11institutetext: INAF – Osservatorio Astronomico di Roma, Via Frascati 33,
I-00078 Monte Porzio Catone (RM), Italy.
11email<EMAIL_ADDRESS>22institutetext: ASI – Space Science Data
Center, Via del Politecnico snc, I-00133 Roma, Italy. 33institutetext: INAF –
Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via A. Corti 12,
I-20133 Milano, Italy. 44institutetext: Università di Bologna, Dip. di Fisica
e Astronomia “A. Righi”, Via P. Gobetti 93/2, I-40129 Bologna, Italy.
55institutetext: INAF – Osservatorio di Astrofisica e Scienza dello Spazio di
Bologna, Via P. Gobetti 93/3, I-40129 Bologna, Italy. 66institutetext: INAF –
Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, I-34143 Trieste,
Italy. 77institutetext: CSIC – Instituto de Astrofísica de Andalucía, Dep.to
de Astronomía Extragaláctica, Glorieta de la Astronomía s/n, E-18008 Granada,
Spain. 88institutetext: Max-Planck Institut für Astronomie, Königstuhl 17,
D-69117 Heidelberg, Germany. 99institutetext: Instituto de Astrofísica de
Canarias, C/ Vía Láctea s/n, E-38205 La Laguna (Tenerife), Spain.
1010institutetext: Universidad de La Laguna, Dep.to de Astrofísica, Av.da
Astrofísico F. Sánchez s/n, E-38206 La Laguna (Tenerife), Spain.

# Capturing dual AGN activity and kiloparsec-scale outflows
in IRAS 20210+1121

F. G. Saturni Capturing dual AGN activity and kiloparsec-scale outflows in
IRAS 20210+1121Capturing dual AGN activity and kiloparsec-scale outflows in
IRAS 20210+1121Capturing dual AGN activity and kiloparsec-scale outflows in
IRAS 20210+1121Capturing dual AGN activity and kiloparsec-scale outflows in
IRAS 20210+1121 G. Vietri Capturing dual AGN activity and kiloparsec-scale
outflows in IRAS 20210+1121Capturing dual AGN activity and kiloparsec-scale
outflows in IRAS 20210+1121Capturing dual AGN activity and kiloparsec-scale
outflows in IRAS 20210+1121Capturing dual AGN activity and kiloparsec-scale
outflows in IRAS 20210+1121 E. Piconcelli Capturing dual AGN activity and
kiloparsec-scale outflows in IRAS 20210+1121Capturing dual AGN activity and
kiloparsec-scale outflows in IRAS 20210+1121 C. Vignali Capturing dual AGN
activity and kiloparsec-scale outflows in IRAS 20210+1121Capturing dual AGN
activity and kiloparsec-scale outflows in IRAS 20210+1121Capturing dual AGN
activity and kiloparsec-scale outflows in IRAS 20210+1121Capturing dual AGN
activity and kiloparsec-scale outflows in IRAS 20210+1121 M. Bischetti
Capturing dual AGN activity and kiloparsec-scale outflows in IRAS
20210+1121Capturing dual AGN activity and kiloparsec-scale outflows in IRAS
20210+1121 A. Bongiorno Capturing dual AGN activity and kiloparsec-scale
outflows in IRAS 20210+1121Capturing dual AGN activity and kiloparsec-scale
outflows in IRAS 20210+1121 S. Cazzoli Capturing dual AGN activity and
kiloparsec-scale outflows in IRAS 20210+1121Capturing dual AGN activity and
kiloparsec-scale outflows in IRAS 20210+1121 C. Feruglio Capturing dual AGN
activity and kiloparsec-scale outflows in IRAS 20210+1121Capturing dual AGN
activity and kiloparsec-scale outflows in IRAS 20210+1121
F. Fiore Capturing dual AGN activity and kiloparsec-scale outflows in IRAS
20210+1121Capturing dual AGN activity and kiloparsec-scale outflows in IRAS
20210+1121 B. Husemann Capturing dual AGN activity and kiloparsec-scale
outflows in IRAS 20210+1121Capturing dual AGN activity and kiloparsec-scale
outflows in IRAS 20210+1121 C. Ramos Almeida Capturing dual AGN activity and
kiloparsec-scale outflows in IRAS 20210+1121Capturing dual AGN activity and
kiloparsec-scale outflows in IRAS 20210+1121Capturing dual AGN activity and
kiloparsec-scale outflows in IRAS 20210+1121Capturing dual AGN activity and
kiloparsec-scale outflows in IRAS 20210+1121

(Received 2021 May 24 / Accepted 2021 Jul 26)

The most standard scenario for the evolution of massive galaxies across cosmic
time assumes a correspondence based on the interplay between active galactic
nuclei (AGN) feedback, which injects large amounts of energy into the host
environment, and galaxy mergers, with their ability to trigger massive star
formation events and accretion onto supermassive black holes. Interacting
systems hosting AGN are useful laboratories for obtaining key insights into
both phenomena. In this context, we present an analysis of the optical
spectral properties of IRAS 20210+1121 (I20210), a merging system at
$z=0.056$. According to X-ray data, this object comprises two interacting
galaxies, each hosting an obscured AGN. The optical spectra confirm the
presence of AGN features in both galaxies. In particular, we are able to
provide a Seyfert classification for I20210 North. The spectrum of I20120
South shows broad blueshifted components associated with the most intense
emission lines that indicate the presence of an ionized outflow, for which we
derive a maximum velocity of $\sim$2000 km s-1, an extension of $\sim$2 kpc,
and a mass rate of $\sim$0.6 M⊙ yr-1. We also report the existence of an
ionized nebular component with $v\sim 1000$ km s-1 at $\sim$6.5 kpc southwards
of I20210 South, which can be interpreted as disrupted gas ejected from the
host galaxy by the action of the outflow. I20120 therefore exhibits a double
obscured AGN, with one of them showing evidence of ongoing events for AGN-
powered outflows. Future spatially resolved spectroscopy will allow for an
accurate mapping of the gas kinematics in this AGN pair and evaluate the
impact of the outflow on both the interstellar medium and the galaxy
environment.

###### Key Words.:

galaxies: active – galaxies: groups: general – galaxies: groups: individual:
IRAS 20210+1121 – galaxies: Seyfert – quasars: emission lines – quasars:
supermassive black holes

##  1 Introduction

The past history of formation and evolution of present-day massive galaxies is
a key point to consider on the path to obtaining a fuller understanding of the
functioning of the Universe. In this context, the study of processes operating
on galaxy-wide scales, such as the presence of active galactic nuclei (AGN;
e.g., Lynden-Bell 1969) or events related to galaxy mergers (e.g., Hernquist
1989), is crucial to improving our knowledge of the mechanisms that are able
to boot, maintain, enhance, and quench star formation in galaxies – thereby
shaping the entire environment in the process.

It is now widely accepted that AGN activity and galaxy mergers are among the
most effective phenomena regulating star formation in massive galaxies at
nearly all redshifts; namely, the first injects large amounts of energy that
are able to originate powerful gas winds in the surrounding environment (e.g.,
Di Matteo et al. 2005; Cattaneo et al. 2009; Fabian 2012), while the second
takes place by triggering massive star formation and starburst events in
molecular gas-rich clouds (e.g., Sanders & Mirabel 1996). Both simulations of
the evolutionary history of the Universe in the framework of the $\Lambda$-CDM
model (e.g., Davis et al. 1985; Springel et al. 2005; Croton et al. 2006) and
observations that confirm their contribution to simultaneously shaping
galactic environments (e.g., Sanders et al. 1988; Kormendy & Ho 2013; Ellison
et al. 2019) have confirmed the major role that such processes play in galaxy
formation and evolution.

Figure 1: Image in false colors of the I20210 system, obtained by combining
the grizy exposures of the Pan-STARRS1 survey (PS1; Chambers & Pan-STARRS Team
2016) centered on the sky coordinates of I20210S ($\alpha_{\rm J2000}=$ 20 23
25.4, $\delta_{\rm J2000}=+$11 31 34.7). The isophotes of the XMM-Newton
Optical Monitor (OM) UVW1 mosaic exposure (green solid lines) taken
simultaneously to the X-ray data analyzed by Piconcelli et al. (2010) – along
with some of the associated CCD count levels – are drawn onto the PS1 image to
highlight weak features. The TNG slit direction and position (magenta dot-
dashed lines) are also indicated along with the positions and directions of
the trace centers (white dashed lines) identified to extract the 1D spectrum
of each object.

Within this general picture, however, several values related to how exactly
AGN energetics and mergers directly impact the star formation history of
galaxies are still missing. For instance, it is still unknown whether
radiation-powered gas outflows are ubiquitous to all AGN (e.g., Elvis 2000) or
whether they affect only a fraction of the AGN lifetime (e.g., Farrah et al.
2007), along with what their effectiveness is with regard to altering the
physical and dynamical status of gas reservoirs on several spatial scales
(e.g., Scannapieco & Oh 2004; Cicone et al. 2018). In addition, the relative
dominance of one process onto the other for moving large gas masses and
triggering or quenching star formation has been found to be dependent on the
details of the AGN emission mode, the galaxy’s surrounding environment and its
star formation history (e.g., Hopkins et al. 2006; Heckman & Best 2014).
Therefore, the study of interacting galactic systems with the presence of
multiple AGN (e.g., Veilleux et al. 2002) offers an extremely interesting
possibility for understanding the properties and links between such competing
mechanisms. The most common objects of this kind are dual AGN, in which an
active nucleus is hosted in both members of a pair of interacting galaxies
with separation on the scale of $5\div 20$ kpc (see e.g., De Rosa et al. 2018,
and references therein).

Figure 2: 2D spectral sections of the I20210 system. Left panel: The H$\beta$
and [O III] region. Right panel: The H$\alpha$+[N II] and [S II] region. In
both panels, the trace centers of the main components are identified for
reference (green dashed lines). The elliptical fits to the [O III] and
H$\alpha$+[N II] emissions from the South Nebula are reported (green ellipses)
along with the respective best-fit parameters and statistical uncertainties.
In the right panel, the vertical features are sky lines, whereas the extrusion
close to the [S II] emission of I20210N is a cluster of saturated pixels that
is excluded from the IRAF extraction of the 1D spectrum. Figure 3: Optical
spectra of the I20210 components. Top panel: I20210N. Middle panel: I20210S.
Bottom panel: The spatially extended South Nebula. In all panels: (i) the
detected signal is reported along with its rms uncertainty (cyan bands); (ii)
the zero-flux level (dashed line) is indicated; and (iii) the positions of
major emission (top) and absorption features (bottom) are labeled accordingly.

In this work, we present the results of the optical spectroscopic analysis of
the $z=0.056$ dual AGN IRAS 20210+1121 (I20210 hereafter; Perez et al. 1990,
P90 hereafter), which is composed of two interacting galaxies oriented in the
N-S direction and separated by $12^{\prime\prime}.2$ (i.e. $\sim$13.3 kpc;
Davies et al. 2002; Arribas et al. 2004). Considered at first as being
composed of a Seyfert 2 with asymmetric emission lines (the southern
component) and a normal galaxy (the northern component), X-ray observations
performed with XMM-Newton revealed that this system is actually a merger
between two obscured AGN hosts (Piconcelli et al. 2010), in which the southern
member is an ultraluminous infrared galaxy (ULIRG; e.g., Sanders et al. 1988).
Additionally, despite having access to spectroscopic data in the near-infrared
(Burston et al. 2001), the optical spectrum of the northern member was still
unobserved, due to its faintness compared to the southern galaxy (Heisler &
Vader 1995). The image of the I20210 system, obtained by combining the grizy
exposures from the Pan-STARRS1 survey (PS1; Chambers & Pan-STARRS Team 2016),
is shown in Fig. 1. This image already gives us an idea of the complex
structure of the system, displaying a luminous bridge that connects the two
galaxies.

This paper is organized as follows. We describe the observation and data-
reduction process in Sect. 2. We characterize the extracted spectrum of the
northern galaxy in Sect. 3, as well as that of the southern one in Sect. 4. We
discuss the relevant physical properties of the structural components of the
southern I20210 member in Sects. 5 and 6. We estimate the supermassive black
hole (SMBH) mass of both I20210 members in Sect. 7. Finally, we summarize our
findings in Sect. 8. For simplicity, we abbreviate the names of the two
galaxies to I20210N (northern member) and I20210S (southern member) hereafter.
Throughout the article, we adopt a $\Lambda$-CDM cosmology with $H_{0}=70$ km
s-1 Mpc-1, $\Omega_{\rm M}=0.3$ and $\Omega_{\Lambda}=0.7$.

##  2 Observations and data reduction

Observations of the I20210 optical spectra were carried out on 2010 August 01
at the Telescopio Nazionale Galileo (TNG; Canarian Islands, Spain). The
spectra were simultaneously obtained with the $B$-band grism (wavelength range
$\lambda\lambda$3000 – 8430 Å, dispersion of 2.52 Å px-1,
$\lambda/\Delta\lambda=585$, implying a resolution of 9.8 Å that corresponds
to $\sim$510 km s-1) of the DOLoRes instrument (point-spread function PSF
$\sim 0^{\prime\prime}.85$), coupled to the $1^{\prime\prime}.5$ slit. To this
end, the instrument configuration was rotated to a position angle of 166∘ in
order to align the slit along the system axis connecting the two nuclei. The
two exposures of 600 s each (total exposure time of 1200 s) were then reduced
with standard IRAF procedures to extract and calibrate the one-dimensional
spectra. We show the slit position and orientation (P.A. $=166^{\circ}$ east
of north) along with the directions of the apertures used to extract the
spectra of each object in Fig. 1, superimposed to the PS1 image of the system.
The resulting spectra have signal-to-noise ratios of ${\rm S}/{\rm N}\sim
23.6$ (I20210N) and $\sim$33.2 (I20210S), respectively, as computed in line-
free continuum regions (Rosales-Ortega et al. 2012).

During the extraction and calibration procedures of the 1D spectra, we found
that a spectrum emitted from a third location was visible southwards of
I20210S, at a projected distance of $\sim$6′′ (corresponding to $\sim$6.5 kpc
given the distance scale of 1.087 kpc/′′ at $z=0.056$) from its trace center.
This additional spectrum, already identified by P90 in their low-resolution
data as extended emission in the I20210S host galaxy (South Nebula,
hereafter), is shown in 2D form in Fig. 2: it exhibits the main transitions
detected in I20210S (H$\beta$ $\lambda$4862, [O III]
$\lambda\lambda$4959,5007, H$\alpha$+[N II] $\lambda\lambda$6548,6583 and [S
II] $\lambda\lambda$6716,6731) detached by $\sim$6′′ from the I20210S nuclear
spectrum, extending over $\sim$2′′ (i.e. $\sim$2.3 kpc) in the N–S direction
and blueshifted by $\sim$450 km s-1 with respect to the systemic rest frame.
We thus extracted and calibrated it in the same way as we do for the spectra
of the main components.

Since the I20210 system is viewed through the Galactic plane, we dereddened
the spectra with the Milky Way extinction curve by Pei (1992) and $A_{\rm
V}=0.6$ (P90). The final spectra obtained in this way are shown in Fig. 3. A
visual inspection of the (so-far undetected) I20210N optical spectrum reveals
prominent [O II] $\lambda$3727, [O III] $\lambda\lambda$4959,5007,
H$\alpha$+[N II] $\lambda\lambda$6548,6583 and [S II]
$\lambda\lambda$6716,6731 emission lines, as well as the lack of the H$\beta$
$\lambda$4862 feature. Such a spectrum shows strong similarities with those of
typical Seyfert 2 galaxies, such as NGC 1667 (Ho et al. 1993, 1995; Jones et
al. 2009) and Mrk 1018 (Osterbrock 1981).

##  3 Characterization of I20210 North

We first proceed to estimate the amount of intrinsic dust extinction in each
object. To this end, we decided to measure the reddening $E(B-V)$ of the AGN
spectrum through the Balmer decrement $F_{{\rm H}\alpha}/F_{{\rm H}\beta}$
(e.g., Miller & Mathews 1972):

$\frac{F_{{\rm H}\alpha}}{F_{{\rm H}\beta}}=\frac{I_{{\rm H}\alpha}}{I_{{\rm
H}\beta}}\cdot 10^{-0.4E(B-V)(1+R_{V})(\kappa_{\alpha}-\kappa_{\beta})},$ (1)

with the intrinsic ratio $I_{{\rm H}\alpha}/I_{{\rm H}\beta}$ depending on the
physical conditions of the emitting gas only (see e.g., Gaskell & Ferland
1984, and refs. therein), and with $R_{V}$ and $\kappa_{\lambda}$ determined
by the adopted extinction model. Since no evidence for narrow lines associated
with H$\beta$ is visible bluewards of the [O III] doublet in the I20210N
spectrum, we first proceed to model the underlying continuum in order to
recover the Balmer emission from the narrow-line region (NLR) of I20210N.

###  3.1 I20210N continuum and emission-line fitting

We modeled the I20210N continuum under the assumption of a negligible AGN
contribution to the continuum emission. This is justified by the fact that the
I20210N central engine ($L_{\rm X}=4.7\times 10^{42}$ erg s-1) is highly
obscured by a column density $N_{\rm H}\sim 5\times 10^{23}$ cm-2 (Piconcelli
et al. 2010) and, therefore, no light from accretion activity is visible. We
subtract the stellar continuum with absorption lines from the I20210N galaxy
spectrum by using the penalized-pixel fitting public code pPXF (Cappellari &
Emsellem 2004; Cappellari 2012, 2017). The spectrum is fitted with a linear
combination of stellar spectra templates from the MILES library (Vazdekis et
al. 2010), which contains single stellar population synthesis models covering
the same wavelength range as the I20210N spectrum with a full width at half
maximum (FWHM) resolution of 2.54 Å. This procedure also yields information
about the kinematics status of the stellar population in the galaxy through
the stellar velocity dispersion $\sigma_{v}^{*}$.

Figure 4: Model of the rest-frame spectrum of I20210N. Upper panel: Full
I20210N spectrum (black solid line), along with the best-fit starlight model
adopted for continuum subtraction (yellow dot-dashed line), the best-fit
reddened emission profiles (green short-dashed lines), the global spectral
model (red solid line), and the masks applied to the telluric absorption lines
(grey bands) shown superimposed to the data. Lower panels: Zoom on the
continuum-subtracted emission lines (black solid line), shown along with the
global best fit (red solid line) and the best-fit single components (green
short-dashed lines) for the blended [O II] doublet, H$\alpha$+[N II] and [S
II] transitions. The standardized residuals after the best-fit subtraction are
also shown in separate windows below each spectral region. In all panels, the
zero-level flux (black long-dashed line) is indicated; in the panel with
H$\alpha$+[N II] and [S II], the masks applied to the telluric absorption
lines (grey bands) are shown superimposed to the data.

We rebinned the MILES templates ($\lambda/\Delta\lambda\sim 2.5$ Å) to match
the DOLoRes spectral resolution of $\sim$10 Å. We include low-order additive
(4th-degree) and multiplicative (1st-degree) Legendre polynomials to adjust
the continuum shape of the templates to the observed spectrum. During the
fitting procedure, strong emission features are masked out and the spectra are
shifted to the rest frame. The pPXF best-fit model is chosen through
$\chi^{2}$ minimization. To estimate the uncertainty on the velocity
dispersion, we produced $10^{3}$ realizations of the I20210N spectrum by
adding noise to the pPXF best-fit model; this noise is drawn from a Gaussian
distribution with dispersion equal to the rms of the input spectrum. We then
iterate the pPXF fitting procedure over such mock spectra and compute the
error associated with $\sigma_{v}^{*}$ as the standard deviation of the
parameter posterior distribution. In doing so, we find a best-fit
$\sigma_{v}^{*}=390\pm 50$ km s-1. The residual spectrum obtained by
subtracting off the best-fit stellar model from the spectrum is then used to
derive emission-line properties. This procedure allowed us to recover the
H$\beta$ narrow emission and therefore compute the Balmer decrement $F_{{\rm
H}\alpha}/F_{{\rm H}\beta}$. Both the fitted starlight continuum and the
residual emission-line spectrum of I20210N are shown in Fig. 4.

We note that the derived stellar-velocity dispersion value is very high
compared to what is expected for typical galaxies: for example, a search in
the catalogue of galactic dynamics by Forbes & Ponman (1999) yields only two
elliptical/S0 objects with $\sigma_{v}^{*}>300$ km s-1. Similarly, the stellar
velocity dispersions measured by Falcón-Barroso et al. (2017) in a large
sample of galaxies from the CALIFA survey and by Perna et al. (2021) in a
sample of nearby ULIRGs never exceed $\sim$200 km s-1. Nevertheless, objects
exhibiting exceptional values of $\sigma_{v}^{*}$ exist: it is, for instance,
the case of NGC 6240 ($\sigma_{v}^{*}\sim 360$ km s-1; Doyon et al. 1994),
which is indeed a final-state merging system. Therefore, the stellar velocity
dispersion value found in I20210N may indicate that the internal kinematics of
the galaxy is deeply altered by the gravitational interaction with I20210S.

We then fit the relevant emission lines with Gaussian profiles through the IDL
minimization package MPFIT (Markwardt 2009). All the narrow components are
simultaneously fitted considering them as emitted at the same distance from
the AGN, that is, with equal FWHM in the velocity space. In addition, we fix
the intensities of the faint components of the [O III] and [N II] doublets to
a ratio $1/3.06$ with the respective dominant component (e.g., Osterbrock &
Ferland 2006). In order to compute meaningful uncertainties of measurement for
the free parameters, we iterate this process over $10^{3}$ Monte-Carlo (MC)
realizations of each line spectrum. Such realizations have fluxes at each
wavelength altered by a random quantity extracted from a Gaussian
distribution, which is centered at the specific flux value and wide as the
corresponding 1$\sigma$ rms flux error.

The best fit of the I20210N emission lines is shown in Fig. 4, along with the
corresponding standardized residuals111The standardized residuals are computed
everywhere as $\left[F_{\lambda}-F_{\lambda}^{\rm(BF)}\right]/\sigma_{F}$,
where $F_{\lambda}^{\rm(BF)}$ is the best-fit model of the line flux and
$\sigma_{F}$ is the standard deviation of the dimensional residuals
$F_{\lambda}-F_{\lambda}^{\rm(BF)}$ (e.g., Cook & Weisberg 1982).. The value
of the Balmer decrement derived from this procedure is $2.97\pm 0.31$,
compatible within errors to both the intrinsic ratio $I_{{\rm
H}\alpha}/I_{{\rm H}\beta}\sim 2.85$ typical of [H II] region-like objects and
the AGN ratio of 3.1 (Veilleux & Osterbrock 1987). Therefore, we can assume
that the NLR of I20210N is viewed along a non-reddened line of sight, with
$E(B-V)\sim 0$. The best-fit parameters of the narrow emission lines are
reported in Table 1, with FWHM corrected for the instrumental broadening
$\Delta v_{\rm inst}\sim 510$ km s-1 corresponding to the DOLoRes resolution
of $\sim$10 Å:

${\rm FWHM}_{\rm corr}=\sqrt{{\rm FWHM}_{\rm obs}^{2}-\Delta v_{\rm
inst}^{2}}.$ (2)

| I20210N ($\chi^{2}/\nu_{\rm d.o.f.}=427/405$) | I20210S ($\chi^{2}/\nu_{\rm d.o.f.}=442/405$) | South Nebula ($\chi^{2}/\nu_{\rm d.o.f.}=228/209$)
---|---|---|---
Transition | Flux ($10^{-14}$ erg s-1 cm-2) | FWHM (km s-1) | Flux ($10^{-14}$ erg s-1 cm-2) | FWHM (km s-1) | Blueshift (km s-1) | Flux ($10^{-14}$ erg s-1 cm-2) | FWHM (km s-1) | Blueshift (km s-1)
| N | B | N | B | —N–B— |  |  |
[O II] $\lambda\lambda$3726,3729 | $1.060\pm 0.055$ | $690\pm 40$ | $10.4\pm 1.1$ | $10.6\pm 4.4$ | $530\pm 90$ | * | * | — | — | —
[O III] $\lambda$4363 | — | — | $0.500\pm 0.054$ | $0.42\pm 0.17$ | ” | * | * | — | — | —
H$\beta$ | $0.214\pm 0.010$ | ” | $13.72\pm 0.94$ | $6.3\pm 2.1$ | ” | $1960\pm 470$ | $330\pm 180$ | $0.0691\pm 0.0036$ | $710\pm 330$ | $550\pm 150$
[O III] $\lambda$5007 | $1.058\pm 0.047$ | ” | $85.2\pm 5.4$ | $32.1\pm 9.8$ | ” | $2080\pm 310$ | $390\pm 120$ | $0.1193\pm 0.0073$ | ” | ”
[O I] $\lambda$6300 | $0.193\pm 0.010$ | ” | $4.13\pm 0.20$ | $2.39\pm 0.57$ | ” | $1910\pm 350$ | $230\pm 150$ | — | — | —
H$\alpha$ | $0.623\pm 0.028$ | ” | $42.5\pm 2.0$ | $19.5\pm 4.3$ | ” | * | * | $0.1418\pm 0.0071$ | ” | ”
[N II] $\lambda$6583 | $0.863\pm 0.039$ | ” | $33.7\pm 1.6$ | $8.0\pm 2.2$ | ” | * | * | $0.0334\pm 0.0047$ | ” | ”
[S II] $\lambda$6717 | $0.436\pm 0.021$ | ” | $6.80\pm 0.31$ | $2.98\pm 0.66$ | ” | * | * | $0.0371\pm 0.0012$ | ” | ”
[S II] $\lambda$6731 | $0.366\pm 0.018$ | ” | $7.21\pm 0.32$ | $5.0\pm 1.0$ | ” | * | * | $0.0273\pm 0.0010$ | ” | ”
$E(B-V)$ |  $\sim$0 |  | $0.271\pm 0.019$ | $0.195\pm 0.091$ |  |  |  |  $\sim$0 |  |
| I20210N | I20210S | Outflow | South Nebula
Ratio | Value | Uncertainty | Value | Uncertainty | Value | Uncertainty | Value | Uncertainty
$\log{\left(\mbox{{[O\,{\scriptsize III}]}}/\mbox{{H$\beta$}}\right)}$ | $0.686$ | $0.040$ | $0.798$ | $0.061$ | $0.71$ | $0.28$ | $0.237$ | $0.049$
$\log{\left(\mbox{{[N\,{\scriptsize II}]}}/\mbox{{H$\alpha$}}\right)}$ | $0.142$ | $0.039$ | $-0.114$ | $0.044$ | $-0.39$ | $0.21$ | $-0.628$ | $0.083$
$\log{\left(\mbox{{[S\,{\scriptsize II}]}}/\mbox{{H$\alpha$}}\right)}$ | $0.110$ | $0.041$ | $-0.489$ | $0.044$ | $-0.39$ | $0.19$ | $-0.343$ | $0.037$
$\log{\left(\mbox{{[O\,{\scriptsize I}]}}/\mbox{{H$\alpha$}}\right)}$ | $-0.509$ | $0.042$ | $-0.933$ | $0.052$ | $-0.91$ | $0.20$ | — | —
$\log{\left(\mbox{{[O\,{\scriptsize II}]}}/\mbox{{H$\beta$}}\right)}$ | $0.695$ | $0.043$ | $-0.120$ | $0.077$ | $0.23$ | $0.33$ | — | —
$\log{\left(\mbox{{[O\,{\scriptsize III}]}}/\mbox{{[O\,{\scriptsize II}]}}\right)}$ | $-0.001$ | $0.042$ | $0.913$ | $0.073$ | $0.48$ | $0.31$ | — | —
-Transition not detected in the spectrum.
${}^{{\rm{}^{\prime\prime}}}$Value fixed to be equal to the first non-null one
upwards in the column.
∗Value anchored to the best fit of the [O III] $\lambda$5007 broad component.

Table 1: Top: Best-fit parameters of the dereddened diagnostic emission lines
in the optical spectrum of I20210N, I20210S NLR (“N” components) and
outflowing emission (“B” components), and emission from the South Nebula. The
integrated fluxes presented here have been dereddened by the indicated amount
of $E(B-V)$ with the SMC extinction by Pei (1992). Bottom: Values of the
diagnostic line ratios with the corresponding uncertainties.

Figure 5: BPT diagnostic diagrams showing the position of I20210N (red
circle), I20210S (green square) and its outflow (blue star), and the South
Nebula (yellow triangle), along with the relative uncertainties on top of the
SDSS data from the OSSY database (Oh et al. 2015, grey dots). As a reference,
in the first three panels, the extreme-starburst and Seyfert-LINER
classification boundaries by Kewley et al. (2001, solid lines) are indicated,
along with the pure star-formation boundary by Kauffmann et al. (2003, long-
dashed line), the alternative Seyfert-LINER relation by Cid Fernandes et al.
(2010, dotted line), and the redshift-dependent relation at $z\sim 0.13$ by
Kewley et al. (2013a, dot-dashed line) in the [O III]/H$\beta$-to-[N
II]/H$\alpha$ diagram. In the [O III]/H$\beta$-to-[O II]/H$\beta$ diagram, the
star-forming and Seyfert-LINER boundaries by Lamareille (2010, triple dot-
dashed line) are indicated, along with the mixed-region boundary (short-dashed
line).

###  3.2 I20210N classification

To assess the nature of the AGN hosted in I20210N, we computed the [O
III]/H$\beta$, [N II]/H$\alpha$, [S II]/H$\alpha$, [O I]/H$\alpha$ and [O
II]/H$\beta$ logarithmic line ratios from the best-fit emission line
parameters. The derived values are presented in Table 1, along with their
uncertainties. For completeness, we also report the value of the [O III]/[O
II] ratio that is used to further classify active galaxies (e.g., Heckman
1980; Kewley et al. 2006).

The values derived for I20210N are plotted in the BPT diagrams shown in Fig.
5, superimposed to the values for SDSS-DR7 objects retrieved from the OSSY
database (Oh et al. 2011, 2015). To discriminate between the different classes
of emission-line galaxies (star-forming, Seyferts, LINERs), we adopt from the
current literature the relations defining the boundaries between types of
galactic activity: (i) the extreme-starburst relation and the Seyfert-LINER
boundaries by Kewley et al. (2001) in the original diagrams by Baldwin et al.
(1981); (ii) the star-forming, Seyfert-LINER and mixed-region boundaries by
Lamareille (2010) in the [O III]/H$\beta$-to-[O II]/H$\beta$ diagram; (iii)
for the [O III]/H$\beta$-to-[N II]/H$\alpha$ diagram, the pure star-formation
boundary by Kauffmann et al. (2003), the alternative Seyfert-LINER relation by
Cid Fernandes et al. (2010) and the redshift-dependent star formation boundary
by Kewley et al. (2013a, see also ) computed for $z=0.128\pm 0.044$, that is,
the average redshift value of the OSSY catalog (Oh et al. 2015).

Figure 5 shows that the I20210N line ratios are generally consistent with
those found for Seyfert galaxies in the [O III]/H$\beta$-to-[N II]/H$\alpha$
and [O III]/H$\beta$-to-[O II]/H$\beta$ diagrams, while they remain
intermediate between a Seyfert and a LINER in the [O III]/H$\beta$-to-[S
II]/H$\alpha$ diagram, where they sit on the Seyfert-to-LINER boundary by
Kewley et al. (2001). Due to their intermediate nature, Heckman (1980) calls
these kinds of objects “transition galaxies” that lie between Seyfert 1.9
(Osterbrock 1981) and LINERs. However, the severe unresolved blending that
affects the H$\alpha$+[N II] emission system may offer an alternative
explanation: in fact, decreasing intrinsic H$\alpha$ intensity would
eventually move the position of I20210N in both the [O III]/H$\beta$-to-[S
II]/H$\alpha$ and [O III]/H$\beta$-to-[O I]/H$\alpha$ diagrams in the Seyfert
region (neglecting the shift in the [O III]/H$\beta$-to-[N II]/H$\alpha$
diagram given by the consequent increase in the [N II] emission). In addition,
the hard X-ray luminosity of $\sim$5$\times$1042 erg s-1 measured for I20210N
by Piconcelli et al. (2010) helps in breaking this uncertainty, pushing the
classification of I20210N towards a Seyfert 2 galaxy.

Figure 6: Model of the rest-frame spectrum of I20210S. Upper panel: Full
I20210S spectrum (black solid line), along with the local power laws adopted
for continuum subtraction (yellow dot-dashed lines), the best-fit reddened
narrow (green short-dashed lines) and broad emission profiles (blue dotted
lines), and the global spectral model (red solid line) shown superimposed to
the data. The intervals used for the local continuum subtraction under the
emission lines are marked, along with the masks applied to the [O I]+[Fe X]
emission and to the telluric H2O absorption line (grey bands). Lower panel:
Zoom on the continuum-subtracted emission lines (black solid line), shown
along with the global best fit (red solid line) and the best-fit narrow (green
short-dashed lines) and broad components (blue dotted lines). The standardized
residuals after the best-fit subtraction are also shown in separate windows
below each spectral region. In all panels, the zero-level flux (black long-
dashed line) is indicated; in the panel with [O I], the mask applied to the [O
I]+[Fe X] emission line is shown superimposed to the data, as well as the one
for the H2O telluric absorption in the panel with H$\alpha$+[N II] and [S II]
(grey bands).

##  4 Characterization of I20210 South

For both the nuclear spectrum and the South Nebula, we applied the same
fitting procedure as done for I20210N. Therefore, we first proceed to model
the I20210S continuum emission. We assume for I20210S an AGN-dominated object,
that is, one exhibiting a power-law continuum reddened by foreground dust;
such an assumption is well motivated by the quasar-like energetics of I20210S
(e.g., $L_{\rm bol}\sim 10^{45}$ erg s-1; Piconcelli et al. 2010).

###  4.1 I20210S nuclear continuum and emission-line fitting

The optical spectrum of I20210S is heavily reddened by intrinsic dust, which
is particularly evident by the lack of a flux rise bluewards the [O II]
emission. However, due to the presence of a large number of emission and
telluric lines that greatly reduce the intervals of featureless spectral
regions, we decided not to perform a global continuum fit to be subtracted
from the spectrum. Instead, we selected sections of the I20210S spectrum free
of major features and adjacent to the lines of interest, and interpolated them
with local power laws in order to remove the underlying AGN emission (see Fig.
6).

Then we used the MC fitting procedure to model the emission lines with
Gaussian profiles, which are shown in Fig. 6, along with the corresponding
standardized residuals. Differently from I20210N, we used two components for
each transition to account for the total emission profile in the spectrum, as
was also done by Arribas et al. (2014). The main narrow components obey the
prescriptions presented in Sect. 3.1; the additional emission, as already
found by P90, consists in a broad line blueshifted by $\sim$400 km s-1 with
FWHM $\sim 2000$ km s-1. The possibility that such features are an effect of
the orientation of the I20210S narrow-line region (NLR) as described in
Bisogni et al. (2017) is ruled out. In fact, for emission lines associated
with permitted transitions the asymmetries would be redshifted with respect to
the line center (see e.g., their figure 3). Therefore, we can conclude that
I20210S exhibits evidence of an ionized gas outflow.

Figure 7: Best-fit profiles of the emission lines from the South Nebula in the
rest frame. Left panel: The H$\beta$+[O III] spectral region. Right panel: The
H$\alpha$+[N II] and [S II] spectral region. In both panels, the global
emission profile (red solid line) is shown superimposed to the line spectrum
(black histogram) along with the single components of the H$\alpha$+[N II] and
[S II] blended profiles (green short-dashed lines), and the zero-flux level
(black long-dashed line) is indicated. The standardized residuals after the
best-fit subtraction are also shown in separate windows below each spectral
region.

To account for this additional emission, we include the broad components in
the fit of the I20210S line profiles anchoring the blueshift and FWHM values
of the transitions affected by severe blending – namely, the [O II]
$\lambda\lambda$3726,3729 doublet, the H$\gamma$+[O III] $\lambda$4363, the
H$\alpha$+[N II] system and the [S II] doublet – to those of the [O III]
$\lambda$5007 (see Table 1). This choice is motivated by the fact that the [O
III] emission has the highest S/N; in the cases where an anchoring to its
parameters is adopted, only the line amplitude is left free to vary. In
addition, we estimate the amount of intrinsic dust extinction for the NLR and
the outflow separately since the two regions are, in principle, located at
different distances from the central engine and can thus be affected by
different amounts of reddening.

The Balmer ratio derived from the MC fit for the narrow components is $F_{{\rm
H}\alpha}/F_{{\rm H}\beta}=4.108\pm 0.079$, corresponding to $E(B-V)_{\rm
NLR}=0.271\pm 0.019$ mag for the AGN intrinsic ratio $I_{{\rm
H}\alpha}/I_{{\rm H}\beta}=3.1$ (Veilleux & Osterbrock 1987) and the SMC
extinction by Pei (1992), whereas a ratio $F_{{\rm H}\alpha}/F_{{\rm
H}\beta}=3.80\pm 0.34$ for the outflow yields $E(B-V)_{\rm out}=0.195\pm
0.091$. Finally, we applied the extinctions derived in this way to deredden
the corresponding emission-line amplitudes. The best fit of the reddened
I20210S spectrum is shown in Fig. 6, whereas the best-fit parameters of both
its NLR and outflow emission are reported in Table 1. On average, the I20210S
wind has an outflow velocity $\Delta v=330\pm 170$ km s-1 and ${\rm
FWHM}=2000\pm 390$ km s-1: such values are a factor of $\sim$2.4 higher than
the corresponding mean parameters found by Arribas et al. (2014) in ULIRGs
hosting AGN (see their table 2), and more in line with those found by
Rodríguez Zaurín et al. (2013) for ionized outflows in nearby ULIRGs (see
their Table 2) and by Zakamska et al. (2016) in high-$z$ reddened quasars (see
their table 1) where the emission-line profiles are modeled using multiple
Gaussians.

###  4.2 South Nebula spectrum

Next we estimated the intrinsic reddening of the South Nebula. The detection
of both H$\beta$ and H$\alpha$ narrow transitions allows us to apply the MC
line-fitting procedure described in the case of I20210N (see Fig. 7 and Sect.
3.1). Lacking any trace of an underlying continuum that could have been used
in the determination of the reddening law, we adopted the SMC extinction by
Pei (1992), as in the case of the parent nucleus. This in turn yields $F_{{\rm
H}\alpha}/F_{{\rm H}\beta}=2.05\pm 0.10$, which is lower than the intrinsic
ratio of 2.85 valid for [H II] regions. Also, the low associated error of
measurement potentially indicates a poorly determined estimate of the Balmer
ratio, likely due to the uncertainties in extracting a continuum-less spectrum
that is $\sim$100 times less intense than the I20210S nuclear emission.
Therefore, the issue of determining the South Nebula intrinsic reddening is
clearly a matter that ought to be left to more sensitive, spatially resolved
spectroscopic future data; in the following, we consider it compatible with
$E(B-V)\sim 0$.

We then fit the parameters of the five emission features that are clearly
identified, namely H$\beta$, [O III], H$\alpha$, [N II] and [S II], without
applying any dereddening (see Table 1). Interestingly, the South Nebula
exhibits a blueshift of $550\pm 150$ km s-1 with respect to the systemic
redshift and a FWHM of $710\pm 330$ km s-1. Such features are a clear
indication of highly disrupted gas (Bellocchi et al. 2013), similar to that
found by Ramos Almeida et al. (2017) in the Teacup Galaxy ($L_{\rm[OIII]}\sim
5\times 10^{42}$ erg s-1 according to Reyes et al. 2008, to be compared with
$L_{\rm[OIII]}\sim 6.5\times 10^{42}$ erg s-1 for I20210S) at comparable
distances ($\sim$5.6 kpc) from the central engine.

###  4.3 I20210S classification

As done in Sect. 3.2 for I20210N, we computed the line ratios for all the
regions decomposed from the spectrum of I20210S, namely, the NLR, the outflow
and the South Nebula, and we placed them in the relevant BPT diagrams to
obtain a first discrimination between an AGN or star-formation powered
emission. A visual inspection confirms that the NLR properties are fully
consistent with their AGN nature, as well as with the outflow emission falling
well inside the AGN region shown to be in agreement with the scenario of an
ionized wind driven by the nuclear activity.

The South Nebula sits close enough to the boundary between AGN and star-
forming galaxies to prevent its straightforward inclusion among the AGN-
powered processes. However, the kinematic properties of this region (velocity
blueshift of $\sim$500 km s-1, FWHM of $\sim$700 km s-1) may actually be
interpreted as being due to the I20210S outflow, which has stripped out
ionized gas from the I20210S nucleus. The possibility that the South Nebula is
an extended NLR component blown out of the central engine by radiation
pressure is in principle supported by studies that ubiquitously find NLRs
extended over $\sim$10 kpc from the central engine in both Type 1 and Type 2
quasars (see e.g. Husemann et al. 2013, and refs. therein); in the case of
I20210S, this is less likely and it is expected, rather, to be extended, non-
outflowing gas associated with extreme mergers that is due to the fact that
blown-out NLRs typically show ${\rm FWHM}<250$ km s-1 (Bellocchi et al. 2013).

Our data do not allow a deeper exploration of the spectral properties of the
South Nebula. Therefore, we point out that due to the intermediate values of
its diagnostic line ratios between AGN and star-forming galaxies, this
detached emitting region is a very interesting environment in which the
effects of AGN feedback may be at work in pushing the gas outside the central
region of the host galaxy (negative feedback) while also triggering some
amount of star formation into it (positive feedback; see e.g., Maiolino et al.
2017). Thus, it is worthy of further investigation with high-quality spatially
resolved spectroscopy.

##  5 Physical properties of the outflow in I20210S

Next we were able to characterize the physics of the I20120S ionized wind. To
this aim, we estimated the outflowing mass $M_{\rm out}$ and the mass loss
rate $\dot{M}$ of the ionized gas following the method presented in Kakkad et
al. (2016) and Bischetti et al. (2017). Under the assumptions that (i) the AGN
wind is free, spherically or biconically symmetric, and mass-conserving; (ii)
the AGN wind has mass-outflow rate and velocity independent on the outflow
radius (Rupke et al. 2002, 2005); and (iii) most of the oxygen consists of O2+
ions, we can use the relation by Carniani et al. (2015):

$\log{\left(\frac{M_{\rm out}}{{\rm
M}_{\odot}}\right)}=7.6+\log{\left(\frac{C}{10^{{\rm[O/H]}-{\rm[O/H]}_{\odot}}}\right)}+\log{\left(\frac{L^{\rm
out}_{\rm[OIII]}}{10^{44}\mbox{ erg s}^{-1}}\right)}-\log{\left(\frac{\langle
n_{e}\rangle}{10^{3}\mbox{ cm}^{-3}}\right)}$

(3)

where $C=\langle n_{e}\rangle^{2}/\langle n_{e}^{2}\rangle$,
${\rm[O/H]}-{\rm[O/H]}_{\odot}$ is the gas metallicity relative to the solar
value, $L^{\rm out}_{\rm[OIII]}$ is the outflowing [O III] $\lambda 5007$
luminosity, and $\langle n_{e}\rangle$ is the average electron density. The
latter is, in turn, related to the electron temperature, $T_{e}$, which can be
derived from the line ratios $\left(I_{4959}+I_{5007}\right)/I_{4363}$ of the
outflow emission (Osterbrock & Ferland 2006):

$\frac{I_{4959}+I_{5007}}{I_{4363}}\approx
7.90\cdot\exp{\left(\frac{32,900\mbox{ K}}{T_{e}}\right)}.$ (4)

From the decomposition of the emission lines in the spectrum of I20210S
through the fit with multiple Gaussian components described in Sect. 3 (see
Fig. 6), we compute $\left(I_{4959}+I_{5007}\right)/I_{4363}=100\pm 70$, which
corresponds to $T_{e}=12,900\pm 3600$ K and is in agreement with the value of
$\sim$104 K generally assumed for AGN outflows (see e g. Perna et al. 2017,
and refs. therein).

Figure 8: Continuum-subtracted I20210S off-axis H$\beta$+[O III] spectra
extracted at a 5-px offset in the northern direction (left panel) and at an
8-px offset in the southern direction (right panel). In each panel, the best-
fit model is shown (red solid line) along with the profiles of the narrow
(green short-dashed line) and broad components (blue dotted line), and the
zero-flux level is indicated (black long-dashed line). The standardized
residuals after the best-fit subtraction are also shown in separate windows
below each spectral region. The fit to the H$\beta$ emission is not accounted
for the calculations of $\chi^{2}$ and $p_{F}$, which are performed on the [O
III] doublet only (see text), and is shown here for visual purposes only.

The electron density $\langle n_{e}\rangle$ is then related to the ratio
$I_{6717}/I_{6731}$ between the components of the [S II] doublet through:

$\frac{I_{6717}}{I_{6731}}=1.49\cdot\frac{1+3.77x}{1+12.8x},$ (5)

with $x=10^{-2}\langle n_{e}\rangle T_{e}^{-1/2}$ (Weedman 1968; Osterbrock &
Ferland 2006; Sanders et al. 2016). However, we compute a ratio
$I_{6717}/I_{6731}=0.60\pm 0.25$ for the ionized wind which is on the
saturating side of Eq. 5, and it only allows us to establish a lower limit at
95% probability of $\langle n_{e}\rangle\gtrsim 4000$ cm-3 to the outflow
electron density. This might either be an indication of a high electron
density or just a consequence of the severe blending that affects the [S II]
region at the low spectral resolution of DOLoRes, preventing us from deriving
a reliable estimate of $\langle n_{e}\rangle$. The same issue holds for the [O
II] doublet, which could have been used in place of the [S II] for such a
measurement (Osterbrock & Ferland 2006) but is even more blended because of
its peak separation of $\sim$3 Å only.

As an alternative possibility for deriving solid estimates of $\langle
n_{e}\rangle$ for the I20210S outflow, we also consider the application of the
trans-auroral ratio (TR) method by Rose et al. (2018). This method, based on
the evaluation of the line ratios – [S II]4068,4076/[S II]6717,6731 and [O
II]3726,3729/[O II]7319,7331 – allows us to obtain at once both $\langle
n_{e}\rangle$ and the intrinsic reddening $E(B-V)$ of the emitting gas. We
thus fit the trans-auroral doublets [S II] $\lambda\lambda$4068,4076 and [O
II] $\lambda\lambda$7319,7331 through the MC procedure with two narrow and two
broad components each, fixing their widths to the corresponding values for the
[O III] $\lambda\lambda$4959,5007 (see Table 1). This yields ${\rm
TR}(\mbox{{[S\,{\scriptsize II}]}})_{\rm out}=0.192\pm 0.051$ and ${\rm
TR}(\mbox{{[O\,{\scriptsize II}]}})_{\rm out}=1.72\pm 0.94$. Having derived
for the outflow a ionization parameters $\log{U}_{\rm out}=-3.09\pm 0.47$ from
its relation to the [O III]/H$\beta$ and [N II]/H$\alpha$ ratios (Baron &
Ménard 2019, BM19 hereafter), we can finally compare its TRs to the
simulations presented in Davies et al. (2020, see their figure 7), obtaining
$\langle n_{e}\rangle_{\rm out}=10,400^{+4000}_{-5200}$ cm-3 and $E(B-V)_{\rm
out}=0.34^{+0.24}_{-0.15}$.

Quantity | Value | Units
---|---|---
| Outflow | South Nebula |
$T_{e}$ | $12,900\pm 3600$ |  $\sim$10,000 | K
$\langle n_{e}\rangle$ |  $\gtrsim$5000 |  $\sim$100 | cm-3
$L_{\rm[OIII]}^{\rm out}$ | $(2.44\pm 0.74)\times 10^{42}$ | $(9.05\pm 0.55)\times 10^{39}$ | erg s-1
$v_{\rm max}$ | $2160\pm 380$ | $1100\pm 430$ | km s-1
$R_{\rm out}$ | $2.20\pm 0.14$ | $6.52\pm 0.43$ | kpc
$t_{\rm dyn}$ | $0.99\pm 0.27$ | $5.8\pm 2.6$ | Myr
$M_{\rm out}$ | $\left(1.94^{+0.69}_{-0.51}\right)\times 10^{5}$ | $\sim 3\times 10^{4}$ | M⊙
$\dot{M}$ | $0.59^{+0.46}_{-0.26}$ | $\sim 6\times 10^{-3}$ | M⊙ yr-1
$\dot{E}_{\rm kin}$ | $\left(0.86^{+1.27}_{-0.54}\right)\times 10^{42}$ | $\sim 2\times 10^{39}$ | erg s-1
$\dot{P}_{\rm out}$ | $\left(0.80^{+0.88}_{-0.44}\right)\times 10^{34}$ | $\sim 4\times 10^{31}$ | erg cm-1

Table 2: Summary of the relevant physical properties of the ionized outflow
discovered in the I20210S optical spectrum and of the South Nebula. Upper
section: Quantities that are independent of the electron density. Lower
section: Quantities dependent on the electron density, for which a value of
$\langle n_{e}\rangle=5000$ cm-3 (Rose et al. 2018) is assumed in the case of
the outflow. Note that the quoted errors of measurement are only indicative of
the magnitude of the statistical uncertainties, not the systematics, affecting
the computed values (see Sect. 5).

As pointed out in the literature (Rose et al. 2018; Spence et al. 2018; Davies
et al. 2020), the TR method allows us to probe denser gas with respect to the
use of the “traditional” [S II] doublet, whose emission is likely produced at
the ionization front where the electron density significantly decreases. This
issue is probably at the base of the high values of ionized gas mass and mass
outflow rate recently found in AGN winds (e.g., Carniani et al. 2015; Kakkad
et al. 2016; Bischetti et al. 2017; Perna et al. 2017), for which values of
$10^{2}$ cm-3 $\lesssim\langle n_{e}\rangle\lesssim 10^{3}$ cm-3 are usually
assumed. Such an assumption is justified from measurements of the outflow
electron density based on the [S II] method: for example, Arribas et al.
(2014) get $\langle n_{e}\rangle\sim 400$ cm-3 for the outflowing emission in
ULIRGs, whereas Perna et al. (2020) find $\sim$200 cm-3 in the archetypal
ULIRG Arp 220. For comparison, the values of $\langle n_{e}\rangle$ found by
Rose et al. (2018) for AGN-driven outflows in ULIRGs fall in the range
$3000\div 56,000$ cm-3, with a median value of $\sim$5000 cm-3. Also, Kakkad
et al. (2018) obtained spatially resolved values of $\langle n_{e}\rangle$ up
to $\sim$2000 cm-3 for ionized winds in nearby radio-selected Seyfert
galaxies. Due to the limited DOLoRes spectral resolution and the severe
blending that affects the I20210S trans-auroral emission lines with nearby
features (e.g., the H$\delta$ close to the [S II] $\lambda\lambda$4068,4076,
the He I blueward and the [Ni II] redward of the [O II]
$\lambda\lambda$7319,7331), we cannot draw any firm conclusion on the
reliability of the I20210S outflow electron density derived with the TR
method. Therefore, in the following discussion of the physical properties of
the I20210S ionized wind, we adopted $\langle n_{e}\rangle\sim 5000$ cm-3
(Rose et al. 2018) as our main reference when computing all the related
quantities.

With $L^{\rm out}_{\rm[OIII]}=(2.44\pm 0.74)\times 10^{42}$ erg s-1 obtained
from the outflow [O III] flux reported in Table 1, and the further assumptions
of $C\approx 1$ and ${\rm[O/H]}\sim{\rm[O/H]}_{\odot}$ (i.e., solar
metallicity), Eq. 3 yields $M_{\rm
out}=\left(1.94^{+0.69}_{-0.51}\right)\times 10^{5}$ M⊙. Clearly, this value
and those based on it are affected by the assumption on $\langle
n_{e}\rangle$. We then derive the expression of the outflowing mass rate
$\dot{M}$ from the fluid-field continuity equation as done in Bischetti et al.
(2017), in order to provide a local estimate of this quantity at the outflow
termination radius $R_{\rm out}$ (e.g., Feruglio et al. 2015):

$\dot{M}=3\frac{M_{\rm out}v_{\rm max}}{R_{\rm out}}.$ (6)

In order to estimate the spatial extension of the outflow, we performed a
series of adjacent, 1-px wide (i.e., $\sim$0.27 kpc, owing to the DOLoRes
angular scale of $0.252$ arcsec px-1 and the scale distance of 1.087 kpc
arcsec-1 at $z=0.056$) extractions of the I20210S spectrum along its 2D trace
in the high-S/N region of H$\beta$+[O III] .

First, we fit a Gaussian function to the trace profile at the [O III] peak to
get the trace width $\sigma=1.67\pm 0.01$ px (i.e., $\sim$0.46 kpc). Then, we
extracted 1D off-axis spectra in both the northern and southern direction
offset by 3 to 12 px ($\sim$0.8 to $\sim$3.3 kpc) from the aperture center
(Perna et al. 2015; Bischetti et al. 2017) in order to both exclude the signal
enclosed in the instrumental PSF of $0^{\prime\prime}.85$
($0^{\prime\prime}.43$ in each direction, i.e., 2.5 px) and avoid overlap with
either I20210N or the South Nebula. Finally, we calibrated such spectra with
the same wavelength dispersion and sensitivity function applied to the I20210S
average spectrum, and applied the MC fitting procedure to the continuum-
subtracted [O III] emission, only letting the line amplitudes free to vary –
narrow FWHM, broad FWHM and blueshifts are fixed to the values reported in
Table 1. We produced the MC fit for both the two-component model and a
comparison single-component model of the emission lines, in which the broad
emission from the outflow is neglected. In this way, we are able to identify
through a statistical $F$-test the transition region where the outflow signal
becomes negligible with respect to the NLR emission: specifically, we define
the significance threshold of the outflow by requesting an $F$-test
probability $p_{F}>0.90$.

The statistical analysis yields significant emission associated with the
outflow up to 5 px ($\sim$1.3 kpc) in the northern direction, with
$p_{F}\gtrsim 0.93$ ($\chi^{2}/\nu_{\rm d.o.f}=76/73$); at larger distances
along this direction, the signal emitted from the ionized wind quickly becomes
indistinguishable from the noise, and thus has no impact on the best fit
($p_{F}=0$). Instead, the outflow emission in the southern direction remains
significant out to 8 px ($\sim$2 kpc, $p_{F}\gtrsim 0.93$, $\chi^{2}/\nu_{\rm
d.o.f}=78/73$). The “terminal” line spectra extracted at 5-px northward and
8-px southward offset are shown in Fig. 8. From this point on, we therefore
adopt the distance $R_{\rm out}=2.20\pm 0.14$ kpc (i.e. $8.0\pm 0.5$ px) as
our fiducial value for the termination radius of the I20210S ionized wind
within the applicability limits of the $F$-test statistics. This choice is
motivated by the observation that an 8-px offset produces an extraction lying
beyond 3$\sigma$ pixels from the trace center, and thus fiducially outside the
2.5-px PSF radius.

Then we calculate $v_{\rm max}=|\Delta v|_{\rm[OIII]}^{\rm
out}+2\sigma_{\rm[OIII]}^{\rm out}=2160\pm 380$ km s-1 (see Bischetti et al.
2017, and references therein) from the outflowing [O III] $\lambda$5007
parameters reported in Table 1. In this way, from Eq. 6, we obtain
$\dot{M}=0.59^{+0.46}_{-0.26}$ M⊙ yr-1. Finally, we derive the outflow kinetic
power $\dot{E}_{\rm kin}$, the dynamical time scale $t_{\rm dyn}$ and the
outflow momentum rate $\dot{P}_{\rm out}$ as:

$\displaystyle\dot{E}_{\rm kin}=\frac{1}{2}\dot{M}v_{\rm max}^{2},$ (7)
$\displaystyle t_{\rm dyn}\approx\frac{R_{\rm out}}{v_{\rm max}},$ (8)
$\displaystyle\dot{P}_{\rm out}=\dot{M}v_{\rm max},$ (9)

which yield $\dot{E}_{\rm kin}=\left(0.86^{+1.27}_{-0.54}\right)\times
10^{42}$ erg s-1, $t_{\rm dyn}=0.99\pm 0.27$ Myr and $\dot{P}_{\rm
out}=\left(0.80^{+0.88}_{-0.44}\right)\times 10^{34}$ erg cm-1, respectively.
We report all these quantities in Table 2, highlighting that, given the high
reference value of $\sim$5000 cm-3 adopted for the outflow $\langle
n_{e}\rangle$, the electron-density dependent parameters might be
underestimated by a factor of $\sim$10$\div$50.

It should also be noted that although the quantities presented in Table 2 are
given along with the errors, these should actually be interpreted as rough
estimates, since the systematic uncertainties insisting on Eq. 3 exert a bias
on them at the level of $1\div 2$ orders of magnitude (Bischetti et al. 2017).
Given this caveat, the values of $M_{\rm out}$, $\dot{M}$ and $\dot{E}_{\rm
kin}$ obtained for the outflow of I20210S are in line with those found by
Rupke & Veilleux (2013) for a sample of nearby galaxy mergers (see also
Rodríguez Zaurín et al. 2013; Spence et al. 2018). In order to definitively
assess the AGN nature of the I20210S outflow, we compare its kinetic power to
the expected ejected mass rate, $\dot{M}_{\rm SN}$, energy output,
$\dot{E}_{\rm SN}$, and momentum injection, $\dot{P}_{\rm SN}$, of starbursts
associated with supernova (SN) explosions (Brusa et al. 2015). According to
Veilleux et al. (2005), such quantities are related to the host galaxy’s star
formation rate (SFR) by:

$\dot{M}_{\rm SN}\lesssim 0.26\left(\frac{{\rm SFR}}{{\rm M}_{\odot}\mbox{
}{\rm yr}^{-1}}\right),$ (10) $\dot{E}_{\rm SN}\lesssim 7\times
10^{41}\left(\frac{{\rm SFR}}{{\rm M}_{\odot}\mbox{ }{\rm yr}^{-1}}\right),$
(11) $\dot{P}_{\rm SN}\lesssim 5\times 10^{33}\left(\frac{{\rm SFR}}{{\rm
M}_{\odot}\mbox{ }{\rm yr}^{-1}}\right),$ (12)

whereas the SFR is linked to the host-galaxy IR ($8\div 1000$ $\mu$m)
luminosity $L^{*}_{\rm IR}$ by (Kennicutt 1998; Kennicutt & Evans 2012;
Kennicutt & De Los Reyes 2021):

$\frac{{\rm SFR}}{{\rm M}_{\odot}\mbox{ }{\rm yr}^{-1}}=3.9\times
10^{-44}\left(\frac{L^{*}_{\rm IR}}{{\rm erg}\mbox{ }{\rm s}^{-1}}\right).$
(13)

We estimate $L^{*}_{\rm IR}\sim 3.4\times 10^{44}$ erg s-1 for I20210S from
the values for the total IR luminosity of the I20210 system and the AGN IR
luminosity for the single members presented in Imanishi & Saito (2014), who
estimated the contributions to the total galaxy IR emission coming from the
active nucleus through photometric aperture size at high spatial resolution
(see their Tables 1, 3, and 5). This in turn yields ${\rm SFR}\sim 13$ M⊙
yr-1, and hence $\dot{M}_{\rm SN}\lesssim 3.4$ M⊙ yr-1, $\dot{E}_{\rm
SN}\lesssim 9\times 10^{42}$ erg s-1 and $\dot{P}_{\rm SN}\lesssim 6.5\times
10^{34}$ erg cm-1. Such values are $\sim$6 to $\sim$10 times higher than those
listed in Table 2. Therefore, a starburst at work in I20210S is potentially
able to produce the observed ionized outflow. However, Veilleux et al. (2005)
note that Eqs. 10 to 12 give the limit values for a thermalization efficiency
of 100% – that is, when none of the starburst-injected energy is radiated
away. Since typical starburst thermalization efficiencies are of the order of
$\sim$10% (see Veilleux et al. 2005, and references therein), the actual
energy output from SNe is expected to be (at most) in line with the values
listed in Table 2. This fact, in combination with an IR emission powered by
the AGN (Imanishi & Saito 2014), leads us to conclude that the wind in I20210S
is likely AGN-driven, although a non-negligible contribution from star
formation cannot be ruled out. Establishing the main driving mechanism of the
I20210S outflow is even further challenged by the uncertainty in its electron
density, which biases the derivation of the physical properties that can be
directly compared with the expected SN energetics; future spatially resolved
observations of I20210S will therefore also be of paramount importance in
precisely assessing the nature of its ionized wind.

Figure 9: Schematic representation of the emitting components observed in the
spectrum of I20210S: the high-velocity outflow (green shaded area) – assumed
biconical for simplicity, since only one slit position is available and the
PSF size prevents us from resolving the morphology of such a small structure –
and the South Nebula (magenta solid ellipse), plotted on top of the PS1 grizy
image of the galaxy. The slit position and orientation (white dot-dashed
lines) are reported; in the upper left corner, the diameter of
$0^{\prime\prime}.85$ of the DOLoRes PSF (white dashed circle) is also
indicated.

Broadened emission lines in ULIRGs hosting Seyfert nuclei are a common
feature. Rodríguez Zaurín et al. (2013) reported that up to 94% of nearby
objects of this kind ($z<0.175$) show strongly blueshifted ($\Delta v>150$ km
s-1) [O III] broad emission components (${\rm FWHM}>500$ km s-1) that are
emitted by near-nuclear ($R_{\rm out}\lesssim 3.5$ kpc) warm ionized outflows.
At the same time, while they are fully detectable in optical and UV spectra,
such outflows are usually not capable of injecting enough power into the
surrounding environment of AGN to effectively affect the host-galaxy ISM and
star formation. In the face of a required $\dot{E}_{\rm kin}$ of the order of
0.5% to 5% of the total AGN radiant energy (Di Matteo et al. 2005; Hopkins &
Elvis 2010), Fiore et al. (2017) showed, in fact, that the majority of near-
nuclear warm outflows clusters around $\dot{E}_{\rm kin}\sim 0.001L_{\rm bol}$
(see their Figure 1). The I20210S outflow appears to be consistent with this
scenario, given its $\dot{E}_{\rm kin}/L_{\rm bol}$ ratio of $\sim$0.002.
However, its power can still be sufficient to locally affect the star
formation rate in some host-galaxy regions, as demonstrated by the anti-
correlation found between the distribution of star-forming clouds and wind
zones in AGN hosts over resolved spatial regions that are $\sim$3$\div$7-kpc
wide (Cano-Díaz et al. 2012; Carniani et al. 2015; Cresci et al. 2015). Given
its proximity, brightness, and spatial structure, I20210 therefore stands as
an extremely peculiar laboratory in which the impact and interplay of ongoing
galaxy merging on both star formation and AGN activity could be investigated
in great detail.

##  6 Physical properties of the South Nebula

In this section, we briefly discuss the properties of the South Nebula. As
described in Sect. 4.2, this region exhibits interesting intermediate
ionization properties between AGN-powered (FWHM $\sim 700$ km s-1, velocity
blueshift of $\sim$500 km s-1) and star-forming gas clouds. We present the
schematic structure of I20210S in Fig. 9, overlapping the position and
extension – within the spectrograph slit – of both the outflow and the South
Nebula to the PS1 image of the galaxy. From this picture, it is evident that
the outflow propagating southwards extends outside enough of the innermost
nuclear region to touch the inner regions of the disk structure, potentially
interacting with the I20210S baryonic reservoir. Furthermore, the South Nebula
is located on both the extensions of the outflow and the I20210S Western
spiral arm, which makes it additionally interesting.

According to Heckman et al. (2000), a galaxy with $L_{\rm IR}\sim 3\times
10^{45}$ erg s-1 as I20210S (Piconcelli et al. 2010) has an average rotational
velocity $\langle v_{\rm rot}\rangle$ of $\sim$250 km s-1; at a distance of
$\sim$2.2 kpc from the central engine, this translates to an escape velocity
$v_{\rm esc}\sim 550$ km s-1 when assuming a galactic radius of $\sim$6.5 kpc
(i.e., the distance of the South Nebula). For comparison, the wind has a
maximum velocity that is about four times higher (see Table 2), and the South
Nebula itself exhibits $v_{\rm max}=1100\pm 430$ km s-1 (about twice as high).
Therefore, this implies that the nebula is being ejected outside the host
galaxy by the ionized outflow, which also triggers possible star formation
activity via quasar feedback as suggested by the placement of the South Nebula
line ratios on the AGN-star formation boundary (see Sect. 4.3).

It is also interesting to consider its physical properties, as done in Sect. 5
for the main outflow. To this end, we assume $T_{e}\sim 10^{4}$ K, which
according to Eq. 5 translates to $\langle n_{e}\rangle\sim 100$ cm-3 for the
South Nebula [S II] ratio $I_{6717}/I_{6731}=1.36\pm 0.10$ (see Tab 1). This,
in turn, yields $M_{\rm out}\sim 3\times 10^{4}$ M⊙, given
$L_{\rm[OIII]}=(9.05\pm 0.55)\times 10^{39}$ erg s-1 from the [O III] flux
listed in Table 1, and finally $\dot{M}=M_{\rm out}v_{\rm max}/R_{\rm out}\sim
6\times 10^{-3}$ M⊙ yr-1 (Bischetti et al. 2017) for $R_{\rm out}=6.52\pm
0.43$ kpc (see Fig. 2). We report all of these quantities in Table 2 along
with the corresponding $t_{\rm dyn}$, $\dot{E}_{\rm kin}$ and $\dot{P}_{\rm
out}$, to allow for a direct comparison with the values that hold for the
outflow.

##  7 I20210 SMBH mass estimates

In order to evaluate the SMBH mass $M_{\rm BH}$ in both objects, we used the
approach detailed in BM19 for Type II AGN, in which obscuration prevents us
from detecting the broad components of permitted emission lines. In this case,
the following single-epoch relation linking $M_{\rm BH}$ to a BLR virial shape
factor $\varepsilon$ that summarizes the uncertainities on the real BLR
geometry, the monochromatic AGN luminosity $\lambda L_{\lambda}(5100\mbox{
\AA})$ at 5100 Å and the broad H$\alpha$ FWHM gives:

$\log{\left(\frac{M_{\rm BH}}{{\rm
M}_{\odot}}\right)}=\log{\varepsilon}+6.90+0.54\cdot\log{\left[\frac{\lambda
L_{\lambda}(5100\mbox{ }\AA)}{10^{44}\mbox{ }{\rm erg}\mbox{ }{\rm
s}^{-1}}\right]}+2.06\cdot\log{\left[\frac{{\rm FWHM}^{\rm(BLR)}_{{\rm
H}\alpha}}{10^{3}\mbox{ }{\rm km}\mbox{ }{\rm s}^{-1}}\right]}$

(14)

The validity of this relation holds as long FWHM${}^{\rm(BLR)}_{{\rm
H}\alpha}$ and the [O III]/H$\beta$ ratio are measured for AGN-dominated
systems and are therefore related by the following logarithmic linear
relation:

$\log{\left(\frac{{\rm[O\mbox{ {\scriptsize III}}]}}{{\rm
H}\beta}\right)}=(0.58\pm 0.07)\cdot\log{\left[\frac{{\rm
FWHM}^{\rm(BLR)}_{{\rm H}\alpha}}{{\rm km}\mbox{ }{\rm
s}^{-1}}\right]}-(1.38\pm 0.38)$

(15)

According to Figs. 3 and 4 of BM19, this happens for AGN-dominated systems
with [O III]/H$\beta$ $\gtrsim 0.55,$ where the line intensities are not
contaminated by star formation in the host galaxy. Since, based on Table 1, we
have $\log{(\mbox{{[O\,{\scriptsize III}]}}/\mbox{{H$\beta$}})}\sim 0.7$ for
I20210N and $\sim$0.8 for I20210S, respectively, we can apply Eqs. 14 and 15
to both I20210 members. We do not quote any errors for the following
estimations of physical quantities involved in the determination of $M_{\rm
BH}$, since the measurement method is indirect and is therefore subject to
uncertainties of at least $\sim$0.5 dex (see BM19 and references therein).

Quantity | I20210N | I20210S | Units
---|---|---|---
$L^{\rm(NLR)}_{{\rm H}\beta}$ | $(1.622\pm 0.075)\times 10^{40}$ | $(1.040\pm 0.072)\times 10^{42}$ | erg s-1
$L_{\rm bol}$ | $5.2\times 10^{43}$ | $3.6\times 10^{45}$ | erg s-1
$\lambda L_{\lambda}(5100\mbox{ }\AA)$ | $7.0\times 10^{42}$ | $2.7\times 10^{44}$ | erg s-1
${\rm FWHM}^{\rm(BLR)}_{{\rm H}\alpha}$ | 3650 | 5700 | km s-1
$M_{\rm BH}$ | $2.9\times 10^{7}$ | $5.2\times 10^{8}$ | M⊙
$\lambda_{\rm Edd}$ | 0.01 | 0.05 | —
$\sigma_{v}^{*}$ | $390\pm 50$ | — | km s-1
$M_{*}$ |  $\lesssim$1.5$\times 10^{12}$ | — | M⊙

Table 3: Physical parameters of the AGN hosted in the I20210 members.
Quantities derived from the application of proportionality relations (Eq. 14
to Eq. 18) are reported without errors due to the uncertainties of at least
$\sim$0.5 dex affecting them (see BM19 and references therein).

For I20210N, we derive $\lambda L_{\lambda}(5100\mbox{ \AA})$ using its
absorption-corrected hard X-ray luminosity $L_{2-10\mbox{ }{\rm
keV}}=4.7\times 10^{42}$ erg s-1 (Piconcelli et al. 2010) via Eq. 5 from
Maiolino et al. (2007):

$\log{L_{2-10\mbox{ }{\rm keV}}}=0.721\cdot\log{\left[\lambda
L_{\lambda}(5100\mbox{ }\AA)\right]}+11.78.$ (16)

This yields $\lambda L_{\lambda}(5100\mbox{ }\AA)\simeq 7.0\times 10^{42}$ erg
s-1. The FWHM of the invisible H$\alpha$ broad component is estimated from Eq.
15; in this way, we find ${\rm FWHM}^{\rm(BLR)}_{{\rm H}\alpha}\simeq 3650$ km
s-1, corresponding to $M_{\rm BH}\simeq 2.9\times 10^{7}$ M⊙ if the value of
$\varepsilon=1.075$ by Reines & Volonteri (2015), which is representative of
assuming the mean virial factor $\langle f\rangle=4\varepsilon=4.3,$ derived
by Grier et al. (2013) by measuring the stellar velocity dispersion in the
host galaxies of powerful nearby quasars.

For I20210S, we decided to avoid directly obtaining $\lambda
L_{\lambda}(5100\mbox{ \AA})$ from the observed spectrum due to the
uncertainty on its intrinsic reddening; similarly, we did not apply Eq. 16 to
indirectly compute it, since only a lower limit to $L_{2-10\mbox{ }{\rm
keV}}\gtrsim 5\times 10^{43}$ erg s-1 is reported in Piconcelli et al. (2010).
Instead, we relied on Eq. 6 from BM19:

$\log{L_{\rm bol}}=\log{L^{\rm(NLR)}_{{\rm
H}\beta}}+3.48+\max{\left\\{0,\mbox{
}0.31\cdot\left[\log{\left(\frac{{\rm[O\mbox{ {\scriptsize III}}]}}{{\rm
H}\beta}\right)}-0.6\right]\right\\}}$

(17)

and Eq. 6 by Netzer (2009):

$\log{\left[\lambda L_{\lambda}(5100\mbox{ }\AA)\right]}=1.09\cdot\log{L_{\rm
bol}}-5.23.$ (18)

From the value for I20210S of $L^{\rm(NLR)}_{{\rm H}\beta}=(1.040\pm
0.072)\times 10^{42}$ erg s-1 computed from the H$\beta$ flux listed in Table
1, we were thus able to derive $L_{\rm bol}\simeq 3.6\times 10^{45}$ erg s-1
and $\lambda L_{\lambda}(5100\mbox{ }\AA)\simeq 2.7\times 10^{44}$ erg s-1.
The value for $L_{\rm bol}$ estimated in this way is fully compatible with the
value of $\sim$3$\times 10^{45}$ erg s-1 assumed by Piconcelli et al. (2010)
on the basis of the I20210S infrared luminosity in the range $10\div 100$
$\mu$m (Sargsyan et al. 2011). Finally, Eq. 15 yields ${\rm
FWHM}^{\rm(BLR)}_{{\rm H}\alpha}\simeq 5700$ km s-1, which translates into
$M_{\rm BH}\simeq 5.2\times 10^{8}$ M⊙.

These SMBH masses imply an Eddington luminosity $L_{\rm Edd}\sim 3.7\times
10^{45}$ erg s-1 for I20210N and $\sim$6.6$\times 10^{46}$ erg s-1 for
I20210S, respectively. Combining them with the AGN bolometric luminosities of
$\sim$5.2$\times 10^{43}$ erg s-1 for I20210N and $\sim$3.6$\times 10^{45}$
for I20210S, we obtain Eddington ratios $\lambda_{\rm Edd}=L_{\rm bol}/L_{\rm
Edd}\sim 0.01$ for I20210N and $\sim$0.05 for I20210S. Although these values
are affected by large uncertainties, such ratios are consistent with the
galaxy classification from the BPT diagrams, with I20210S clearly falling in
the Seyfert region and with I20210N exhibiting more mixed properties. In
addition, we combine the system of equations in Eq. 9 from BM19 to infer the
stellar mass $M_{*}$ of the I20210N host galaxy from its $\sigma_{v}^{*}$ of
390 km s-1 (derived in Sect. 3.1), finding $M_{*}\sim 1.5\times 10^{12}$ M⊙.
We summarize all these parameters in Table 3.

The value of $M_{*}$ derived for I20210N is $\sim$50 times larger than that
typically expected from the $M_{\rm bulge}$-to-$M_{\rm BH}$ relation ($M_{\rm
bulge}\simeq 10^{3}M_{\rm BH}$; e.g., Magorrian et al. 1998; Häring & Rix
2004; Gültekin et al. 2009); however, we highlight that the I20210N kinematics
is likely altered by its gravitational interaction with I20210S, thus making
its measured $\sigma_{v}^{*}$ unreliable for the purposes of estimating the
stellar mass. Therefore, the value of $M_{*}$ derived in this way should only
be treated as an (overestimated) upper limit to the I20210N baryonic content.

##  8 Summary and conclusions

In this article, we present an optical spectroscopic analysis of the AGN pair
hosted in the interacting system IRAS 20210+1121 (I20210; P90; Heisler & Vader
1995; Burston et al. 2001; Davies et al. 2002; Arribas et al. 2004; Piconcelli
et al. 2010) at $z=0.056$. This study is based on spectroscopy taken through a
slit aligned along the nuclei of the two interacting galaxies. The high-
quality data taken at the Telescopio Nazionale Galileo allowed us to perform a
detailed study of the light emission from both components and from their
surrounding environment in the rest-frame wavelength range of 3500 – 7300 Å,
with the possibility of a comprehensive characterization of this interacting
galaxy pair. Here we summarize our main findings:

  * •

I20210N, the northern member of the I20210 system, can be definitively
classified as a Seyfert 2 galaxy with an exceptional stellar velocity
dispersion of $\sigma_{v}^{*}\sim 400$ km s-1, hosting an AGN powered by a
black hole with $M_{\rm BH}\sim 3\times 10^{7}$ M⊙ that radiates at 1% of its
Eddington limit.

  * •

The environment around I20210S, the southern component is a powerful Type II
quasar with $M_{\rm BH}\sim 5\times 10^{8}$ M⊙ radiating at 5% of its
Eddington limit and revealed to be highly structured, with an ionized outflow
and a detached gaseous nebula (the South Nebula) alongside the nuclear
emission.

  * •

The physical properties of the ionized outflow derived from the analysis of
the broad emission-line components ($T_{e}\sim 10^{4}$ K, $\langle
n_{e}\rangle\gtrsim 5000$ cm-3, $v_{\rm max}\sim 2000$ km s-1, $R_{\rm
out}\sim 2$ kpc, $M_{\rm out}\sim 2\times 10^{5}$ M⊙, $\dot{M}\sim 0.6$ M⊙
yr-1) are in line with those found in other powerful AGN hosted in ULIRGs
(Rodríguez Zaurín et al. 2013; Rupke & Veilleux 2013; Kakkad et al. 2018;
Spence et al. 2018). This suggests that the I20210S AGN activity has
potentially a direct impact on the host-galaxy environment through quasar
feedback; however, these results need to be further investigated with higher
resolution spectral observations in order to constrain the value of the wind
electron density and thus allow for a better characterization of the feedback
mechanism to the star formation activity in I20210S (e.g., Carniani et al.
2015; Fiore et al. 2017);

  * •

The South Nebula exhibits dynamical properties consistent with those of highly
disrupted gas stripped out of the I20210S nucleus (velocity blueshift of
$\sim$500 km s-1, FWHM of $\sim$700 km s-1 that is similar to the case of the
Teacup Galaxy (Ramos Almeida et al. 2017) and coupled to intermediate
ionization properties between AGN-powered and star-forming gas. Such features
qualify this region as a very interesting target for a deeper investigation of
the potential feedback processes – either triggered by AGN activity or by the
galaxy merger – at work in I20210S.

Thanks to the above properties, the I20210 system can be characterized as a
very interesting target in the local Universe that ought to be investigated
with dedicated multi-wavelength follow-ups aimed at a detailed study of the
effects of AGN feedback coupled to host-galaxy interaction on the AGN
surrounding environment. In particular, obtaining higher resolution spectra
($\lambda/\Delta\lambda\gtrsim 1500$) is crucial to improving the emission-
line diagnostics of the I20210S components (nucleus, outflow, South Nebula)
and to allow for a precise evaluation of the I20210S outflow physical
conditions. Furthermore, integral-field spectroscopic observations are
required to accurately constrain both the morphology and interplay of outflows
and any off-nuclear emitting region in I20210S.

###### Acknowledgements.

We thank our anonymous referee for their helpful comments. GV, EP, CV, MB, CF
and FF acknowledge support from PRIN MIUR project “Black Hole winds and the
Baryon Life Cycle of Galaxies: the stone-guest at the galaxy evolution
supper”, contract #2017PH3WAT. GV also acknowledges financial support from
Premiale 2015 MITic (PI: B. Garilli). CRA acknowledges financial support from
the Spanish Ministry of Science, Innovation and Universities (MCIU) under
grant with reference RYC-2014-15779, from the European Union’s Horizon 2020
research and innovation programme under Marie Skłodowska-Curie grant agreement
No 860744 (BiD4BESt), from the State Research Agency (AEI-MCINN) of the
Spanish MCIU under grants ”Feeding and feedback in active galaxies” with
reference PID2019-106027GB-C42, ”Feeding, feedback and obscuration in active
galaxies” with reference AYA2016-76682-C3-2-P, and ”Quantifying the impact of
quasar feedback on galaxy evolution (QSOFEED)” with reference EUR2020-112266.
CRA also acknowledges support from the Consejería de Economía, Conocimiento y
Empleo del Gobierno de Canarias and the European Regional Development Fund
(ERDF) under grant with reference ProID2020010105 and from IAC project
P/301404, financed by the Ministry of Science and Innovation, through the
State Budget and by the Canary Islands Department of Economy, Knowledge and
Employment, through the Regional Budget of the Autonomous Community. Based on
observations made with the Italian Telescopio Nazionale Galileo (TNG) operated
on the island of La Palma by the Fundación Galileo Galilei of the INAF
(Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de
los Muchachos of the Instituto de Astrofísica de Canarias. IRAF is distributed
by the National Optical Astronomy Observatories, which is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA) under
cooperative agreement with the National Science Foundation. Reproduced with
permission from Astronomy & Astrophysics, © ESO.

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