# Directly tracing cool filamentary accretion over $>100$ kpc into the
interstellar medium of a quasar host at $z\approx 1$

Sean D. Johnson Department of Astronomy, University of Michigan, 1085 S.
University, Ann Arbor, MI 48109, USA Sean D. Johnson<EMAIL_ADDRESS>Joop
Schaye Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden,
the Netherlands  Gregory L. Walth IPAC, California Institute of Technology,
Mail Code 314-6, 1200 E. California Blvd., Pasadena, CA 91125  Jennifer I-Hsiu
Li Department of Astronomy, University of Michigan, 1085 S. University, Ann
Arbor, MI 48109, USA  Gwen C. Rudie The Observatories of the Carnegie
Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA
Hsiao-Wen Chen Department of Astronomy & Astrophysics, The University of
Chicago, Chicago, IL 60637, USA  Mandy C. Chen Department of Astronomy &
Astrophysics, The University of Chicago, Chicago, IL 60637, USA  Benoît Epinat
Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France  Canada-France-Hawaii
Telescope, CNRS, 96743 Kamuela, Hawaii, USA  Massimo Gaspari Department of
Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ
08544-1001, USA  Sebastiano Cantalupo Department of Physics, University of
Milan Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy  Wolfram
Kollatschny Institut für Astrophysik, Universität Göttingen, Friedrich-Hund
Platz 1, 37077 Göttingen, Germany  Zhuoqi (Will) Liu Department of Astronomy,
University of Michigan, 1085 S. University, Ann Arbor, MI 48109, USA  Sowgat
Muzahid Inter-University Centre for Astronomy and Astrophysics (IUCAA), Post
Bag 4, Ganeshkhind, Pune 411 007, India

(Received ?; Revised ?; Accepted ?)

###### Abstract

We report the discovery of giant (50$-$100 kpc) [O II] emitting nebulae with
the Multi-Unit Spectroscopic Explorer (MUSE) in the field of TXS 0206$-$048, a
luminous quasar at $z=1.13$. Down-the-barrel UV spectra of the quasar show
absorption at velocities coincident with those of the extended nebulae,
enabling new insights into inflows and outflows around the quasar host. One
nebula exhibits a filamentary morphology extending over 120 kpc from the halo
toward the quasar and intersecting with another nebula surrounding the quasar
host with a radius of 50 kpc. This is the longest cool filament observed to-
date and arises at higher redshift and in a less massive system than those in
cool-core clusters. The filamentary nebula has line-of-sight velocities $>300$
${\rm km\,s}^{-1}$ from nearby galaxies but matches that of the nebula
surrounding the quasar host where they intersect, consistent with accretion of
cool inter- or circum-galactic medium or cooling hot halo gas. The kinematics
of the nebulae surrounding the quasar host are unusual and complex, with
redshifted and blueshifted spiral-like structures. The emission velocities at
5$-$10 kpc from the quasar match those of inflowing absorbing gas observed in
UV spectra of the quasar. Together, the extended nebulae and associated
redshifted absorption represent a compelling case of cool, filamentary gas
accretion from halo scales into the extended interstellar medium and toward
the nucleus of a massive quasar host. The inflow rate implied by the combined
emission and absorption constraints is well below levels required to sustain
the quasar’s radiative luminosity, suggesting anisotropic or variable
accretion.

††journal: ApJL

##  1 Introduction

Observed scaling relations between the interstellar medium (ISM) and star
formation in massive star-forming galaxies imply ISM depletion times of a few
Gyr at low-$z$ (for a review, see Kennicutt & Evans 2012; cf. Leitner &
Kravtsov 2011) and under 1 Gyr at $z>1$ (e.g. Tacconi et al., 2013). These ISM
depletion timescales are smaller than the age of the Universe, indicating that
galaxies must accrete gas from external reservoirs to enable future star
formation and black hole growth. This fresh material can be supplied via
mergers (e.g. Moreno et al., 2021), accretion of cool ($T\lesssim 10^{5}$ K)
inter-/circum-galactic medium (IGM/CGM; e.g., Dekel et al., 2009), or cooling
of hot ($T\gtrsim 10^{6}$ K) gaseous halos (e.g., Correa et al., 2018).
Despite the importance of gas accretion in galaxy evolution, direct and
unambiguous observations of accretion are rare (for a review, see Putman,
2017).

The Milky Way represents a unique case where we can study gas accretion onto a
galaxy over many phases and angles. The kinematics of the Milky Way’s
extraplanar gas exhibit clear signs of accretion in both neutral and ionized
phases (for a review, see Putman et al., 2012), with total inferred mass
inflow rates consistent with the Galaxy’s star formation rate (e.g. Lehner &
Howk, 2011).

Beyond the Milky Way, most studies of inflows rely on absorption features in
galaxy spectra that are redshifted relative to the galaxy systemic velocity
(for a review, see Rubin, 2017). Despite the difficulty in detecting
redshifted gas in “down-the-barrel” spectra, surveys at $z\lesssim 1.5$ have
revealed likely inflows for ${\approx}80$ galaxies (e.g. Sato et al., 2009;
Krug et al., 2010; Coil et al., 2011; Rubin et al., 2012; Martin et al.,
2012). However, the locations of the inflowing gas relative to the galaxy are
not directly constrained, leaving their origins and fate unknown.

In contrast, the background absorption spectroscopy often used to characterize
the CGM (for reviews, see Chen et al., 2017; Tumlinson et al., 2017) informs
the spatial distribution of the CGM and IGM around galaxies but carries little
direct information on the radial direction of gas flows for individual
systems. In rare cases, absorption spectroscopy can help differentiate inflows
from outflows through modeling of velocity shear in multi-sightline data
(e.g., Chen et al., 2014; Lopez et al., 2018), detailed metallicity gradients
(e.g. Fu et al., 2021), or correspondence with galactic rotation (e.g. Ho et
al., 2017; Zabl et al., 2019), but all of these cases require model
assumptions. Observations that carry direct information on both the morphology
and radial direction of gas flows in the same systems have the potential to
significantly improve our understanding of accretion onto galaxies.

Wide-field integral field spectrographs (IFS) such as MUSE (Bacon et al.,
2010) and KCWI (Martin et al., 2010) enable unprecedented morphological and
kinematic maps of giant IGM/CGM nebulae through H I Ly$\alpha$ emission at
$z\gtrsim 2$ (e.g. Cantalupo et al., 2014; Borisova et al., 2016; Cai et al.,
2019; O’Sullivan et al., 2020; Chen et al., 2021; Fossati et al., 2021) and
rest-frame NUV$-$optical emission features at lower redshift (Epinat et al.,
2018; Johnson et al., 2018; Boselli et al., 2019; Rupke et al., 2019; Chen et
al., 2019; Burchett et al., 2021; Helton et al., 2021; Zabl et al., 2021;
Leclercq et al., 2022). At $z\lesssim 1$, IFS provide highly complete galaxy
redshift surveys and access to non-resonant emission lines to directly trace
ionized gas morphology and kinematics. To date, IFS data enabled the discovery
of halo-scale nebulae arising from large-scale outflows, cool intragroup
medium, and stripping of ISM during galaxy interactions including two nebulae
with additional insights from nearby intervening absorption spectroscopy (Chen
et al., 2019; Zabl et al., 2021). Moreover, wide-field IFS enable measurements
of the velocity structure function which provide unique insights into
turbulence in diffuse gas (e.g., Li et al., 2020; Chen et al., 2022). However,
observations of gas accretion with wide-field IFS remain ambiguous with the
exception of a giant Ly$\alpha$ nebulae at $z=3.3$ with evidence of infall
coming from morphology (Rauch et al., 2011, 2016) and the Ly$\alpha$ emission
profile (Vanzella et al., 2017).

Here, we report the emission detection of an accreting gas filament extending
over $>120$ proper kpc (pkpc) from the halo into the ISM around TXS
0206$-$048, a luminous quasar at $z\approx 1$. An archival down-the-barrel UV
spectrum of the quasar breaks the inflow/outflow degeneracy that limits
intervening absorption studies and reveals inflowing absorption at velocities
similar to the nearby nebular emission. The Letter proceeds as follows. In
Section 2, we describe the MUSE, Hubble Space Telescope  (HST), and Magellan
observations and data reduction. In Section 3, we characterize the properties
of TXS 0206$-$048 and its host group. In Section 4, we describe the giant
nebulae in the quasar environment and the coincidence with inflowing gas seen
in down-the-barrel absorption. In Section 5, we summarize our findings and
discuss their implications. All magnitudes are given in the AB system.
Throughout, we adopt a $\Lambda$ cosmology with $\Omega_{\rm m}\,{=}\,0.3$,
$\Omega_{\Lambda}\,{=}\,0.7$, and $H_{0}\,{=}\,70\,{\rm
km\,s^{-1}\,Mpc^{-1}}$.

##  2 Observations, data reduction, and galaxy measurements

Investigators studying the IGM and CGM (e.g., Tejos et al., 2014; Finn et al.,
2016) obtained high quality UV absorption spectra of TXS 0206$-$048 with the
Cosmic Origins Spectrograph (COS; Green et al., 2012) due to its UV
brightness, long redshift pathlength, and availability of archival data from
the Gemini Deep Deep Survey (GDDS; Abraham et al., 2004). We obtained the COS
G130M and G160M spectra of TXS 0206$-$048 (PI: Morris; PID: 12264) from MAST
and reduced them following procedures described in Johnson et al. (2013) and
Chen et al. (2018) to improve the wavelength calibration.

The field near TXS 0206$-$048 was observed with the Advanced Camera for
Surveys (ACS) aboard HST with the F814W filter by the GDDS survey for a total
of 32 ksec (PI: Abraham; PID: 9760) and for 7.2 ksec by a Sagittarius Stream
program (PI: van der Marel; PID 12564), but the quasar falls just $4$ arcsec
from the edge of the field-of-view (FoV). To identify and measure the
morphologies of faint galaxies near the quasar sightline, we obtained an
additional 2.1 ksec of exposure with ACS$+$F814W (PI: L. Straka; PID: 14660).
We reduced and coadded the ACS imaging for the field using DrizzlePac
(Hoffmann et al., 2021) including Tweakreg for alignment and Astrodrizzle to
combine them. The effective wavelength of ACS$+$F814W corresponds to a rest-
frame wavelength $\approx 3700$ Å at $z=1.13$.

To provide deep galaxy redshift survey data, we acquired a total of 8 hours of
exposure under $0.7$ arcsec seeing conditions on the field of TXS 0206$-$048
with MUSE as part of the MUSE-QuBES survey (PI: J. Schaye, PID: 094.A-0131,
094.A-0131). We reduced the data using the GTO pipeline (Weilbacher et al.,
2014) and sky subtraction tools (Soto et al., 2016) as described in Johnson et
al. (2018). To ensure robustness, we also reduced the MUSE data using CubEx
(Borisova et al., 2016; Cantalupo et al., 2019) and the ESO pipeline
(Weilbacher et al., 2020) and found consistent conclusions. We scaled the
variance array reported by the GTO pipeline by a factor of $1.4$ to better
reflect empirical variance (see Herenz et al., 2017).

To detect faint emission near the quasar, we performed quasar light
subtraction as described in Helton et al. (2021). We then identified galaxies
in the MUSE FoV by running source extractor (Bertin & Arnouts, 1996) on both
the ACS image of the field and a median image formed from the MUSE datacube.
Finally, we extracted spectra using MPDAF (Piqueras et al., 2017).

While the MUSE FoV is wide for an IFS, its FoV radius corresponds to a
projected distance of $d=250-350$ pkpc from the quasar at $z=1.13$. We
supplemented the MUSE data with a wider-field galaxy redshift survey using the
LDSS3 spectrograph on Magellan following procedures outlined in Johnson et al.
(2019). The LDSS3 FoV extends to $d\approx 1.5$ proper Mpc at the $z=1.13$.

For both the MUSE and LDSS3 spectra, we measured redshifts by fitting the
observed spectra with linear combinations of the first four galaxy
eigenspectra from Bolton et al. (2012) as described in Johnson et al. (2018)
and Helton et al. (2021). To prevent spatially coincident extended nebular
emission from biasing galaxy redshifts, we measured galaxy redshifts based
purely on stellar absorption when possible by masking strong emission lines.
For galaxies without sufficient continuum signal-to-noise, we measured
redshifts with the [O II] doublet. Finally, for galaxies with secure
spectroscopic redshifts, we measured absolute magnitudes in the rest-frame
$B$-band and 4000 Å break strength ($D_{n}(4000)$; Balogh et al., 1999).

##  3 Quasar Properties & environment

TXS 0206$-$048 is a luminous, core-dominated radio-loud quasar (Becker et al.,
2001) at a redshift of $z=1.1317\pm 0.0002$ based on a measurement of the [O
II] emission-line centroid in its MUSE spectrum and adopting the rest-frame
effective [O II] doublet centroid and systemic uncertainty from Hewett & Wild
(2010). To estimate the quasar’s luminosity and black hole mass, we fit the
MUSE quasar spectrum near the Mg II emission line with a power-law continuum,
Fe II template, and 3 Gaussian emission-line components using PyQSOFit (Guo et
al., 2019). The measured monochromatic continuum luminosity at rest-frame 3000
Å implies a bolometric luminosity of $\log L_{\rm bol}/{\rm
erg\,s^{-1}}\approx 47.2$ using bolometric corrections from Richards et al.
(2006). The Mg II line width and luminosity result in an inferred black hole
mass of $\log M_{\rm BH}/M_{\odot}\approx 9.6$ using the single epoch virial
theorem estimator from Shen et al. (2011).

Table 1: Summary of galaxies in the field of TXS 0206$-$048 at $z\,{\approx}\,z_{\rm QSO}$. ID | R.A. | Decl. | $m_{\rm F814W}$ | redshift | redshift | $\Delta\theta$ | $d$ | $\Delta v$ | $D_{n}(4000)$ | $M_{B}$ |  $\log L_{B}/L_{*}$aaWe adopt an absolute magnitude of $M_{B}=-21.5$ for an $L_{*}$ galaxy based on the luminosity function measurement from Faber et al. (2007). This corresponds to a luminosity of $\approx 5\times 10^{10}\ L_{\rm\odot}$.
---|---|---|---|---|---|---|---|---|---|---|---
| (J2000) | (J2000) | (AB) |  | type | (′′) | (pkpc) | ($\rm km\,s^{-1}$) |  | (AB) |
quasar | 02:09:30.77 |  $-$04:38:26.1 | $-$ | $1.1317\pm 0.0002$ | [O II] | $0.0$ | $0$ | $0$ | – | – | –
G1 | 02:09:30.42 |  $-$04:38:29.8 | $23.82\pm 0.02$ | $1.1275\pm 0.0002$ | stellar | $6.3$ | $51$ | $-590$ | $1.31\pm 0.05$ | $-20.4$ | $-0.5$
G2 | 02:09:30.92 |  $-$04:38:18.6 | $25.26\pm 0.08$ | $1.1313\pm 0.0004$ | stellar | $7.9$ | $65$ | $-60$ | $1.13\pm 0.08$ | $-18.9$ | $-1.0$
G3 | 02:09:30.59 |  $-$04:38:18.4 | $23.34\pm 0.03$ | $1.1296\pm 0.0002$ | stellar | $8.2$ | $68$ | $-300$ | $1.06\pm 0.03$ | $-20.6$ | $-0.4$
G4 | 02:09:31.35 |  $-$04:38:27.7 | $25.77\pm 0.13$ | $1.1397\pm 0.0002$ | [O II] | $8.8$ | $72$ | $+1130$ | $1.17\pm 0.25$ | $-18.4$ | $-1.2$
G5 | 02:09:30.14 |  $-$04:38:27.1 | $25.43\pm 0.09$ | $1.1365\pm 0.0002$ | [O II] | $9.3$ | $77$ | $+680$ | $0.76\pm 0.21$ | $-18.5$ | $-1.2$
G6 | 02:09:30.90 |  $-$04:38:15.5 | $24.26\pm 0.10$ | $1.1295\pm 0.0002$ | stellar | $10.8$ | $89$ | $-310$ | $1.21\pm 0.07$ | $-19.8$ | $-0.7$
G7 | 02:09:31.49 |  $-$04:38:21.1 | $23.68\pm 0.04$ | $1.1287\pm 0.0002$ | stellar | $11.9$ | $98$ | $-420$ | $1.38\pm 0.07$ | $-20.5$ | $-0.4$
G8 | 02:09:30.27 |  $-$04:38:16.6 | $24.93\pm 0.06$ | $1.1300\pm 0.0002$ | [O II] | $12.0$ | $99$ | $-240$ | $1.02\pm 0.08$ | $-19.0$ | $-1.0$
G9 | 02:09:30.95 |  $-$04:38:14.4 | $23.97\pm 0.05$ | $1.1272\pm 0.0002$ | stellar | $12.1$ | $99$ | $-630$ | $1.32\pm 0.06$ | $-20.2$ | $-0.5$
G10 | 02:09:31.01 |  $-$04:38:13.1 | $24.09\pm 0.06$ | $1.1260\pm 0.0002$ | stellar | $13.5$ | $111$ | $-800$ | $1.23\pm 0.07$ | $-20.1$ | $-0.6$
G11 | 02:09:31.48 |  $-$04:38:15.8 | $25.20\pm 0.09$ | $1.1299\pm 0.0002$ | [O II] | $14.9$ | $122$ | $-250$ | $1.04\pm 0.10$ | $-18.7$ | $-1.1$
G12 | 02:09:29.99 |  $-$04:38:15.7 | $25.53\pm 0.11$ | $1.1305\pm 0.0002$ | [O II] | $15.7$ | $129$ | $-170$ | $1.03\pm 0.20$ | $-18.4$ | $-1.2$
G13 | 02:09:31.80 |  $-$04:38:21.0 | $24.56\pm 0.08$ | $1.1351\pm 0.0002$ | [O II] | $16.3$ | $134$ | $+510$ | $0.98\pm 0.09$ | $-19.4$ | $-0.9$
G14 | 02:09:29.66 |  $-$04:38:25.1 | $24.13\pm 0.06$ | $1.1271\pm 0.0002$ | stellar | $16.6$ | $137$ | $-650$ | $1.01\pm 0.10$ | $-19.8$ | $-0.7$
G15 | 02:09:29.66 |  $-$04:38:26.0 | $25.04\pm 0.10$ | $1.1232\pm 0.0002$ | [O II] | $16.6$ | $136$ | $-1200$ | $1.02\pm 0.16$ | $-18.9$ | $-1.0$
G16 | 02:09:31.82 |  $-$04:38:34.0 | $25.23\pm 0.09$ | $1.1264\pm 0.0002$ | [O II] | $17.6$ | $145$ | $-750$ | $0.90\pm 0.21$ | $-18.7$ | $-1.1$
G17 | 02:09:30.39 |  $-$04:38:45.0 | $25.30\pm 0.10$ | $1.1307\pm 0.0002$ | [O II] | $19.6$ | $161$ | $-140$ | $0.96\pm 0.13$ | $-18.6$ | $-1.2$
G18 | 02:09:30.71 |  $-$04:38:47.6 | $24.46\pm 0.05$ | $1.1299\pm 0.0002$ | [O II] | $21.5$ | $177$ | $-250$ | $1.02\pm 0.07$ | $-19.5$ | $-0.8$
G19 | 02:09:29.44 |  $-$04:38:41.5 | $25.61\pm 0.07$ | $1.1343\pm 0.0002$ | [O II] | $25.1$ | $206$ | $+370$ | $0.97\pm 0.28$ | $-18.3$ | $-1.3$
G20 | 02:09:31.10 |  $-$04:38:00.5 | $23.18\pm 0.01$ | $1.1368\pm 0.0002$ | stellar | $26.1$ | $215$ | $+720$ | $1.48\pm 0.04$ | $-21.4$ | $+0.0$
G21 | 02:09:29.15 |  $-$04:38:43.5 | $23.17\pm 0.02$ | $1.1259\pm 0.0002$ | stellar | $29.8$ | $244$ | $-820$ | $1.01\pm 0.03$ | $-20.8$ | $-0.3$
G22 | 02:09:31.59 |  $-$04:38:54.4 | $23.42\pm 0.02$ | $1.1331\pm 0.0002$ | stellar | $30.8$ | $253$ | $+200$ | $1.48\pm 0.05$ | $-21.1$ | $-0.1$
G23 | 02:09:28.85 |  $-$04:38:41.1 | $23.36\pm 0.05$ | $1.1259\pm 0.0002$ | stellar | $32.4$ | $266$ | $-820$ | $1.10\pm 0.04$ | $-20.7$ | $-0.3$
G24 | 02:09:28.80 |  $-$04:38:40.6 | $25.04\pm 0.06$ | $1.1345\pm 0.0002$ | [O II] | $32.8$ | $269$ | $+390$ | $0.93\pm 0.08$ | $-18.9$ | $-1.0$
G25 | 02:09:31.16 |  $-$04:39:02.8 | $23.58\pm 0.03$ | $1.1349\pm 0.0003$ | stellar | $37.1$ | $305$ | $+450$ | $1.47\pm 0.12$ | $-21.0$ | $-0.2$
G26 | 02:09:28.29 |  $-$04:38:18.9 | $22.36\pm 0.01$ | $1.1311\pm 0.0004$ | stellar | $37.8$ | $310$ | $-80$ | $1.40\pm 0.10$ | $-22.2$ | $+0.3$
G27 | 02:09:30.95 |  $-$04:37:29.7 | $23.00\pm 0.03$ | $1.1354\pm 0.0004$ | [O II] | $56.5$ | $465$ | $+520$ | $1.12\pm 0.21$ | $-21.1$ | $-0.2$

To characterize the group environment of TXS 0206$-$048, we identified 27
galaxies in our spectroscopic survey with secure redshifts and velocities
within $\pm 1500$ ${\rm km\,s}^{-1}$ of the quasar systemic redshift. Table 1
summarizes the properties of these galaxies including their right ascension
(R.A.), declination (decl.), apparent magnitude in the F814W filter ($m_{\rm
F814W}$), redshift, redshift measurement type ([O II] emission or stellar
absorption), projected angular ($\Delta\theta$) and physical distance ($d$)
from the quasar sightline, line-of-sight velocity relative to the quasar
($\Delta v$), strength of the 4000 Å break ($D_{n}(4000)$), rest-frame B-band
absolute magnitude ($M_{B}$), and $B$-band luminosity relative to $L_{*}$
based on the $z=1.1$ luminosity function from Faber et al. (2007).

To estimate the mass of the quasar host group, we measured the velocity
dispersion of the group members including the quasar. We found a mean group
velocity of $\Delta v_{\rm group}=-130$ ${\rm km\,s}^{-1}$ relative to the
quasar and a velocity dispersion of $\sigma_{\rm group}\approx 550$ ${\rm
km\,s}^{-1}$. Assuming that the group is relaxed, this line-of-sight velocity
dispersion implies a dynamical mass of $\log M_{\rm dyn}/M_{\odot}\approx
13.7$ using the cluster dispersion-to-mass relation from Munari et al. (2013).
This is consistent with halo mass expectations based on the black hole mass of
TXS 0206$-$048 and the black hole mass$-$halo mass relation inferred by
Gaspari et al. (2019). The luminosity weighted group center is $\approx 60$
pkpc West and $\approx 20$ pkpc North of the quasar assuming the quasar host
has a luminosity of $L_{B}=1-3\ L_{*}$ which is typical of luminous AGN (e.g.
Zakamska et al., 2006). The location of the luminosity weighted group center
is driven away from that of the quasar primarily by one luminous galaxy, G26,
which falls outside of the MUSE field-of-view. The MUSE galaxy redshift survey
is deeper than the LDSS3 survey, so the group center may be biased toward the
center of the MUSE field-of-view. A full image of the group is included in the
Appendix.

Figure 1: Top panels: Continuum-subtracted, narrow-band images around the [O
II] doublet with wavelength ranges chosen to highlight nebulae in the
environment of TXS 0206$-$048\. The wavelength range and corresponding
approximate [O II] line-of-sight velocity relative to the quasar systemic are
labelled in the top right corner of each panel. In each panel, the locations
of galaxies in the quasar host environment are marked with black circles and
the quasar position is marked by a star. Bottom left panel: [O II] surface
brightness contours overlayed on the HST ACS/F814W image of the field marking
surface brightness levels of $0.15,0.3,0.6,1.2,$ and $2.4\times 10^{-17}\ {\rm
erg\,s^{-1}\,cm^{-2}\,arcsec^{-2}}$ after smoothing with a
$1^{\prime\prime}\times 1^{\prime\prime}$ boxcar. The [O II] contours are
formed by summing the three images in the top panels. Bottom middle and right
panels: [O II] line-of-sight velocity and line-of-sight velocity dispersion
maps of the nebulae around TXS 0206$-$048 calculated with doublet fitting as
described in the text. The locations of group members (quasar) are marked by
black circles (black star) in the middle panel and the color interior to the
symbols shows the objects systemic line-of-sight velocity relative to the
quasar, $\Delta v$. The approximate luminosity-weighted group center is marked
with an asterisk in all panels.

##  4 Giant nebulae around TXS 0206$-$048

Table 2: Summary of properties of the nebulae around TXS 0206$-$048

name | $\Delta\theta$ | $d$ | $\Delta v$ | $\sigma$ | length-scale | area | [O II] surface brightnessaaThe total fluxes and luminosities are integrated within isophotal areas with surface brightness greater than $10^{-18}\ {\rm erg\,s^{-1}\,cm^{-2}\,{arcsec}^{-2}}$. For the Host nebula, we masked a circular region with radius $r=1^{\prime\prime}$ when measuring the total [O II] flux and luminosity to avoid large residuals from the quasar subtraction. If this region is not masked, the measured flux and luminosity increase by a factor of two. | [O II] fluxaaThe total fluxes and luminosities are integrated within isophotal areas with surface brightness greater than $10^{-18}\ {\rm erg\,s^{-1}\,cm^{-2}\,{arcsec}^{-2}}$. For the Host nebula, we masked a circular region with radius $r=1^{\prime\prime}$ when measuring the total [O II] flux and luminosity to avoid large residuals from the quasar subtraction. If this region is not masked, the measured flux and luminosity increase by a factor of two. | [O II] luminosityaaThe total fluxes and luminosities are integrated within isophotal areas with surface brightness greater than $10^{-18}\ {\rm erg\,s^{-1}\,cm^{-2}\,{arcsec}^{-2}}$. For the Host nebula, we masked a circular region with radius $r=1^{\prime\prime}$ when measuring the total [O II] flux and luminosity to avoid large residuals from the quasar subtraction. If this region is not masked, the measured flux and luminosity increase by a factor of two.
---|---|---|---|---|---|---|---|---|---
| $\rm arcsec$ | (pkpc) | (${\rm km\,s}^{-1}$) | (${\rm km\,s}^{-1}$) | (pkpc) | (pkpc2) | ($\rm erg\,s^{-1}\,cm^{-2}\,arcsec^{-2}$) | ($\rm erg\,s^{-1}\,cm^{-2}$) | $(\rm erg\,s^{-1})$
N.E. nebula | $15.0$ | $120$ | $-240$ | $180$ | $50$ | $900$ |  $0.1$ to $1.0\times 10^{-17}$ | $5.1\times 10^{-17}$ | $3.6\times 10^{41}$
S. nebula | $15.0$ | $120$ |  $-250$ to $-110$ | $170$ | $60$ | $1100$ |  $0.1$ to $1.0\times 10^{-17}$ | $5.8\times 10^{-17}$ | $4.1\times 10^{41}$
N. filament |  $6.0$ to $20.5$ |  $50$ to $170$ |  $-250$ to $+330$ |  $100$ to $230$ | $120$ | $2000$ |  $0.1$ to $0.3\times 10^{-17}$ | $5.4\times 10^{-17}$ | $3.8\times 10^{41}$
Host nebula |  $0$ to $6$ |  $0$ to $50$ |  $-300$ to $+540$ |  $100$ to $300$ | $100$ | $6800$ |  $0.1$ to $24.3\times 10^{-17}$ |  $1.2\times 10^{-15}$aaThe total fluxes and luminosities are integrated within isophotal areas with surface brightness greater than $10^{-18}\ {\rm erg\,s^{-1}\,cm^{-2}\,{arcsec}^{-2}}$. For the Host nebula, we masked a circular region with radius $r=1^{\prime\prime}$ when measuring the total [O II] flux and luminosity to avoid large residuals from the quasar subtraction. If this region is not masked, the measured flux and luminosity increase by a factor of two. | $8.5\times 10^{42}$

At the redshift of TXS 0206$-$048, the [O II] $\lambda\lambda$3727, 3729
doublet, which traces cool ($T\sim 10^{4}$ K) ionized gas, is observed at
$\approx 7940$ Å. To identify [O II] emitting nebulae around the quasar, we
performed continuum subtraction on the quasar-light subtracted MUSE datacube
by fitting low-order polynomials to each spaxel over a wavelength interval of
7800 to 8100 Å after masking the 50 Å region around [O II]. We then subtracted
the continuum fit from each spaxel to produce a emission-line datacube with a
typical 3-$\sigma$ detection limit of $10^{-18}\ {\rm
erg\,s^{-1}\,cm^{-2}\,arcsec^{-2}}$ for a line with a width $\sigma=200$ ${\rm
km\,s}^{-1}$ averaged over an $r=0.5^{\prime\prime}$ aperture.

The continuum subtracted datacube reveals the presence of four distinct,
$50-100$ pkpc scale ionized nebulae emitting in [O II] in the quasar host
group and several smaller-scale nebulae closely associated with group members.
To visualize these nebulae, the top three panels of Figure 1 display continuum
subtracted [O II] surface brightness maps integrated over three 10 Å intervals
which correspond to line-of-sight velocities of $\Delta v\approx-670$ to
$-290$, $-290$ to $90$, and $90$ to $460$ ${\rm km\,s}^{-1}$ relative to the
quasar systemic velocity. We note that the [O II] doublet separation
corresponds to $\approx 200$ ${\rm km\,s}^{-1}$ so this velocity
correspondence is approximate. Throughout the paper, we refer to these nebulae
by their morphology and location relative to the quasar as the Northeast
(N.E.) nebula, the South (S.) nebula, the North (N.) filament, and the Host
nebula (see Figure 1). In addition to these, there is a possible second
filamentary feature extending south of the Host nebula seen in Figure 1.
However it is less prominent than the N. filament and may be an extension of
the Host nebula’s arm-like feature.

To visualize the morphologies of the nebulae relative to galaxies in the
group, the bottom left panel of Figure 1 displays the HST image of the field
overlaid with [O II] surface brightness contours computed from the sum of the
three images shown in the top three panels. The nebulae are labeled by their
name and approximate length scale in the bottom-left panel. To better quantify
the kinematics of these nebulae, we also performed [O II] doublet fitting to
the datacube as described in Johnson et al. (2018) and Helton et al. (2021).
The bottom middle and bottom-right panels of Figure 1 display the line-of-
sight velocity relative to the quasar and line-of-sight velocity dispersion
(corrected for the MUSE line spread function (Bacon et al., 2017)),
respectively. In most cases, the emission is too broad or too low in S/N to
measure the [O II] 3729-to-3727 doublet ratio, which can vary between 0.35 and
1.5. Uncertainty in the doublet ratio introduces a systematic uncertainty of
$\approx 50-80$ ${\rm km\,s}^{-1}$ in the [O II] velocity centroid. Table 2
summarizes the properties of the nebulae, and we discuss each in turn in the
following.

###  4.1 Northeast nebula

The N.E. nebula is located $\Delta\theta\approx 15^{\prime\prime}$ or
$d\approx 120$ pkpc from TXS 0206$-$048, has a length scale of $\approx 50$
pkpc and [O II] surface brightness ranging from $\approx 0.1$ to $1.0\times
10^{-17}\ {\rm erg\,s^{-1}\,cm^{-2}\,arcsec^{-2}}$. The N.E. nebula exhibits a
head-tail morphology oriented approximately East-West with a surface
brightness peak that is spatially coincident with a group member galaxy, G11.
Together, the morphology and spatial coincidence with G11 suggest that the
N.E. nebula arises from ram pressure stripping of the ISM of G11 as it moves
through the hot halo of the quasar host group. Such “jelly-fish” galaxies are
often observed in galaxy clusters and groups (e.g. Fumagalli et al., 2014;
Poggianti et al., 2017; Chen et al., 2019; Boselli et al., 2019) and around
quasar hosts at $z\approx 0.5$ (e.g. Johnson et al., 2018; Helton et al.,
2021) where tidal stripping also plays a role (see also Decarli et al., 2019,
for a case of likely tidal stripping around a quasar at $z=6$). We caution
that the continuum S/N of G11 is too low to measure a stellar-absorption-based
redshift.

###  4.2 South nebula

The S. nebula is located $\Delta\theta=15^{\prime\prime}$ or $d\approx 120$
pkpc South of TXS 0206$-$048, extends over a length-scale $\approx 60$ pkpc,
and exhibits [O II] surface brightness of $\approx 0.1$ to $1.0\times
10^{-17}\ {\rm erg\,s^{-1}\,cm^{-2}\,arcsec^{-2}}$. While the surface
brightness contours of the S. nebula are somewhat elongated, its surface
brightness peak is not coincident with any continuum sources in the HST image.
At the location of S. nebula, the HST image is sensitive to galaxies of
$M_{B}=-16$ at $z=1.13$. The morphology and lack of associated galaxies
suggest that the S. nebula is a collection of cool intragroup medium clouds in
the massive halo (e.g. Nelson et al., 2020), similar to others observed around
quasars (e.g. Johnson et al., 2018; Helton et al., 2021) at $z\approx 0.5$.
However, we note that the S. nebula could also represent ram pressure debris
stripped from the ISM of a dwarf galaxy fainter than $L_{B}\approx
0.006L_{*}$. The brightest galaxy without a robust redshift near the S. nebula
falls along the Northern edge of faintest [O II] surface brightness contour
shown in the bottom left panel of Figure 1. This galaxy has an apparent
magnitude of $m_{\rm F814W}=26.0$ which would correspond to an absolute
magnitude of $M_{B}=-18.1$ at $z=1.13$, but it lacks the strong nebular
emission expected from a galaxy experiencing on-going ram pressure stripping.

###  4.3 North filament

The N. filament extends from $\Delta\theta\approx 20.5^{\prime\prime}$ or
$d=170$ pkpc North of the quasar toward it and intersects with the Host nebula
$6^{\prime\prime}$ or $d=50$ pkpc from the quasar. Despite its length, the N.
filament is narrow with a width that ranges from $1^{\prime\prime}$ to
$3^{\prime\prime}$ or $\approx 8$ to $24$ pkpc. The N. filament is fainter
than the other nebulae in the field with a peak [O II] surface brightness of
$0.3\times 10^{-17}\ {\rm erg\,s^{-1}\,cm^{-2}\,arcsec^{-2}}$. The kinematics
of the N. filament are complex, and vary from $\Delta v\approx-250$ ${\rm
km\,s}^{-1}$ at its northern-most points to $\Delta v\approx+350$ ${\rm
km\,s}^{-1}$ where it intersects with the Host nebula. The N. filament is
spatially coincident with nearby galaxies, extending $\approx 30$ pkpc to the
North of G10 and connecting G9, G6, and G2 before intersecting with the Host
nebula. Kinematically, the velocity of the N. filament appears distinct from
G2, G6, G9, and G10 but matches that of the Host nebula where they intersect,
as seen in Figure 1.

Optically emitting outflows from radio-loud AGN are commonly observed with
orientations aligned with radio lobes (e.g. Nesvadba et al., 2017) which could
explain the morphology of the N. filament. However, these outflows typically
exhibit broad line widths of $500<{\rm FWHM}<1500$ ${\rm km\,s}^{-1}$ while
the N. filament exhibits a median width of FWHM $\approx 300$ ${\rm
km\,s}^{-1}$. Furthermore, radio observations from FIRST (Becker et al.,
2001), VLASS (Lacy et al., 2020), and the XXL Survey GMRT 610 Mhz continuum
observations (Smolčić et al., 2018) reveal no evidence of a jet or lobe
aligned with the N. filament despite $3\sigma$ detection limits of 450, 210,
and 140 $\mu$Jy per beam respectively. Starburst driven outflows and radio-
quiet AGN driven outflows typically exhibit wide opening angles (e.g. Liu et
al., 2013; Rupke et al., 2019; Burchett et al., 2021; Zabl et al., 2021)
inconsistent with the morphology of the N. filament. Together, the kinematics,
morphology, and lack of detected jets disfavor an outflow origins for the N.
filament.

Figure 2: Line-of-sight velocity of the Host nebula and N. filament as a
function of projected distance from the quasar compared with nearby galaxies.
Points corresponding to the Host nebula are shown as blue pentagons and those
corresponding to the N. filament are shown as red squares. The N. filament
measurements are made following the filament contours seen in Figure 1 with
$0.6-1.2^{\prime\prime}$ radius extraction apertures depending on the width of
the filament at each location. The measurements for the Host nebula are made
moving North from the quasar toward the N. filament with $0.6"$ radius
extractions. Galaxies G2, G6, G9, and G11 overlap spatially with the N.
filament and their stellar velocities vs projected distances are shown in
black circles in the bottom panel for comparison with the nebulae the quasar
is marked by a blue star. Despite their spatial coincidence seen in Figure 1,
the line-of-sight [O II] velocities of the N. filament differ from nearby
galaxies by $\gtrsim 300$ ${\rm km\,s}^{-1}$. The luminosity-weighted group
velocity is indicated by the red dashed line.

The morphology of the N. filament can be explained if it arises from
overlapping nebulae resulting from on-going ram-pressure stripping of group
members or cool, filamentary accretion. The on-going ram pressure stripping
scenario can explain the spatial coincidence with galaxies but would require a
somewhat contrived chance alignment. Further, the significant differences
between the [O II] emission velocities of the nebulae vs. stellar absorption
velocities of galaxies G2, G6, G9, and G10 disfavors ongoing stripping of ISM.
To quantify the velocity differences, Figure 2 displays the [O II]-based
velocities for the N. filament and the stellar absorption-based velocities of
G2, G6, G9, and G10 versus projected distance from the quasar. The galaxy and
nebular line-of-sight velocities differ by more than $300$ ${\rm km\,s}^{-1}$,
even at the locations of the galaxies. This mis-match is inconsistent with
previous observations of nebulae arising from ongoing ram pressure stripping
of ISM (Johnson et al., 2018; Boselli et al., 2019; Chen et al., 2019; Helton
et al., 2021).

The long, narrow morphology, lack of kinematic correspondence with galaxies,
and matching velocity to the northern edge of the Host nebula all suggest the
N. filament arises from cool, filamentary accretion. Such cool accretion could
result from several mechanisms including (1) a cooling flow from the hot halo
of the quasar host group, (2) cool, filamentary gas accreting from the IGM, or
(3) accreting cool CGM and intragroup medium from a less massive galaxy group
as it falls into the quasar host system. Filamentary cooling flows in cool
core clusters are observed in optical emission lines including [O II] (e.g.
McDonald et al., 2010, 2012). In this case, the N. filament would be the
longest and highest redshift known such flow despite arising in a less massive
system with a velocity dispersion $\approx 3\times$ lower than that of the
current record holder, the Phoenix Cluster (McDonald et al., 2012). Spatial
coincidence with group members is not typical for cooling flows in cool-core
clusters. Instead, the filament could represent chaotic cold accretion from
the hot halo (e.g. Gaspari et al., 2018) if interactions between G2, G6, G9,
G10, and the quasar host induce turbulenct cooling. In this case, the complex
velocity shear would reflect bulk motion of the hot halo.

Alternatively, the N. filament’s morphology and coincidence with galaxies can
be explained by cool accretion of an intergalactic medium filament connecting
galaxies G2, G6, G9, and G10 or CGM from a galaxy group containing G2, G6, G9,
and G10 as the group is accreted by the quasar host system. In both cases, the
velocity difference between the nebulae and nearby galaxies can be explained
if the gas experienced ram pressure deceleration over sufficiently long
timescales. Bulk motion of hot halos can include rotation (e.g. Hodges-Kluck
et al., 2016; Oppenheimer et al., 2018) or other complex patterns which could
explain the velocity shear of the N. filament. The accreting CGM scenario
would require tidal forces to explain the elongated morphology.

An IGM filament would explain the morphology and potentially connect galaxies.
The ratio of the stream minor axis radius to the estimated host halo virial
radius ($\approx 560$ pkpc) is $\approx 1-4\%$, consistent with predictions
for cool inflowing IGM streams from Mandelker et al. (2020). The N. filaments
properties can also be explained by CGM or intragroup medium in the presence
of interactions. In particular, if the filament originated as CGM or
intragroup medium around G2, G6, G9, and G10, the elongated morphology could
be the result of tidal and ram pressure forces experienced as the group
members and surrounding gas fell toward the quasar host. In summary, the
morphology and kinematics of the N. filament are consistent with cool
filamentary accretion from the IGM, from the CGM/intragroup medium around a
group of galaxies being accreted by the quasar host system, or interaction-
induced cooling of hot gas.

###  4.4 Host nebula

Figure 3: Summary of emitting and absorbing gas kinematics near the nucleus.
The image in the center panel displays the [O II] surface brightness map
zoomed-in and scaled to highlight structures near the nucleus. The quasar
position is marked by a blue star and pixels touching the star are subject to
large uncertainties in quasar light subtraction. The top right panel shows
normalized flux of [Ne III] versus line-of-sight velocity from the quasar
while the three panels on the left display [Ne III] emission for circular
apertures offset from the quasar centroid. The regions corresponding to each
spectrum are shown in black circles and connected to the corresponding
spectral panel by a dashed black line. The bottom two panels on the right
display O V (middle right) and N IV (bottom right) narrow, associated
absorption from the COS FUV spectrum of TXS 0206$-$048\. The COS spectrum
reveals both outflowing (blue-shifted) and inflowing (redshifted) absorption
and photoionization analysis of the absorbers from Finn et al. (2014)
indicates that the absorbing gas arises at distances of 2$-$6 pkpc from the
nucleus. For reference, the velocity ranges of the outflowing and inflowing
material observed in the quasar absorption spectrum are shown in blue and red
highlight respectively. The inflowing component exhibits a line-of-sight
velocity of $\approx+200$ ${\rm km\,s}^{-1}$ that is consistent with the
velocity field seen in [Ne III] emission seen at $5-10$ kpc from the quasar in
the panels on the left. The outflowing absorbing gas shows velocities that are
comparable to the nuclear [Ne III] but exceed the blueshifts seen in emission
at $d=5-10$ kpc seen in the left three panels.

The Host nebula is approximately centered on the quasar and extends to a
radius of $\Delta\theta=6^{\prime\prime}$ or $d\approx 50$ pkpc from the
quasar with [O II] surface brightness levels ranging from $0.1$ to $24\times
10^{-17}\ {\rm erg\,s^{-1}\,cm^{-2}\,arcsec^{-2}}$. The kinematics of the Host
nebula are complex with line-of-sight velocities ranging from $\approx-300$ to
$+540$ ${\rm km\,s}^{-1}$ and distinct spiral-like structure seen in Figure 1.

The radial extent, morphological correspondence with the quasar, and high peak
surface brightness, suggest that the Host nebula represents extended, ionized
ISM and diffuse gas around the quasar host. However, the Host nebula’s
kinematics are more complex than the canonical “spider” diagram expected for
rotating disks. Quantitative analysis of the velocity structure function of
the Host nebula demonstrates that it follows expectations for Kolmogorov
turbulence with isotropic, homogeneous, and incompressible gas (see Chen et
al., 2022).

The unusual spiral structure visible in the surface brightness panels (top)
and velocity map (bottom middle) in Figure 1 may be a signature of tidal arms
from a past interaction, possibly one that helped fuel the quasar. However,
the bright quasar limits our ability to search for interaction signatures that
would confirm this scenario.

To investigate whether there is a significant, large-scale inflow or outflow
associated with the Host nebula, we take advantage of the archival COS
spectrum of the quasar, which enables clean differentiation of outflowing
(blueshifted) and inflowing (redshifted) gas along the quasar sightline. The
COS spectrum reveals narrow associated absorption, including both outflowing
and inflowing components detected in an array of ions. The middle and bottom
panels on the right of Figure 3 show O V $\lambda 629$ and N IV $\lambda 569$
absorption as a function of line-of-sight velocity relative to the quasar.
Finn et al. (2014) conducted ionization analysis of these absorbers and found
that the absorbing clouds likely arise at distances of $\gtrsim 2$ pkpc from
the nucleus. This is only slightly smaller than the angular resolution of the
seeing limited MUSE data ($0.7^{\prime\prime}$ corresponds to $\approx 6$ pkpc
at $z=1.13$), providing a unique opportunity to jointly study the emitting and
absorbing gas near the quasar.

The regions of the Host nebula near the quasar are bright enough to be
observed in [Ne III] $\lambda 3869$ emission, which enables more precise
velocity measurements. To search for kinematic correspondence between the
emitting gas and associated absorbers, Figure 3 displays peak-normalized [Ne
III] emission as a function of line-of-sight velocity for a central ($d<5$
pkpc) extraction centered on the quasar in the top right panel and extractions
at $d=5-10$ pkpc immediately to the N. (left/top panel), E. (left/middle
panel), and S. (left/bottom panel).

The [Ne III] emission at $d<5$ pkpc from the quasar sightline peaks between
the quasar systemic velocity and that of the inflowing component seen in
associated absorption in O V, N IV, and other ions. The circum-nuclear [Ne
III] emission also exhibits a prominent blue wing extending from $\Delta
v\approx-200$ to $-400$ ${\rm km\,s}^{-1}$, similar to the outflowing
absorbers. The similarity in velocity range observed between the associated
inflow/outflow observed in absorption and the circumnuclear [Ne III] suggests
that the emissions may trace different phases and locations of the the same
gas flows traced in absorption.

Further from the nucleus, the [Ne III] emission at $d=5-10$ pkpc shows
decreased prominence of the blue wing, suggesting that the faster outflow
component at $\Delta v<-200$ ${\rm km\,s}^{-1}$ may be confined to central
regions of the host. On the other hand, the redshifted emission component is
more prominent and peaks near the associated inflow velocity, particularly in
the N. extraction. The more extended nature of the redshifted emission and
kinematic coincidence with the associated inflow suggest a common origin and a
relation to Host nebulae. In this case, the emitting gas near the nucleus
would have to be in front of the quasar and oriented towards us within the
quasar ionizing radiation cone to be illuminated and for the line-of-sight
velocity to approximately match the radial velocity of the down-the-barrel
inflow.

##  5 Discussion and Conclusions

Based on a combination of deep MUSE observations and archival HST data, we
discovered four distinct and giant ionized nebulae in the environment of TXS
0206$-$048, a luminous quasar at $z=1.13$. Some of these nebulae are likely
related to an inflow detected in UV absorption. Two of the nebulae are well
separated from the quasar host galaxy and likely arise from on-going ram
pressure stripping of a group member (N.E. nebula) and cool clouds in the
intragroup medium (S. nebula), extending observations of large-scale streams
observed in groups and around quasar hosts to higher redshift (e.g. Hess et
al., 2017; Johnson et al., 2018; Helton et al., 2021).

The two other giant nebulae include a $\approx 120$ pkpc filament extending to
the N. of the quasar (N. filament), which intersects–both spatially and
kinematically–with a $100$ pkpc diameter nebula (Host nebula) surrounding the
quasar host itself. Immediately around the quasar, the Host nebula exhibits
velocities similar to inflowing absorbing gas observed in UV spectra of the
quasar. The morphology and kinematics of the N. filament and Host nebula and
coincidental inflowing gas constitute strong evidence of large-scale, cool
filamentary accretion from halo scales into the quasar host and toward the
nucleus.

If the [O II] emitting gas is in pressure equilibrium with a hot halo, we can
gain insights into its physical conditions. Hydrodynamical simulations predict
global pressure equilibrium, although small-scale fluctuations from subsonic
turbulence can occur (e.g. van de Voort & Schaye, 2012; Gaspari et al., 2014),
though observational results are mixed (see Werk et al., 2014; Stern et al.,
2016; Zahedy et al., 2019; Butsky et al., 2020; Qu et al., 2022). Adopting the
group mass from Section 3 and the generalized NFW pressure profile from Arnaud
et al. (2010), we estimate hot halo pressure of $P_{\rm hot}/k_{\rm B}\approx
5\times 10^{4}\ {\rm K\,cm^{-3}}$ at $\approx 100$ pkpc for the N. filament.
Based on Cloudy v17.03 (Ferland et al., 2017) photoionization equilibrium
models for gas illuminated by the quasar which dominates over the expected UV
background at these distances (e.g. Faucher-Giguere & -A., 2019), we expect a
temperature of $\log T/{\rm K}=4-4.5$ and a density of $n_{e}\approx 1-5$
${\rm cm^{-3}}$.

Estimating the surface brightness of [O II] given a density and total ionized
column, $N({\rm H\,II})$, is complicated by the unknown ionization state and
metallicity of the gas. However, we expect a line ratio of [O II]/H$\alpha\sim
1$ over a fairly wide range of conditions for AGN photoionized gas (e.g.
Groves et al., 2004), enabling us to roughly estimate the ionized gas column
because H$\alpha$ surface brightness can be approximated as ${\rm
SB_{H\alpha}}\approx 1.7\times 10^{-16}C\left(\frac{\langle n_{e}\rangle}{{\rm
cm^{-3}}}\right)\left(\frac{N({\rm H\,II})}{3\times 10^{20}{\rm
cm^{-2}}}\right)\left(\frac{1}{1+z}\right)^{4}$ $\rm
erg\,s^{-1}\,cm^{-2}\,arcsec^{-2}$ where $C$ is the clumping factor,
$C=\langle n_{e}^{2}\rangle/\langle n_{e}\rangle^{2}$. Assuming $n_{e}\sim 3\
{\rm cm^{-3}}$ and $C\approx 1$, the surface brightnesses of the N. filament
corresponds to approximate ionized gas columns of $N({\rm H\,II})\sim 10^{19}\
{\rm cm^{-2}}$. With the same assumptions, we estimate total ionized gas
masses of $M_{\rm g}\sim 10^{9}$ and $2\times 10^{10}\ M_{\odot}$ for the N.
filament and Host nebula, respectively based on the total line luminosity
following Greene et al. (2011). These are significantly lower than the
estimates of ionized gas mass assuming the same density and near unity volume
filling factor which results in $M_{\rm gas}\sim 10^{12}M_{\odot}$ for the N.
filament assuming a cylinder with length of $l=100$ pkpc and radius of $r=10$
pkpc and $M_{\rm gas}\sim 10^{13}M_{\odot}$ for the Host nebula assuming a
uniform sphere with radius $r=50$ pkpc. These volume-based estimates
significantly exceed not only the previous luminosity-based estimates but also
the expected baryon budget for the group, requiring lower mean density and
higher clumping factor (e.g., Cantalupo et al., 2019). With a clumping factor
of $C\approx 100$, the discrepancy between the two mass estimates can be
resolved. Insights into the density structure of the gas require new emission
observations.

Given its low surface brightness and possible column density, the N. filament
may arise in gas analogous to Lyman limit systems and damped Ly$\alpha$
absorbers but in a more overdense, higher pressure environment and subjected
to intense ionizing radiation from the quasar. The $\approx 80\times$ higher
peak surface brightness of the inner regions of the Host nebula suggests
significantly higher density or ionized gas column, consistent with ISM. The
inflowing absorption detected in the spectrum of TXS 0206$-$048 combined with
the morphology of the extended, emitting gas of the Host nebula provides an
opportunity to estimate the accretion rate in a quasar host assuming that the
absorbing gas and extended emission are tracing different phases and locations
along a coherent gas flow. Following Weiner et al. (2009), we assume a thin,
spherical shell which results in an inflow rate of $\frac{dM}{dt}\sim\left(22\
M_{\odot}\,{\rm yr^{-1}}\right)\left(\frac{N({\rm H})}{10^{20}\,{\rm
cm^{-2}}}\right)\left(\frac{R}{5\,{\rm pkpc}}\right)\left(\frac{v}{300\,{\rm
km\,s^{-1}}}\right)$ where $N({\rm H})$ is the total hydrogen column density
of the inflow, $R$ is the radius of the shell, and $v$ is the inflow velocity.
While it is an oversimplification, we chose the thin-shell for ease of
comparison with previous results (e.g. Arav et al., 2013) and to place a
conservative upper limit on the inflow rate. Adopting a uniform, spherical
flow of radius $R$ would reduce the inflow rate by a factor of three. With a
total column of $N({\rm H})\sim 10^{17}\,{\rm cm^{-1}}$ based on the
ionization analysis of Finn et al. (2014), a radius of $R<50$ pkpc based on
the maximum observed extent of the Host nebula, and the observed inflow
velocity of $v\approx 150$ ${\rm km\,s}^{-1}$, we infer an upper limit on the
inflow rate of $\frac{dM}{dt}<0.1\ M_{\odot}\,{\rm yr}^{-1}$. The inflowing
absorbing gas column inferred by Finn et al. (2014) is several orders-of-
magnitude below our initial surface-brightness based estimate for the ionized
column. This can be resolved if the emitting gas traces denser, higher column
density gas phase with lower covering factor than the absorbing gas or
equivalently, by a clumping factor of $\approx 100$ as previously suggested.
The H I column for the inflow inferred by Finn et al. (2014) is based on metal
absorption only and assumes approximately solar metallicity due to
unavailability of an H I column measurement.

If the absorbing gas arises closer to the nucleus at $\approx 2$ pkpc (see
Finn et al., 2014), then the inflow rate estimate decreases to
$\frac{dM}{dt}<0.004\ M_{\odot}\,{\rm yr}^{-1}$. An inflow rate of $0.1\
M_{\odot}\,{\rm yr}^{-1}$ corresponds to a luminosity of $\sim 5\times
10^{45}\,{\rm erg\,s^{-1}}$ or a radiative luminosity of $\sim 5\times
10^{44}\,{\rm erg\,s^{-1}}$ assuming a radiative efficiency of 10%. This is
several order-of-magnitude below the observed radiative luminosity of the
quasar indicating a significant difference in accretion rate at $\gtrsim 2$
pkpc compared to nearer the accretion disk. This large difference suggests
highly anisotropic or highly time variable accretion. Time variable accretion
will be reflected in luminosity variability of the quasar, though we note that
TXS 0206$-$048 exhibits 5% level variability in the UV on month timescales
(e.g. Punsly et al., 2016). Accretion mechanisms such as chaotic cold
accretion (e.g. Gaspari et al., 2018) and interaction induced quasar activity
(e.g. Goulding et al., 2018) are expected to produce significant accretion
rate variability on timescales comparable to the dynamical time at a few pkpc.
Alternatively, the observations of accretion rate at $\gtrsim 2$ pkpc than is
orders-of-magnitude lower than that inferred for the quasar engine itself on
much smaller scales could be a signature of effective AGN feedback heating the
CGM to slow accretion.

The discovery and morphokinematic analysis of multiple large nebulae including
a $\approx 100$ pkpc long filament and connected $\approx 100$ kpc diameter
nebulae around a luminous quasar with an inflowing associated absorber
demonstrate the unique insights enabled by observations of non-resonant
emission lines with wide-field IFS when coupled with down-the-barrel
absorption spectroscopy. Developing a better understanding of the origins,
fate, and physical conditions of these gas flows requires observations of
emission lines from a wider variety of ions which will become possible with
upcoming near-IR IFS such as MIRMOS on Magellan (Konidaris et al., 2020) and
HARMONI (Thatte et al., 2021) on the E-ELT.

## Acknowledgements

The authors are grateful for the efforts of the anonymous referee whose
thorough feedback strengthened the paper. SDJ gratefully acknowledges partial
support from HST-GO-15280.009-A, HST-GO-15298.007-A, HST-GO-15655.018-A, and
HST-GO-15935.021-A. MG acknowledges partial support by HST GO-15890.020/023-A,
the BlackHoleWeather program, and NASA HEC Pleiades (SMD-2359).. SC gratefully
acknowledges support from the European Research Council (ERC) under the
European Union’s Horizon 2020 research and innovation programme grant
agreement No 864361. GLW acknowledges partial support from HST-GO-15655.006-A
and HST-GO-16016.002

Based on observations from the European Organization for Astronomical Research
in the Southern Hemisphere under ESO (PI: Schaye, PID: 094.A-0131) and the
NASA/ESA Hubble Space Telescope (PI: Abraham, PID: 9760; PI: Morris, 12264;
PI: van der Marel, PID: 12564; PI: L. Straka; PID: 14660). The COS spectra and
ACS images analyzed in this paper can be accessed via
https://doi.org/10.17909/4dnz-y880 (catalog COS DOI) and
https://doi.org/10.17909/tekk-dx10 (catalog ACS DOI), respectively.

The This paper includes data gathered with the 6.5 meter Magellan Telescopes
located at Las Campanas Observatory, Chile. The paper made use of the
NASA/IPAC Extragalactic Database, the NASA Astrophysics Data System, Astropy
(Astropy Collaboration et al., 2018), and Aplpy (Robitaille & Bressert, 2012).

##  Appendix A Supplemental information on the group hosting TXS 0206$-$048

To provide additional context on the galaxy group hosting TXS 0206$-$048,
Figure 4 displays a wider FoV version of the HST ACS$+$F814W image with
galaxies labelled by group membership based on spectroscopic redshifts as
described in Section 3. The figure also highlights the velocity dispersion of
the group and best-fit Gaussian and the core-dominated nature of the radio
component of TXS 0206$-$048 in inset panel on the top left and top right,
respectively.

Figure 4: Summary of the redshift survey and galaxy group hosting TXS
0206$-$048\. The greyscale image displays the reduced, coadded HST ACS$+$F814W
image of the field centered on TXS 0206$-$048 with orientation and scale
marked in the bottom right. The full $1^{\prime}{\times}1^{\prime}$ MUSE FoV
is marked by the black square with solid outline, and the
$40^{\prime\prime}{\times}40^{\prime\prime}$ region shown in Figure 1 is
marked in dashed line. Galaxies in the group hosting TXS 0206$-$048 are
labelled with red circles and labelled by their ID and velocity in ${\rm
km\,s}^{-1}$ from Table 1. The approximate luminosity-weighted group center is
marked with an asterisk. Galaxies with secure redshifts but foreground and
background to the quasar host group are marked in blue and orange dashed
symbols, respectively. The inset panel on the top right displays the line-of-
sight velocity histogram for group members, including TXS 0206$-$048, in
black, solid line while the best-fitting Guassian with mean line-of-sight
velocity of $\mu=-130$ ${\rm km\,s}^{-1}$ and line-of-sight velocity
dispersion of $\sigma=550$ ${\rm km\,s}^{-1}$ is shown in dashed grey line.
The 1.4 Ghz radio image from the FIRST survey is shown in the top right inset
panel and confirms the core-dominated nature of this radio-loud quasar.

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