last panel of Figure 4. We find that the total
flux density of the source between 88 MHz and 1.4 GHz is best fitted by a
simple power law, with $\alpha^{1.4}_{0.088}=-0.61\pm 0.04$ ($\chi^{2}=13.46$)
at a reference frequency of 200 MHz.

###  3.2 Radio Spectral Analysis

To investigate these spectral shapes on resolved scales and estimate the
spectral age of the radio-emitting plasma, we perform spectral analysis using
the Broadband Radio Astronomy ToolS333http://www.askanastronomer.co.uk/brats
software package (BRATS; Harwood et al., 2013). BRATS is a software for
handling spectral analysis of broadband radio data, including calculations of
resolved spectral index maps, fitting synchrotron spectral aging models to the
observed data, and image reconstruction. We use the original untapered 1 GHz-
wide images between 4.5 and 11.5 GHz as the input for BRATS. BRATS uses an
‘adaptive’ method to identify the regions over which flux densities will be
integrated and analyzed within a source image, with a set of limits defined by
the user. The data are fitted on a pixel-by-pixel basis, and use the default
parameters in BRATS (Harwood et al., 2013). Performing this adaptive source
region identification resulted in a total of 616 regions, each 1 pixel2
(0.1$\times$0.1 arcsec2) in size, collectively covering 6.1 arcsec2. The core
of the radio emission was not included in the final BRATS source regions as
spectral aging models to not apply to the radio emission produced in the
nucleus.

BRATS allows the user to fit spectral aging models to the observed data on a
region-by-region basis. For these models, the user can make some fitting
choices to control the maximum source age to restrict the model fit (‘myears’
parameter), the number of times to decrease the range of ages to fit across
and re-attempt the fit (‘levels’ parameter), and the resolution of each
fitting attempt (‘ageres’ parameter). For our use of BRATS, we imposed a
maximum source age of ‘myears’=35 Myears, the number of fitting levels as
‘levels’=3, and the resolution of each fitting attempt to be ‘ageres’ = 10
increments. The user can then determine which model to use to fit to the data
from the well-known Jaffe & Perola (1973, hereafter the ‘JP’ model), Kardashev
(1962, hereafter the ‘KP’ model), and Tribble (1993) spectral aging models. In
this work we use the JP spectral aging model which allows for a realistic
description of the motions of particles, while still retaining a relatively
simple description of the magnetic field. Specifically, the JP model assumes
random/isotropic pitch angles while the KP model assumes a fixed pitch angle
for all time. Both the JP and KP models assume a constant magnetic field.

The most important physical inputs when calculating the spectral aging model
are the injection index ($\alpha_{inj}$), the magnetic field strength (B) of
the radio source, and the redshift of the source. $\alpha_{inj}$ describes the
initial distribution of electrons injected along the magnetic field lines at
the point of jet disruption/termination. The magnetic field strength, B,
determines the synchrotron radiative losses of the electron distribution. The
redshift of the source determines the magnetic field strength of the Cosmic
Microwave Background (Bc), and thus the inverse-Compton radiative losses of
the electron distribution. Once a model had been created for a source, BRATS
can then compute a reconstructed model image of the source, a spectral age map
and associated errors, and a $\chi^{2}$ map to aid in assessing the goodness-
of-fit across each region.

Figure 5: Left: Reconstructed model image from the best fitting spectral aging
model. Right: Residual map showing the reconstructed modelled emission
subtracted from the observed emission. VLA BnA configuration C band (4-5 GHz)
contours down to 4.0$\sigma$ (50.8, 87.9, 151.9, 262.6, and 453.8 $\mu$Jy per
beam) are included in each image. The beam size of the radio contours is
$1.6\times 0.5$ arcsec2, and is shown in the bottom right corner (dashed
black) of each image. The reader should note the difference in the the
colorbar ranges for each panel. The largest residual (right panel) is
$\sim-0.30$ mJy/beam.

For the magnetic field strength, we assume equipartition conditions. We use
existing low-frequency imaging from the LOFAR Virgo cluster survey (Edler et
al., 2023) to measure a total source flux density of 221 $\pm$ 44 mJy at 150
MHz, indicating an effective magnetic field strength of B = 0.49 $\pm$ 0.03 nT
(assuming B=0.4$\times$Beq; Croston et al., 2005; Ineson et al., 2017).

For the injection index, $\alpha_{inj}$, BRATS provides functionality to
identify the best-fitting $\alpha_{inj}$ across the source. This is done by
fitting the spectral aging model to each region with $\alpha_{inj}$ being an
additional free parameter, and minimizing the sum of the model $\chi^{2}$
values over all 616 regions. For the VLA observations of RGB1, BRATS finds a
best-fitting injection index of $\alpha_{\rm inj}=-0.70^{+0.03}_{-0.02}$
($\chi^{2}=808.42$). With the best-fitting model we create a reconstructed
model image of the source (Figure 5; left), a residual map (Figure 5; right),
a spectral age map (Figure 6; top), a spectral age error map (Figure 6;
middle), and a $\chi^{2}$ map corresponding to the goodness of fit of the
best-fitting spectral aging model (Figure 6; bottom).

Figure 6: Top: Spectral Age map produced from the best fitting spectral age
model run in BRATS. Bottom left: Errors on the spectral ages shown in first
panel. Bottom right: $\chi^{2}$ values describing the goodness of fit of the
best-fitting spectral age model. VLA BnA configuration C band (4-5 GHz)
contours down to 4.0$\sigma$ (50.8, 87.9, 151.9, 262.6, and 453.8 $\mu$Jy per
beam) are included in each image. The beam size of the radio contours is
$1.6\times 0.5$ arcsec2, and is shown in the bottom right corner (dashed
black).

###  3.3 Spectro-polarimetric properties of radio emission

The resulting Stokes Q and U broadband (4-12 GHz) images were used to compute
the polarized intensity, corrected for Ricean bias (Killeen et al., 1986), and
the resulting Stokes Q and U 1 GHz bandwidth images were used to compute the
polarization angle and rotation measure of the observed emission.
Specifically, the 1 GHz images were concatenated into a cube and the CASA
Imagepol tool po.rotationmeasure was used to create a rotation measure (RM)
map, corrected with a Galactic rotation measure of 6.6 rad/m2 (Hutschenreuter
et al., 2022), and a map of the polarized emission position angle (PA) at zero
wavelength. A map of the broadband polarized intensity is shown in Figure 7
with the PA vectors, rotated by 90∘ to represent the $\vec{B}$ field rather
than the actually measured $\vec{E}$ field, overlaid as red arrows. The
average polarization fraction within the radio beam centered on the mean of
the 2D Gaussian fit to the locations of the NE region and SW region are 8.9%
with a standard deviation of 3.7%, and 20.2% with a standard deviation of
3.1%, respectively. The Core is not detected in polarized emission in our VLA
data.

Figure 7: VLA BnA configuration C and X band (4-12GHz) polarization fraction
map. The map of the angle of polarized intensity, rebinned such that there are
$\sim$3 pixels/beam is shifted by 90 degrees, consistent with the definition
of the projected magnetic field direction, and overlaid with red arrows. VLA
BnA configuration broad band (4-12 GHz) contours down to 4.0$\sigma$ (24.3,
45.0, 81.9, 150.6, 276.8 $\mu$Jy/beam) are included in each image. The
direction to the core from each hotspot is indicated by the black arrows. The
beam size of the radio contours is $1.6\times 0.5$ arcsec2, and is shown in
the bottom right corner (dashed black). Left: NE region. Right: SW region.
Note that the size of the images are different.

From the 1 GHz bandwidth full Stokes cubes, we computed the fractional
polarization as a function of frequency for each pixel in the polarized
intensity image. A linear fit was performed in each pixel to determine the
slope between the fraction of polarized emission and frequency (in GHz), and
this slope was recorded for each pixel. The average slope value within the
radio beam centered on the mean of the 2D Gaussian fit to the locations of the
NE region is 0.002 with a standard deviation of 0.005, and of the SW region is
0.008 with a standard deviation of 0.006, indicating no more than 1.5% per GHz
change in the fractional polarization with frequency for these radio
components.

With the RM map produced by CASA’s po.rotationmeasure function, we investigate
the change in the polarization position angle as a function of $\lambda^{2}$
to determine whether there are any Faraday screens present within the radio
emitting source or along the line-of-sight to the observer. The average RM
value within the radio beam centered on the mean of the 2D Gaussian fit to the
locations of the NE region is -18.27 rad/m2 with a standard deviation of 28.48
rad/m2 ($\chi^{2}=0.72$), and of the SW region is 17.98 rad/m2 with a standard
deviation of 12.84 rad/m2 ($\chi^{2}=1.40$). The $\chi^{2}$ values for each
component are the median $\chi^{2}$ values within the radio beam at that
location. These RM values are consistent with negligible Faraday rotation in
the hotspots of the source.

###  3.4  Host Optical Morphology

Figure 8 shows a zoomed-in view of the HST F625W image, focused on the host
galaxy, its EELR, and the peak of synchrotron emission in the SW region. The
image reveals compact bright emission, co-spatial with the nucleus of the host
galaxy, and asymmetrically distributed emission to the north and south of the
nucleus. There is an elongated region of enhanced emission north of the
nucleus spanning a direction that is nearly perpendicular to the radio-jet
axis. Two smaller clumps of enhanced emission are visible on either side of
the nucleus (see inset).

The detached cloud, first identified in the APO spectra, is seen to the east
of the SW region, and another detached cloud of similarly diffuse emission is
present to the west of the SW region. We refer to these as South West
Cloud:east (SWC:east) and South West Cloud:west (SWC:west), respectively. We
also identify a compact source surrounded by diffuse emission to the north
west of the host galaxy and we refer to this source as the North West Nebula
(NWN). We manually mask the image to exclude all sources except the host
galaxy, its EELR, and the three nearby regions of diffuse emission which may
be associated with the EELR (SWC:east, SWC:west, and NWN), at a level of
3$\times$ the RMS in a 4 arcsec2 source-free region. The centroid of the host
galaxy is found with the raw image moments of the masked image, excluding the
SWC:east, SWC:west, and NWN. The brightest parts of SWC:east, SWC:west, and
the NWN lie at distances of 28.5 kpc, 37.8 kpc, and 19.46 kpc from the
centroid, respectively. The APO KOSMOS longslit (Table 2) aligned with the
radio jet axis is included in Figure 8 for reference.

Figure 8:  A zoomed-in version of Figure 3 focused on the host galaxy,
detached EELN, and the source of brightest synchrotron emission in the SW
region. The emission is manually masked to include only the host galaxy nebula
and the seemingly associated clouds, then these regions are masked down to
3$\times$ the off-source rms of the HST image. The color scales of the full
optical image and the inset image are Log normalized between 0 and 0.5 and 0
and 1.0, respectively. The radio contour levels are drawn down to 4$\sigma$
(50.8, 87.9, 151.9, 262.6, and 453.8 $\mu$Jy/beam). The beam size of the radio
contours is $1.6\times 0.5$ arcsec2, and is shown in the bottom right corner
(dashed black). The dashed rectangle represents the APO KOSMOS slit (1.18
″wide) aligned with the radio-jet axis, with the direction from the host
galaxy to the NE region indicated by a black arrow. The red star indicates the
image moment centroid.

###  3.5  Host Galaxy & EELR Line Ratios

We use the KOSMOS spectrum to calculate the [O III]$\lambda$5007/H$\beta$ and
[N II]$\lambda$6584/H$\alpha$ ratios to determine the location of RGB1 on the
classic BPT (Baldwin et al., 1981) diagnostic diagram. The host galaxy of RGB1
has BPT ratios of log10([O III]$\lambda$5007/H$\beta)=$0.98 $\pm$ 0.06 and
log10([N II]$\lambda$6584/H$\alpha)=$-0.39 $\pm$ 0.08, placing it above the
theoretical maximum for star forming galaxies (Kewley et al., 2001) and in the
region dominated by AGN-ionized emission. Although the [N II]$\lambda$6584
detection in the SWC:east was not significant enough for a flux measurement,
we find an [O III]$\lambda$5007/H$\beta$ ratio of log10([O
III]$\lambda$5007/H$\beta)=$0.83 $\pm$ 0.12 for the SWC:east, placing it above
the theoretical maximum for star forming galaxies (Kewley et al., 2001) if
log10([N II]$\lambda$6584/H$\alpha$) $>$ -1.0. Furthermore, Shirazi &
Brinchmann (2012) showed that AGN can be more clearly separated from star-
forming galaxies by using the He II$\lambda$4686/H$\beta$ ratio in place of
the [O III]$\lambda$5007/H$\beta$ ratio since the high ionization potential of
nebular He II$\lambda$4686 (54.4 eV) makes it more sensitive to the presence
of an AGN than [O III]$\lambda$5007 (35.1 eV). For the host galaxy we find
log10(He II$\lambda$4686/H$\beta$) = -0.28 +/- 0.09, and for the SWC:east
log10(He II$\lambda$4686/H$\beta$) = -0.33 +/- 0.35, placing them above the
updated theoretical maximum for star forming galaxies (Shirazi & Brinchmann,
2012), in a region where the AGN contribution to powering the emission is
$>$75%.

##  4 Discussion

We have identified a new Green Bean galaxy hosting extended radio emission and
a fractured EELR. We have combined optical long-slit spectra from APO and high
resolution broadband radio observations (4-12 GHz) from the VLA with archival
radio surveys (88 MHz-3.0 GHz) and pre-existing HST broadband imaging (WFC
F625W) to investigate the nature of the EELR around RGB1, explore the feedback
modes of the AGN, and disentangle the morphology of the large scale radio
emission. In what follows, we discuss these results and suggest scenarios to
explain the nature of RGB1.

###  4.1 Powering Mechanism of the EELR

Our optical results suggest that the EELR around RGB1 is likely powered by AGN
photoionization. The SWC:east appears almost completely covered by the APO
long-slit, suggesting the narrow line widths measured for this cloud (Table
2.2.2) are characteristic across the SWC:east structure. Such narrow line
widths combined with the high [O III]/H$\beta$ and He II/H$\beta$ ratios
measured for both the SWC:east and the host galaxy are suggestive of AGN
photoionization powering the emission, as opposed to shock heating (although
this does not preclude the possibility of photoionization by a shock front
that has not yet reached the illuminated region of the cloud, see Section
4.4). In what follows, we assume that the EELR around RGB1 is powered
predominantly by AGN photoionization. Without spectroscopic information for
the NWN, we cannot constrain its powering mechanism, but we note that the
assumption of AGN photoionizations powering this source does not the affect
our main results.

###  4.2 The Ionization Profile of the EELR

For gas that is heated via AGN photoionization, photoionization modeling
(e.g., CLOUDY Ferland et al., 1998, 2013, 2017) can help map the rate of
photons produced in recombination lines, such as H$\alpha$, to the rate of
ionizing photons (Qion) intercepted by gas in the EELR. If the production of
ionizing photons varies on timescales longer than the recombination time of
the observed line, but shorter than the time it takes light to traverse the
EELR, the record of this change will be encoded in the surface brightness
distribution of the recombination line. Essentially, mapping the recombination
line photons to AGN ionizing photons as a function of distance from the
nucelus constrains changes in the rate of ionizing photons with time. While
accurately performing this recombination line mapping requires information on
the gas electron temperatures and densities (see e.g., Finlez et al., 2022), a
lower limit to the expected rate of ionizing photons can be estimated by
assuming that each recombination line photon corresponds to a single
photoionization event (recombination balance).

If the expected Qion derived in this way is underestimated by an amount that
is constant with distance from the nucleus, the Qion vs. distance (lightyears)
profile can still be interpreted in terms of how the ionizing photon rate has
evolved over the light travel time of the EELR. The expectation that Qion
would be underestimated comparably at each distance assumes that the gas
density of the EELR is relatively constant with distance from the nucleus.
This assumption may be appropriate for EELRs which consist of photoionized
tidal debris, particularly for the densest (brightest) clumps of gas. This so-
called ionization history analysis has been demonstrated on HST narrow band
observations of z$\sim$0.05 AGN hosting EELRs with photoionized tidal features
(e.g. Keel et al., 2017).

Specifically, Keel et al. (2017) mapped H$\alpha$ recombination emission to
Qion using narrowband imaging centered on the H$\alpha$ recombination line for
a sample of fading AGN, and available narrowband imaging centered on [O
III]$\lambda\lambda$4959,5007 (assuming [O III]/H$\alpha$=4) for a comparison
sample of currently active AGN. They showed that the fading AGN sample has
different behavior in the derived ionization history, compared to the active
sample, with the former showing a distinct fall in expected ionizing flux from
the outer EELR to the location of the AGN.

Assuming photoionization equilibrium for a centrally located AGN surrounded by
an EELR without unreasonable elongation along the line of sight, the ionizing
photon rate Qion can be estimated as:

$Q_{ion}=\frac{N}{fA}\frac{(4\pi~{}R^{2})}{a^{2}}(4\pi~{}D_{L}^{2})(1+z)$ (1)

where N is the H$\alpha$ image count rate for each pixel in units of photons
s-1, A is the effective telescope area, R is the projected distance of each
pixel from the nucleus (where it is assumed the production of ionizing photons
occurs), a is the area of a single pixel as seen from the nucleus, DL is the
luminosity distance for RGB1, z is the redshift, and f is the fraction of
Hydrogen recombinations that lead to an H$\alpha$ photon. Following Keel et
al. (2017), we let $f=0.29$, which is appropriate for “Case B” recombination.

We manually mask the HST WFC F625W image to exclude all sources except the
host galaxy, its EELR, and the three nearby sources, at a level of 2$\times$
the RMS in a 4 $arcsec^{2}$ source-free region. We then measure the image
counts (electron s-1) in each pixel of our masked image, corrected for the
quantum efficiency of the HST WFC/F625W filter system ($\eta=$0.40 electrons
photon-1 at the observed wavelength of [O III]$\lambda$5007). The corrected
counts (photon s-1) are converted into H$\alpha$ image counts using the
measured ([O III]$\lambda\lambda 4959,5007$+H$\beta$)/H$\alpha$ ratios from
the APO KOSMOS data, assuming other lines falling within the HST WFC F625W
filter have negligible strengths compared to these three. For pixels lying
within a projected distance of d$<$64,660 lightyears we find ([O
III]$\lambda\lambda 4959,5007$+H$\beta$)/H$\alpha$ = 5.2 and for pixels lying
above this distance we find ([O III]$\lambda\lambda
4959,5007$+H$\beta$)/H$\alpha$ = 6.6, which we use to compute their ionizing
luminosities.

The projected distance of each pixel (R) underestimates the true distances of
each volume of gas (pixel) from the nucelus by a factor of $1/sin(i)$, where
$i$ is the angle between the volume of gas and the line of sight. This means
that the time delay between the arrival of nuclear emission to the observer
and the arrival of reprocessed emission from a volume of the EELR lying at an
angle of $i$ to the line-of-sight is: $(R/c)\times(1-cos(i))/sin(i)$ (Keel et
al., 2012b), where c is the speed of light. The Qion profile as a function of
distance from the assumed location of the AGN (the brightest pixel) in the HST
WFC/F625W image is shown in Figure 9.

Figure 9:  Pixel-by-pixel inferred ionization history of RGB1. We mark the
locations of the brightest pixel in the masked image (solid red line), and the
brightest pixels in the SWC:east (dashed red), SWC:west (dashed pink), and NWN
(dotted orange). The lower envelope is caused by the 3$\times$ off-source RMS
cutoff used in the HST F625W image.

The lower envelope of this ionization profile is caused by the 3$\times$ off-
source RMS cutoff used for the analysis. The upper envelope corresponds to the
densest (brightest) gas clumps at each distance, and thus gives us the
strictest limits on the minimum Qion required to power the H$\alpha$
recombination emission. To reconstruct this upper envelope, one can bin the
pixels in distance by 1000 light years and select the max Qion value in each
bin. The temporal resolution of the ionization history derived in this way is
limited by the recombination timescale of the observed line, which depends on
the gas density in the EELR. This means the temporal resolution of the
ionization history should degrade with distance from the source of ionizing
photons (Keel et al., 2017). Following Keel et al. (2017), we smooth our max
Qion values by 0.14$\times$R. The scaling of 0.14 is consistent with the
recombination time scale ($\sim$8000 years) for the density (n${}_{e}<$50
cm-3) of the prototypical ionization echo, Hanny’s Voorwerp, which lies at a
projected distance of $\sim$57,500 light years from its source of ionizing
photons (Lintott et al., 2009). The smoothed ionization profile of RGB1
(Figure 10) spans $\sim$2 orders of magnitude in Qion and shows a clear
decline from the outer to inner EELR, characteristic of the “fading” signature
seen in Keel et al. (2017).

Figure 10:  Inferred Ionization History (Smoothed max Qion) of RGB1. We mark
the locations of the brightest pixel in the masked image (solid red line), and
the brightest pixels in the SWC:east (dashed red), SWC:west (dashed pink), and
NWN (dotted orange). The dashed green Gaussian profile in the bottom left of
the figure represents the size of the optical continuum, as suggested by the
FWHM (purple) of the Gaussian profile fit to the spatial axis of the 2D
spectrum. The Gaussian profile is arbitrarily normalized.

The change in Qion with distance from the nucleus suggests that the ionizing
photon rate of the AGN has declined by $\sim$2 dex over the past 0.15 Myr.
Without comparable narrowband imaging centered on the H$\beta$+[O
III]$\lambda\lambda$4959,5007 complex, we cannot constrain the continuum
contribution to the broadband emission, therefore, we do not correct for the
continuum, but note that removing it from the hot galaxy would bias the inner
region towards lower Qion estimates, further exaggerating the “fading”
signature.

###  4.3 Timescales of AGN activity

The timescales associated with different states of accretion in AGN allow for
detailed analysis of the contributions of radiative and kinetic feedback to
the energy injected into the environment of the galaxy (Fabian, 2012;
Morganti, 2017). Radiative mode feedback dominates during episodes of high-
Eddington accretion, which produces radiation pressure driven winds and
outflows. Kinetic mode feedback is thought to dominate during the low-
Eddington accretion phase via mechanical driven outflows, which can persist as
remnants in the system long after the AGN has transitioned into a different
accretion state (Schawinski et al., 2015). Evidence of these two modes of
accretion has been observed simultaneously in some radiatively efficient AGN
hosting jet-driven radio structures (high excitation radio galaxies; HERGs,
Laing et al., 1994; Best & Heckman, 2012), and in some cases kinetic mode
feedback has been observed to dominate in systems where one might expect
radiative mode feedback (i.e. in HERGs hosting jet-driven outflows, Villar-
Martín et al., 2017; Villar Martín et al., 2021).

####  4.3.1  The EELR as a Tracer of Radiatively Efficient Accretion

In AGN, the total accretion lifetime is thought to be $\sim 10-1000$ Myrs
(Soltan, 1982; Yu & Tremaine, 2002; Marconi et al., 2004; Hopkins & Hernquist,
2009), but may be broken up into smaller radiatively efficient accretion
episodes of $\sim$0.1 Myrs (Schawinski et al., 2015; King & Nixon, 2015).
Based on our optical results we find that the minimum duration of the
radiatively efficient accretion state responsible for powering the production
of ionizing photons in the RGB1 system is of the order of $\sim$0.1 Myr.
Specifically, the light travel distance between the nucleus of RGB1 and the
south-west clouds suggests that ionizing photons released from the nucleus of
RGB1 would take at least $\sim$0.09 Myr and $\sim$0.12 Myr to photoionize the
extended gas in SWC:east and SWC:west, respectively. Furthermore, the
ionization history analysis suggests that the rate of ionizing photons
produced in the nucleus has declined by two orders of magnitude over the past
$\sim$0.15 Myr.

The emission used to derive these constraints traces AGN heating on scales
comparable to the light travel distance of the EELR ($>$1000 light years),
while a more direct view of the AGN as indicated by AGN-heated dust in the
nuclear torus (10-100 lightyears) is traced by mid-IR (MIR) emission. Type 2
AGN with fading and rising rates of ionizing photon production can be
identified by a comparison of their [O III] EELR luminosity vs. MIR
luminosity. For this relationship, samples of active Type 2 AGN with
radiatively efficient accretion flows will span a positively increasing locus
of [O III] vs. MIR luminosity. AGN that fall above and below this track can be
identified as potential fading and rising candidates, respectively (Esparza-
Arredondo et al., 2020; Pflugradt et al., 2022). Using this technique,
Schirmer et al. (2013) found that all 17 reported GB galaxies lie above this
locus when compared to typical active Type 2 AGN and luminous obscured AGN,
suggesting they host AGN with falling rates of ionizing photon production.

Using the APO KOSMOS spectrum, we find an [O III]$\lambda$5007 luminosity of
log10(L${}^{KOSMOS}_{[OIII]}$) = 42.2704 $\pm$ 0.0042 for RGB1. Given that the
APO spectra are not corrected for slit losses, we also estimate the [O
III]$\lambda$5007 luminosity by using the ([O III]$\lambda\lambda
4959,5007$+H$\beta$+continuum${}_{5446\AA}^{7100\AA}$)/[O III]$\lambda$5007
ratio measured from the optical spectrum to estimate the contribution of [O
III]$\lambda$5007 emission to the HST WFC/F625W image. In doing this, we find
log10(L[OIII]λ5007)= 42.7 $\pm$ 0.26. The estimates from the APO spectrum and
HST image place RGB1’s L[OIII]λ5007 $\sim$0.5 dex and $\sim$0.1 dex below the
least [O III]$\lambda$5007-luminous GB galaxy, respectively. In the MIR, the
Wide-field Infrared Survey Explorer (WISE, Wright et al., 2010) 22$\mu$m
luminosity for RGB1 is log10(L22μm) = 44.28 $\pm$ 0.16, consistent with the GB
galaxies. This suggests that RGB1 would not be selected as a fading AGN
candidate by this type of [O III] vs. MIR inspection.

One reason why the fading behavior of RGB1 is captured by the ionization
profile analysis but not the [O III] vs MIR comparison could be that RGB1 does
not have as much extragalactic gas available to be illuminated by the AGN,
compared to the GB galaxies. If this is the case, this suggests that using an
inspection of [O III]$\lambda$5007 vs. MIR emission to identify fading
candidates may bias the selection towards AGN with more massive EELRs outside
their host galaxy.

The lack of fading behavior in the [O III]$\lambda$5007 vs. MIR inspection may
also be due to differences in the host galaxy properties of RGB1 and the GB
galaxies. The GB galaxies have MIR emission that follows a power law,
suggesting the emission is nuclear in origin (i.e. AGN heated, Schirmer et
al., 2013). In RGB1 the MIR WISE flux densities (Fλ) show a decrease in
brightness between the rest frame wavelengths 2.6-3.5 $\mu$m before
monotonically rising between 3.52-16.9 $\mu$m, suggesting that the AGN is not
dominating the MIR emission. Given that the 22$\mu$m luminosity of RGB1 is
comparable to the GB galaxies, this may indicate higher rates of star
formation or a particularly dusty interstellar medium (ISM) in RGB1. In this
case, a decline in the MIR emission associated with AGN-heated dust may not be
sufficient to allow RGB1 to stand out in an [O III]$\lambda$5007 vs. MIR
inspection. However, the dominance of the AGN in powering the EELR emission
via photoionization combined with the low AGN contribution in the nucleus as
inferred from the MIR emission further supports a decline in the Qion of the
AGN over the past $\sim$0.15 Myr.

####  4.3.2 The Spectral Age of the Radio Emission

Interpretation of the radio spectral aging analysis is not as clear. The
spectral index of the radio core is steep ($\alpha^{10.5}_{4.5}=-1.02\pm
0.09$), while typical spectral indices for active radio galaxy cores are much
flatter, generally $\alpha_{GHz}>-0.5$ (Burns & Gregory, 1982; Hardcastle &
Looney, 2008). This observed steepness could be caused by a recent change in
the accretion rate of the AGN in RGB1.

The spectral slopes of the compact components in the NE and SW regions are
also steep, but consistent with the typical spectral indices measured in
active hotspots and fresh lobes of radio galaxies where the median is
typically $-0.75$ to $-0.85$ (Herbig & Readhead, 1992; Kharb et al., 2008;
O’Dea et al., 2009; Vaddi et al., 2019), and may be as steep as $-1$ in the
case of a strong termination of the jet (“high-loss” hotspots) where the
electrons accelerated by the jet termination lose most of their energy before
being transported into the lobes (Meisenheimer et al., 1989). However, if such
strong shock terminated in the SWC complex, then this seems at odds with the
narrow line widths in the EELR east of the presumed SW region hotspot
(SWC:east; but see Section 4.4 for discussion of a potential jet-cloud
interaction). Alternatively, if the steep spectrum of the radio core is due to
a recent change in the accretion rate, the somewhat steeper spectral slopes of
the NE and SW compact components as compared to the typical values of active
radio lobes may be caused by the initial decline in jet power.

The spectral modeling revealed low age regions in the center of the NE BRATS
region and along the southern edge of the SW BRATS region (Figure 6, top).
These low aged plasma regions, which may indicate where the true hot spots
most likely are, span ages of 0-3 Myrs. Visual inspection of the model fits in
these low aged areas suggests that the regions where the ages are exactly 0
Myr are driven by noisy data and poor model fits (note the higher spectral age
errors and chi squared values associated with the zero-aged regions).
Furthermore, our visual inspection also determined that the low age plasma
regions with ages above 0 Myrs are associated with adequate model fits, but
high spectral age errors ($2-3$ Myrs). This makes it it impossible to
determine whether the hotspots of RGB1 are still active, or remnant with an
off time of $\sim 2-3$ Myrs.

The average age for all pixels included in the NE BRATS regions are 6.20 Myrs
with standard deviation 2.54 Myrs, and in the SW BRATS regions 6.37 Myrs with
standard deviation 1.55 Myrs. These averages suggest that the duration of the
phase responsible for powering the radio jet in RGB1 is at least $\sim$6 Myr,
but likely much longer given that these averages are more heavily weighted
towards hotspot emission (lowest ages) rather than emission from the diffuse
lobe plasma (oldest ages). Resolved spectral age modeling across all scales of
the low frequency emission is imperative to estimating the true age of the
oldest plasma.

####  4.3.3 Timelines of the AGN feedback modes

Our ionization history analysis for RGB1 suggest that this AGN was previously
active in a high-accretion state, i.e. the efficient accretion state
associated with high Eddington-scaled accretion rates and a high production
rate of ionizing photons. Then, over the course of 0.15 Myr, the high-
Eddington accretion rate declined, leaving behind an EELR ionization echo.
Being further from the nucleus, the large scale jet-induced radio emission
associated with RGB1 probes longer timescales of the AGN activity. The low age
regions found by the spectral aging analysis are consistent with active jet
termination in the presumed hotspots, while the spectrally steep, faint
emission from radio core suggests the jet-production potentially being in a
phase of switching off.

Combined multiwavelength studies are a powerful tool in deciphering accretion
stages and timescales in AGN (e.g. Fabbiano et al., 2022; Alcorn et al.,
2023). Recently X-ray, UV, optical, IR, and radio observations have been
combined to characterize the accretion history of the AGN in IC 2497, which
hosts the prototypical ionization echo, “Hanny’s Voorwerp” (Lintott et al.,
2009), hence being particularly relevant here. Multi-wavelength studies of
this system have revealed that the AGN in IC 2497 shows evidence of both a
remnant (t${}_{spec}>$100 Myr; Smith et al., 2022) and a restarted (Józsa et
al., 2009; Rampadarath et al., 2010) phase of jet production, along with signs
of kinetic feedback currently acting on the host galaxy (Keel et al., 2012b;
Sartori et al., 2016). The highly ionized gas in “Hanny’s Voorwerp” is likely
associated with an episode of radiatively efficient AGN activity that ended
$\sim$0.1 Myrs ago, sometime between the two kinetic mode episodes. The
general picture revealed by combining these studies is one where the AGN in IC
2497 has alternated between radiatively inefficient and efficient accretion
states (possibly dipping into quiescent states) for the past $\geq 100$ Myrs.

If the jet power responsible for the radio emission in RGB1 is declining then
the picture painted by combining our results for RGB1 suggests that it may
have been a HERG whose jet power and ionizing photon rate have declined over
the past $\sim$2–3 Myr and $\sim$0.15 Myr, respectively. In this case, it is
not clear whether the AGN transitioned from an accretion state associated with
radio jet production to an accretion state associated with ionizing photon
production before declining in nuclear energy production all together, or
whether the rise and fall in nuclear ionizing photon rate occurred alongside
the radio jet production phase (i.e. feedback modes acting simultaneously). On
the other hand, if the radio emission in RGB1 is associated with active jet
production, then our results suggest that RGB1 was a HERG that transitioned to
a low excitation radio galaxy (LERG) in the past $\sim$0.15 Myr.

###  4.4 A Potential Jet-Cloud Interaction

We observe a superposition of the VLA-detected SW region and the SWC complex
(Figure 8) that is suggestive of a jet-cloud interaction. Hydrodynamical
simulations indicate that interaction of both non-relativistic and
relativistic jets with dense clouds in extragalactic radio sources may cause
significant changes in these components including cloud destruction or
displacement, and jet bending or even termination especially in the case of
weak jets (Higgins et al., 1999; Wang et al., 2000; Choi et al., 2007).

In principle, the observed superposition of the VLA-detected SW region and the
SWC complex may be incidental. The lack of broad line widths in the SWC:east
spectrum can support such a scenario. However, one explanation for the narrow
line widths observed in the cloud could be due to the slit of the optical
spectrum not being fully aligned with the SW region hotspot in our
observations (Figure 8). In 3C 277.3 (Coma A; Miley et al., 1981), which is
undergoing jet-cloud interactions on both sides of its nucleus (van Breugel et
al., 1985), integral field spectroscopic (IFS) observations of the system
found lower H$\alpha$ line widths for gas slightly offset from the northern
jet hotspot (Solórzano-Iñarrea & Tadhunter, 2003). The authors suggest that
this gas has not yet been disturbed by the radio jet, but is being ionized by
radiation produced in the shock (Solórzano-Iñarrea & Tadhunter, 2003; Worrall
et al., 2016). This suggests that we should see evidence of an on-going shock
in the SW region hotspot.

Based on hydrodynamical simulations jet–cloud interaction would produce
sharply increased synchrotron emission at the location (Choi et al., 2007).
The coincident radio components in the VLA observations suggest such an
incidence. However, in our VLBA observations of the SW region we do not detect
any emission from the hotspot or jet knots, with a 3$\sigma$ upper limit of
69.3 $\mu$Jy/beam. While actively replenished hotspots should be of a
brightness temperature detectable at VLBI angular resolutions, it is possible
for them to be resolved out. Based on the shortest baseline for the VLBA
observations, a non-detection of an active hotspot could imply that its size
is larger than 275 mas (1.24 kpc at the redshift of RGB1). However, htspot
sizes detected in VLBI experiments are observed to be 50 pc – 2.5 kpc in size
(e.g. Tingay et al., 2008, and references therein). Alternatively, the hotspot
may simply be too faint to be detected in our observations with their
brightness temperature limit of $7.1\times 10^{4}$ K. In principle this is
enough to detect hotspots of powerful radio galaxies at mas scales (e.g.
Lonsdale & Barthel, 1998; Young et al., 2005; Tingay et al., 2008), but our
radio galaxy is significantly fainter and less powerful.

An alternative suggestion , in the case where the jet power has declined over
the past $\sim$2–3 Myr, is the possibility that shocks induced in the cloud
$\sim$ 2–3 Myrs ago due to the termination of the SW jet may have dissipated
quickly enough to result in such quiescent cloud kinematics seen today. Such a
scenario is also suggested as a possibility for the IC 2497/“Hanny’s Voorwerp”
system by Smith et al. (2022) in order to explain the quiescent kinematics of
“Hanny’s Voorwerp” given that the large-scale radio jet is believed to have
punched a “hole” through the nebula $>$100 Myr ago. Deeper, higher resolution
IFS observations may reveal whether the cloud is being shocked by the jet by
estimating whether the resolved electron temperatures and densities across the
cloud are consistent with shock models (Davies et al., 2015).

###  4.5 Radio Morphology of RGB1

RGB1 was first spotted due to its hybrid-like radio morphology appearing in
mid-resolution radio images, hence a brief discussion on this aspect is called
for here. HyMoRS are an interesting class of object providing insight in the
Fanaroff-Riley dichotomy debate (nature vs. nurture, e.g. Reynolds et al.,
1996; Meier et al., 1997; Laing & Bridle, 2002, 2014; Meliani et al., 2008;
Saripalli, 2012, among many others). With its HyMoRS-like, asymmetric apparent
radio structure, RGB1 could in principle be considered a member of this
morphological class. Radio morphology of the NE region ($\sim 26\arcsec$
distance from the radio core) does resemble a classical FR II source, with
fairly compact structure located at the outermost part of the lobe, presumed
to be a jet termination point, and a more diffuse lobe emission trailing back
towards the core. The most compact component of the SW region peaks in
brightness at a distance $\sim 7.3\arcsec$ from the radio core, with more
diffuse emission extending to the southwest, away from the host galaxy and the
radio core. The polarization properties of the hot spots display signatures
typical for FR II types where the polarization angle is near perpendicular to
the jet direction (signature of plasma compression, Figure 7). Based on these
results and the asymmetry of the radio source, RGB1 could simply be an FR II
radio galaxy viewed at an angle to our line of sight.

A suggestion has been recently put forward that some radio galaxies with the
apparent HyMoRS type and clear hot spots in each of the lobes may be
intrinsically FR II sources with bent jets seen in projection (Harwood et al.,
2020). However, both radio lobes of RGB1 have similar, steep average spectral
indices ($\alpha^{12\rm GHz}_{4{\rm GHz}}\simeq-0.9$; Figure 4), and we do not
see any significant depolarization in either region (Section 3.3); both of
these effects would be expected to be present at least in some degree if the
bent-jet FR II model applied to RGB1 (specifically the one in which jets are
oriented away from the observer, cf. FR I counter-jet viewing angle, Figure 4
in Harwood et al., 2020). However, absence of these effects does not
necessarily rule out the possibility of projection.

It has also been previously discussed that HyMoRS may potentially be FR II
type radio galaxies seen in a transient stage, i.e. during the radio source
switching off, restarting, or undergoing episode of increased activity, in
combination with projection effects (Gopal-Krishna et al., 1996; Marecki,
2012; Kapińska et al., 2017; Harwood et al., 2020). For a radio source viewed
at an angle to the line of sight, the central engine output modulation and
differential light travel time between the far and near hotspots (relative to
the observer) may temporarily produce an apparent HyMoRS. For example, for a
radio source seen in projection that has undergone augmentation of activity in
producing radio jets the near radio lobe and its hotspot should be brighter
than the far one. This is what we observe in RGB1 where the NE hotspot is
nearly twice as bright as the SW one, suggesting such an event could have
happened recently. However, the asymmetry in the brightness of hotspots may be
also due to other causes such as difference in the gas density around the
source, with the brighter hotspot produced in higher density environment (but
see the discussion in previous section on the possible gas–jet interaction in
the SW region).

Polarization properties of the SW region radio emission indicate a highly
turbulent medium (Figure 7); whether the lobe is seen in projection or it is
truly trailing behind the fading hot spot away from the radio core, the
environment on the southern side of the RGB1 seems to be inhomogeneous,
possibly denser, and more turbulent as compared to the northern side. While
our results presented here are inconclusive regarding the viewing angle of the
radio emission in RGB1, should the radio galaxy prove to not be projected,
then the HyMoRS structure would still be caused by an early FR type II jet
termination rather than the jet being intrinsically an FR I type on one side.

##  5 Conclusions

We have presented a multi-wavelength investigation into the nature of the RGB1
system, including the powering mechanisms of its EELR, and the feedback modes
of its AGN. We summarize our results as follows:

  1. 1.

RGB1 is a new Green Bean galaxy hosting extended radio emission and an EELR
that is most likely photoionized by an AGN.

  2. 2.

The minimum duration of the radiatively efficient accretion state in RGB1 is
given by light-travel estimates to be $\sim$0.1 Myr, while the ionization
profile analysis indicates a decline in the rate of ionizing photons produced
in this state over the past $\sim$0.15 Myr. The spectral age estimates of the
extended radio emission suggest the duration of the jet production phase of
the AGN is at least $\sim$6 Myr, but are inconclusive regarding whether the
jet production phase is still ongoing. Combined, these results suggest that
RGB1 was in a phase of jet production prior to the radiatively efficient
accretion phase traced by the current EELR emission. It is unclear whether
RGB1 was a HERG that has transitioned into either a LERG or an inactive galaxy
over the past $\sim$0.15 Myr, or whether the extended radio and optical
emission trace distinct accretion phases that occurred in succession.

  3. 3.

The SWC:east along the radio axis is aligned with the hotspot in the SW region
and displays a blue shift relative to the host galaxy. The cloud does not
display any other evidence of turbulence (e.g. large line dispersion).
Additional optical IFU and X-ray observations can shed light on whether the
jet powering the SW region terminated in the complex, or whether we are
observing a chance alignment between the two.

  4. 4.

While the VLA continuum total intensity image of the source suggest a hybrid
radio morphology, the spectro-polarimetric radio observations suggests that
both hotspots are produced as a result of the jet terminating in the external
medium, consistent with both components being of FR II morphology. Whether the
HyMoRS appearance of RGB1 is caused by an intrinsic FR type II source seen in
projection or by an early termination of the southern jet into a more dense
environment is unclear.

RGB1 is an interesting system that warrants multiwavelength follow-up on
resolved scales to better understand the history and future of the host galaxy
and the fading AGN within.

## Acknowledgements

The authors would like to thank Preshanth Jagannathan, Rick Perley, Natalie
Wells, and Daniel Gondines for helpful discussions.

The initial observation of RGB1 was undertaken as part of the National Radio
Astronomy Observatory (NRAO) VLA Summer Program funded by the National Science
Foundation (NSF) using director’s discretionary time. The authors would like
to acknowledge Jacob Hetrick for their help with the initial data calibration.
Support for the analysis of these observations was provided by the NSF through
the Grote Reber Fellowship Program administered by Associated Universities,
Inc./National Radio Astronomy Observatory. The National Radio Astronomy
Observatory is a facility of the National Science Foundation operated under
cooperative agreement by Associated Universities, Inc.

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