# The TELPERION Survey for Distant [O III] Clouds around Luminous and
Hibernating AGN

William C. Keel,1,2 Alexei Moiseev,3,4 D.V. Kozlova,3 A.I. Ikhsanova,3,5 D.V.
Oparin,3 R.I. Uklein,3 A.A. Smirnova,3 and M.V. Eselevich6
1Dept. of Physics and Astronomy, University of Alabama, Box 870324,
Tuscaloosa, AL 35487, USA
2SARA Observatory, Embry-Riddle Aeronautical University, Daytona Beach FL
32114, USA
3Special Astrophysical Observatory, Nizhny Arkhyz, Russia
4Lomonosov Moscow State University, Sternberg Astronomical Institute,
Universitetsky pr. 13, Moscow 119234, Russia
5Kazan Federal University, Kremlevskaya str.18, 420008 Kazan, Russia
6Institute of Solar-Terrestrial Physics, Russian Academy of Sciences, Siberian
Branch, Irkutsk, Russia E-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present a narrowband [O III] imaging survey of 111 AGN hosts and 17
merging-galaxy systems, in search of distant extended emission-line regions
(EELRs) around AGN (either extant or faded). Our data reach deeper than
detection from the broadband SDSS data, and cover a wider field than some
early emission-line surveys used to study extended structure around AGN.
Spectroscopic followup confirms two new distant AGN-ionized clouds, in the
merging systems NGC 235 and NGC 5514, projected at 26 and 75 kpc from the
nuclei (respectively). We also recover the previously-known region in NGC
7252. These results strengthen the connection between EELRs and tidal
features; kinematically quiescent distant EELRs are virtually always
photoionized tidal debris. We see them in $\approx 10$% of the galaxies in our
sample with tidal tails. Energy budgets suggest that the AGN in NGC 5514 has
faded by$>3$ times during the extra light-travel time $\approx$250,000 years
from the nucleus to the cloud and then to the observer; strong shock emission
in outflows masks the optical signature of the AGN. For NGC 235 our data are
consistent with but do not unequivocally require variation over
$\approx$85,000 years. In addition to these very distant ionized clouds,
luminous and extensive line emission within four galaxies - IC 1481, ESO
362-G08, NGC 5514, and NGC 7679. IC 1481 shows apparent ionization cones, a
rare combination with its LINER AGN spectrum. In NGC 5514, we measure a 7-kpc
shell expanding at $\approx 370$ km s-1 west of the nucleus.

###### keywords:

galaxies: active – galaxies: interactions – galaxies: Seyfert

††pubyear: 2021††pagerange: The TELPERION Survey for Distant [O III] Clouds
around Luminous and Hibernating AGN–The TELPERION Survey for Distant [O III]
Clouds around Luminous and Hibernating AGN

##  1 Introduction

Some active galactic nuclei (AGN) are surrounded by regions of ionized gas
extended beyond the ordinary narrow-line region (NLR), extending well into the
host galaxies’ interstellar medium (ISM) and sometimes even beyond into tidal
features or outflows. These are denoted extended emission-line regions (EELRs;
Wilson 1996 and Stockton, Fu, & Canalizo 2006). Their properties can provide
insight into several facets of AGN - directions of escape of ionizing
radiation, the history of the ionizing luminosity, and interactions with
emerging radio jets. Contributions by volunteers with the Galaxy Zoo project
(Lintott et al., 2008), using the colour contrast between ordinary galaxy
light and [O III] emission in composite images from the Sloan Digital Sky
Survey (SDSS; York et al. 2000), led to identification of 19 EELRs around low-
redshift AGN, most radio-quiet, where about 1/3 showed energy mismatches
between the AGN as observed and the cloud properties suggesting long-term
fading over spans $\approx 10^{4}$ years (Keel et al., 2012). Using
increasingly distant regions of ionized gas around AGN offers a way to probe
their past ionizing luminosity, and thereby examine their variability over
otherwise inaccessible timescales beyond the century or so spanned by our
direct operations, including photographic archives. Results from this sample
motivated us to undertake further surveys in several directions.

To complement continued searches111Ongoing discussion in
https://www.zooniverse.org/projects/zookeeper/galaxy-
zoo/talk/1269/858076?comment=2551725 for AGN-ionized clouds by Galaxy Zoo
volunteers using SDSS, DES, and Pan-STARRS1 images, we have conducted a
narrowband survey in a redshifted [O III] filter to seek fainter clouds which
would be lost against the brighter sky background in the survey passbands,
over wider regions than were studied in many of the early examinations of
EELRs. The study by Knese et al. (2020) became the first phase of this larger
survey, named TELPERION222Nominally an acronym for Tracing Emission Lines to
Probe Extended Regions Ionized by Once-active Nuclei, but more evocative for
readers of Tolkien’s work on the First Age of Middle Earth, where it gives a
connotation of vanished brilliance.. Use of long exposures in narrow filters
allows 1m telescopes to reach deeper in this application than the SDSS
broadband images, thereby complementing the Keel et al. (2012) survey which
examined $\approx 15,000$ galaxies to a higher surface-brightness limit. We
are particularly interested in very large or distant EELR clouds, since these
are the ones where light-travel times let us see longest luminosity histories
for the AGN. As in Keel et al. (2012), we seek AGN-ionized gas at projected
distances $>10$ kpc from the AGN, roughly marking the distinction between the
ISM of the AGN host galaxy and gas emplaced by additional processes (tidal
disturbance or outflows). The circumnuclear ionized regions around some AGN do
extend farther than this, often associated with tidal distortions. The
abbreviation EELR has been used to describe both, so we will call the ones we
seek, outside the host ISM, as distant EELRs.

Known distant EELRs, including those associated with fading AGN, occur around
nuclei with a range of currently-observed activity. We see them around bright
AGN, including both ordinary and “type 2" QSOs, Seyfert galaxies, as well as
LINERs and inactive nuclei (Schweizer et al., 2013). Accordingly, we sought a
balance between sampling wide ranges of host-galaxy properties and the number
of galaxies that could feasibly be observed. We observed subsamples selected
on merger status, and on AGN luminosity broadly defined. The survey recovered
some known EELRs, found two new distant cases, and adds to the statistics of
their occurrence around broad categories of galaxies. For some features of
interest, we have followup spectroscopic data to examine their ionization
source and kinematics, confirming several ionization cones and EELR clouds as
being AGN-photoionized. Both newly-detected distant EELR clouds are associated
with tidal debris, continuing a pattern seen in previous surveys.

We calculate projected sizes and luminosities using a Hubble constant $\rm
H_{0}=70$ km s-1 Mpc-1, $\Lambda_{M}=0.286$, and flat geometry.

##  2 Survey Sample, Observations, and Analysis

###  2.1 Samples

The initial phase of this survey, reported by Knese et al. (2020), included
nearby Seyfert galaxies for which surrounding H I structures were known. This
way, we would know immediately how ionized and neutral structures were
spatially related, and what fractions of extended H I envelopes host distant
EELRs. If the outer gas content is independent of the properties of the AGN,
for example, this would tell what fraction of AGN would produce distant EELRs
if there were suitable neutral gas to provide a tracer when ionized. Building
on the success of that phase in finding a new distant EELR around Mkn 1, out
of 26 Seyfert galaxies, we intended this survey to reach surface brightness
levels 1/10 that seen in, for example, Hanny’s Voorwerp near IC 2497 (Lintott
et al., 2009). For otherwise identical situations, this would allow detection
of gas over three times farther from the AGN and responding to its energy
output at epochs three times longer before the epoch at which we see the AGN
directly. We report here observations of two additional galaxy samples - an
extended version of the Toomre (1977) merger sequence, and a wide-ranging set
of AGN host galaxies.

The redshift range for the survey was selected to use off-the-shelf filters
for economy. For redshifted [O III] emission, we used filters denoted F510,
from Custom Scientific333https://customscientific.com/, with designed peak
transmission at 510 nm and FWHM 10 nm. They were installed first on the SARA
1m telescope at Kitt Peak, and later at the 1m Jacobus Kapteyn Telescope (JKT)
on La Palma, now operated by the SARA consortium (Keel et al., 2017a). In the
$f/7.5$ beam of the Kitt Peak telescope, we expect a blueward shift of the
peak by no more than 8 Å . The half-transmission points correspond to [O III]
$\lambda 5007$ at redshifts $=0.0086-0.0285$, $cz=2580-8570$ km s-1. We used
this redshift range for our target lists in each survey phase.

Since most known (kinematically-quiescent) EELRs are associated with
interacting and merging systems, we examined all galaxies among the well-known
Toomre (1977) mergers within our redshift and declination limits, augmented by
morphologically similar systems from Keel & Wu (1995). These were observed
only from Kitt Peak, as listed in Table 1. The table also collects information
from the literature on the nuclear spectra, to identify optical AGN
signatures.

Table 1: Log of imaging observation for the extended Toomre merger sequence. The UT date of observation is listed for images in redshifted [O III], all from Kitt Peak. As a guide to the merger and interaction stage, columns 5-7 indicate how many separate galaxies, nuclei, and tidal tails appear in optical images. Column 3 indicates which galaxies were included in the original Toomre sequence. The final column shows the adopted classification of the nucleus from optical spectroscopy and the source of this information. Name | Arp | Toomre? | $z$ | Galaxies | Nuclei | Tails | Date obs (YYYYMMDD) | Nucleus, source
---|---|---|---|---|---|---|---|---
NGC 455 | 164 |  | 0.0176 | 1 | 1 | 2 | 20131202 | Quiescent Falco et al. 1999
NGC 2418 | 165 |  | 0.0169 | 1 | 1 | 2 | 20140106 | Passive, Bernloehr (1993)
NGC 2623 | 243 | Toomre | 0.0182 | 1 | 1 | 2 | 20131202 | LINER Keel 1984
NGC 2865 |  |  | 0.0087 | 1 | 1 | 1 | 20131115 | E+A Jones et al. 2009
NGC 2914 | 137 |  | 0.0110 | 1 | 1 | 2 | 20140106 | Quiescent SDSS
Arp 221 | 221 |  | 0.0180 | 1 | 2 | 2 | 20140115 |
NGC 3303 | 192 |  | 0.0201 | 2 | 2 | 1 | 20170418 | LINERs Keel 1985
NGC 3509 | 335 | Toomre | 0.0255 | 1 | 1 | 2 | 20140524 | LINER Falco et al. 1999
NGC 3921 | 224 | Toomre | 0.0200 | 1 | 1 | 2 | 20140515 | LINER Stauffer 1982
NGC 4676 | 242 | Toomre | 0.0220 | 2 | 2 | 2 | 20140106 | LINERs Keel et al. 1985
IC 883 | 193 |  | 0.0239 | 1 | 2 | 2 | 20140106 | LINER/composite Keel 1985
IC 4553 | 220 |  | 0.0182 | 1 | 2 | 1 | 20210211 | LINER Falco et al. 1999
NGC 6240 |  |  | 0.0242 | 1 | 2 | 3 | 20160430 | LINER Keel 1990
NGC 6621/2 | 81 | Toomre | 0.0208 | 2 | 2 | 1 | 20131002 | Starburst+LINER Keel 1996
NGC 7252 | 226 | Toomre | 0.0158 | 1 | 1 | 2 | 20150719 | Quiescent Schweizer et al. 2013
NGC 7585 | 223 |  | 0.0114 | 1 | 1 | 1 | 20131108 | Quiescent Moustakas & Kennicutt 2006
NGC 7592 |  | Toomre | 0.0244 | 2 | 2 | 2 | 20151102 | Composite Moustakas & Kennicutt 2006

We then surveyed a much larger set of AGN host galaxies, regardless of
morphology or interaction status. Desiring a nearly all-sky sample to best use
telescope time, and in view of the limited sky coverage then available for the
SDSS and limited depth for X-ray surveys, we opted for the more miscellaneous
but broad selection of AGN in the VCV catalog (Véron-Cetty & Véron, 2010). In
view of our limited knowledge of what properties are associated with the
incidence of EELRs, we used a simple selection on listed VCV absolute
magnitude (brighter than $M=-20$, from inhomogeneous sources), plus requiring
declination north of $-36^{\circ}$, allowing observations from Kitt Peak for
more than one hour per night. This selection yielded 120 AGN host galaxies, of
which 9 were already observed by Knese et al. (2020) and not re-observed. The
remaining galaxies observed in this study are listed in Table 2. Among these,
NGC 2329 is listed in VCV with a 1-degree error in declination, and MCG
-02-51-008 is misnamed as MCG -5-51-008 (the listed coordinates from VCV
correspond to MCG -02-51-008). Both E and W components of NGC 7592 were
included in this sample, and also appear as one combined entry in the Toomre
sample. The VCV compilation includes AGN classifications from a wide range of
data in the literature; some of these galaxies may not show spectroscopic AGN
signatures when examined more closely.

Some of the galaxies in our samples have extended emission-line material over
very large regions for reasons other than photoionization by an AGN, but their
[O III] emission is weak enough to avoid confusion in this survey. NGC 6240
has a very extensive network of emission-line loops and filaments associated
with its starburst wind (Heckman, Armus, & Miley 1987, Veilleux et al. 2003,
Yoshida et al. 2016, López-Cobá et al. 2020) as seen in H$\alpha$ and [N II],
but we do not detect [O III] outside the main body of the merger. Likewise the
NGC 1275 filament system, variously attributed to shock ionization and cooling
of the intracluster material, has [O III]/H$\beta<1$ (Kent & Sargent 1979,
Hatch et al. 2006). Surveying [O III] rather than, for example, H$\alpha$,
immediately reduces the number of non-AGN-photoionized regions detected.

Table 2: Object list, and log of imaging observations, for the VCV AGN sample. The absolute magnitude $M$ is taken from the VCV catalog, as are the AGN types with expansion from their compact notation. The final column shows the UT dates each was observed, in the order $V$, F510 when both were not from Kitt Peak (KP) on the same night. Data from Cerro Tololo (CT) or the JKT are indicated. $\alpha_{2000}$ | $\delta_{2000}$ | Name | $z$ | AGN type | $M$ | Date Observed ($V$, F510)
---|---|---|---|---|---|---
00 11 06.6 | -12 06 27 | Mkn 938 | 0.019 | Sy 2 | -20.3 | JKT 2016-09-28, KP 2011-11-14
00 18 23.5 | +30 03 47 | NGC 71 | 0.022 | Sy 2 | -20.0 | JKT 2016-09-01, KP 2017-09-19
00 42 52.8 | -23 32 28 | NGC 235 | 0.022 | Sy 1 | -20.6 | JKT 2016-09-28, JKT 2019-11-26
00 55 02.6 | -19 00 17 | ESO 541-G001 | 0.021 | Sy 2 | -21.0 | JKT 2016-09-28, KP 2017-11-14
00 59 40.1 | +15 19 51 | UGC 615 | 0.018 | Sy 2 | -20.1 | CT 2016-11-30, KP 2016-10-09
01 14 07.0 | -32 39 02 | IC 1657 | 0.012 | Sy 2 | -20.4 | CT 2015-12-29, KP 2015-12-29
01 16 03.6 | +04 17 39 | Mkn 565 | 0.021 | Sy? | -21.2 | CT 2016-11-30, KP 2016-10-12
01 31 50.4 | -33 08 09 | ESO 353-G09 | 0.017 | Sy 2 | -20.6 | JKT 2016-09-28, KP 2017-09-19
01 34 26.3 | -15 48 55 | MCG -03.05.007 | 0.020 | Sy | -20.5 | CT 2016-10-31, KP 2017-11-10
01 45 25.4 | -03 49 38 | MCG -01.05.031 | 0.018 | Sy 2 | -20.3 | CT 2016-10-31, KP 2017-09-19
01 52 49.0 | -03 26 48 | MCG -01.05.047 | 0.017 | Sy 2 | -20.1 | CT 2016-10-31, KP 2016-10-09
02 00 14.9 | +31 25 46 | NGC 777 | 0.017 | Sy 2 | -20.6 | KP 2015-10-13
02 01 06.5 | -06 48 56 | NGC 788 | 0.013 | Sy 1 | -20.0 | KP 2015-12-10, KP 2017-11-10
02 34 20.1 | +32 30 20 | NGC 973 | 0.016 | Sy 2 | -20.6 | KP 2015-11-02
02 41 38.7 | -28 10 17 | IC 1833 | 0.017 | Sy 2 | -20.0 | KP 2015-11-12
02 42 35.7 | +34 45 46 | NGC 1050 | 0.013 | Sy 2 | -20.2 | KP 2016-03-10
02 49 03.9 | -31 10 21 | IC 1859 | 0.020 | Sy 2 | -20.2 | KP 2015-12-10
03 11 14.7 | -08 55 19 | NGC 1241 | 0.013 | Sy 2 | -21.0 | KP 2015-10-13
03 19 48.2 | +41 30 42 | NGC 1275 | 0.017 | Sy 1.5 | -21.2 | KP 2015-12-29
03 42 03.6 | -21 14 37 | ESO 548-G81 | 0.015 | Sy 1 | -20.2 | KP 2015-11-12
04 13 49.7 | -32 00 24 | ESO 420-G13 | 0.012 | Sy 2 | -20.2 | CT 2016-01-08, KP 2016-02-11
04 48 37.2 | -06 19 12 | NGC 1667 | 0.015 | Sy 2 | -20.2 | KP 2015-12-10
04 58 54.6 | -00 29 20 | NGC 1713 | 0.015 | LINER | -20.6 | KP 2016-02-27
05 08 19.7 | +17 21 47 | Zw 468.002 | 0.017 | Sy 2 | -20.8 | JKT 2016-10-29, KP 2016-11-25
05 10 13.9 | -29 24 13 | MCG -05.13.010 | 0.023 | LINER | -20.0 | CT 2015-12-29, KP 2015-12-29
05 11 09.0 | -34 23 36 | ESO 362-G08 | 0.016 | Sy 2 | -20.8 | CT 2015-12-29, KP 2016-02-17
06 23 46.5 | -32 13 00 | ESO 426-G002 | 0.022 | Sy 2 | -20.5 | CT 2016-01-08, KP 2016-02-11
06 23 51.0 | -23 11 37 | MCG -04.16.001 | 0.024 | Sy | -20.7 | CT 2016-04-08, KP 2016-11-25
06 32 47.5 | +63 40 23 | UGC 3478 | 0.012 | NLSy 1 | -20.6 | KP 2016-11-12
06 47 45.8 | +74 28 54 | NGC 2258 | 0.014 | LINER | -20.9 | KP 2016-02-17
06 54 35.1 | +50 21 10 | Mkn 373 | 0.020 | Sy 1 | -20.4 | KP 2016-02-27
07 09 08.0 | +48 36 55 | NGC 2329 | 0.019 | LINER | -21.3 | KP 2016-03-10
07 42 32.9 | +49 48 35 | Mkn 79 | 0.022 | Sy 1.2 | -20.0 | KP 2015-11-12
07 44 09.1 | +29 14 49 | UGC 3995 | 0.015 | Sy 2 | -20.6 | JKT 2016-10-29, KP 2017-03-02
07 59 40.2 | +15 23 15 | UGC 4145 | 0.016 | Sy 2 | -20.1 | KP 2016-04-01
08 07 41.0 | +39 00 13 | Mkn 622 | 0.023 | Sy 2 | -20.1 | JKT 2017-01-17, KP 2016-02-11
08 08 47.1 | +00 18 01 | RX J08087+0018 | 0.019 |  | -20.4 | CT 2016-04-08, KP 2017-03-02
08 35 38.8 | -04 05 19 | NGC 2617 | 0.014 | Sy 1.8 | -20.0 | KP 2016-03-09
08 37 26.6 | +40 02 07 | UGC 4498 | 0.024 | Sy? | -20.3 | KP 2016-03-01
08 37 32.7 | +28 42 18 | NGC 2619 | 0.012 | Sy 1 | -20.3 | JKT 2017-01-18, KP 2017-03-02
08 43 38.0 | +50 12 21 | NGC 2639 | 0.011 | LINER | -20.3 | JKT 2017-01-17, KP 2017-03-30
09 19 02.3 | +26 16 11 | NGC 2824 | 0.008 | Sy? | -21.8 | JKT 2017-01-17, KP 2017-03-30
09 20 46.2 | -08 03 21 | MCG -01.24.012 | 0.020 | Sy 2 | -20.8 | CT 2016-04-08, KP 2016-05-03
09 34 43.8 | -21 55 42 | ESO 565-G19 | 0.015 | Sy 2 | -20.2 | CT 2016-02-06, KP 2016-02-11
09 37 41.1 | -22 02 06 | NGC 2945 | 0.016 | Sy | -21.0 | CT 2016-11-11, KP 2017-03-02
09 50 56.5 | -04 59 06 | MCG -01.25.049 | 0.022 | Sy 2 | -20.3 | CT 2016-11-30, KP 2017-04-05
09 51 55.0 | -06 49 23 | NGC 3035 | 0.015 | Sy 1.8 | -20.4 | JKT 2017-01-18, KP 2017-04-18
10 00 40.9 | -31 39 51 | PKS 0958-314 | 0.009 | LINER | -20.7 | KP 2016-03-10
10 02 07.0 | +03 03 27 | IC 588 | 0.023 | Sy 1 | -20.1 | JKT 2017-01-19 KP 2017-04-05
10 37 00.1 | +18 08 08 | NGC 3303 | 0.020 | LINER | -20.0 | JKT 2017-01-19, KP 2017-04-20
10 39 46.3 | -05 28 59 | MCG -01.27.031 | 0.021 | Sy 1 | -20.7 | CT 2016-04-08, KP 2017-04-05
11 07 18.1 | -19 28 17 | NGC 3497 | 0.012 | LINER | -20.0 | CT 2016-02-06, KP 2016-02-11
11 10 48.0 | -28 30 04 | ESO 438-G09 | 0.024 | Sy 1 | -20.4 | CT 2016-04-08, KP 2016-03-02
11 24 02.8 | -28 23 15 | IRAS 11215-2806 | 0.014 | Sy 2 | -20.9 | CT 2016-04-08, KP 2017-03-30
11 27 23.4 | -29 15 27 | ESO 439-G09 | 0.023 | Sy 2 | -20.2 | CT 2016-04-08, KP 2016-04-01
11 35 57.5 | +70 32 07 | NGC 3735 | 0.009 | Sy 2 | -20.2 | KP 2016-03-09
11 40 13.9 | +24 41 49 | NGC 3798 | 0.012 | Sy 1 | -20.4 | JKT 2016-05-05, KP 2016-05-03
11 42 11.3 | +10 16 40 | NGC 3822 | 0.020 | Sy 1.9 | -20.8 | JKT 2016-05-05, KP 2016-05-05
11 43 18.6 | -12 52 41 | NGC 3831 | 0.018 | Sy 2 | -20.7 | CT 2017-02-09, KP 2017-02-27
11 45 11.6 | -09 18 51 | NGC 3858 | 0.019 | Sy 2 | -20.5 | CT 2017-02-09, KP 2017-04-18
11 46 12.2 | +20 23 29 | NGC 3884 | 0.023 | LINER | -21.2 | KP 2016-04-01
11 55 40.1 | -12 01 39 | NGC 3974 | 0.019 | LINER | -20.0 | CT 2017-02-09, KP 2017-03-02
11 55 57.4 | +06 44 56 | NGC 3976 | 0.008 | Sy 2 | -20.4 | KP 2016-02-07
Table 3: continued

VCV AGN sample  $\alpha_{2000}$ $\delta_{2000}$ Name $z$ AGN type $M$ Observed
12 04 43.4 +31 10 38 UGC 7064 0.024 Sy 1.9 -20.2 JKT 2017-01-19, KP 2017-04-18
12 12 18.9 +29 10 46 NGC 4169 0.013 Sy 2 -20.6 JKT 2016-05-6, KP 2016-05-05 12
21 49.0 -24 10 05 ESO 506-G04 0.013 LINER -21.1 CT 2017-01-28, KP 2017-03-30
12 26 16.1 -07 40 50 NGC 4404 0.018 LINER -21.2 JKT 2017-02-04, KP 2017-04-20
12 38 54.6 -27 18 28 ESO 506-G27 0.024 Sy 2 -20.2 CT 2016-04-26, KP 2017-04-05
12 41 44.7 +35 03 45 NGC 4619 0.023 Sy 1.0 -21.5 KP 2016-03-10 12 51 31.8 -26
27 07 MCG -04.30.029 0.011 LINER -20.1 CT 2017-01-28, KP 2017-02-27 12 52 36.4
-21 54 38 ESO 575-IG016 0.023 Sy 2 -20.1 CT 2017-02-29, KP 2017-04-06 13 01
22.8 -30 56 06 NGC 4903 0.016 Sy 2 -21.7 CT 2017-01-28, KP 2017-03-30 13 06
17.3 +29 03 47 NGC 4966 0.024 LINER -20.8 JKT 2017-02-4, KP 2017-04-18 13 08
41.8 -24 22 57 PKS 1306-241 0.014 Sy 2 -20.0 CT 2017-01-28, KP 2017-04-06 13
10 17.3 -07 27 15 MCG -01.34.008 0.022 Sy 2 -20.3 JKT 2017-02-04, KP
2017-04-18 13 19 50.0 -27 24 36 NGC 5078 0.007 LINER -20.6 CT 2017-01-31, KP
2017-04-05 13 22 24.5 -16 43 42 MCG -03.34.064 0.017 Sy 1 -20.4 JKT
2017-03-06, KP 2017-04-06 13 24 50.3 -30 18 28 NGC 5124 0.014 LINER -20.8 CT
2017-01-31, KP 2017-03-30 13 33 26.0 -34 00 56 ESO 383-G18 0.013 Sy 1.8 -20.1
CT 2017-01-31, KP 2017-04-06 13 36 08.5 -08 29 52 NGC 5232 0.023 Sy? -21.4 JKT
2017-02-04, KP 2017-03-02 13 39 22.5 -32 13 27 MCG -05.32.059 0.024 LINER
-21.0 CT 2016-08-04, KP 2017-03-02 13 40 19.7 -23 51 28 NGC 5260 0.022 Sy 2
-21.3 CT 2016-04-26, KP 2017-02-27 13 47 40.1 -30 56 21 NGC 5292 0.015 LINER
-21.1 CT 2016-04-26, KP 2016-05-03 13 50 36.0 +33 42 17 NGC 5318 0.014 Sy?
-20.4 JKT 2016-05-05, KP 2016-06-17 13 53 26.7 +40 16 59 NGC 5353 0.008 Sy?
-21.2 JKT 201608-31, KP 2016-06-17 13 58 38.0 +37 25 28 NGC 5395 0.012 Sy 2
-20.3 KP 2016-04-01 14 01 58.4 -25 32 25 ESO 510-G46 0.021 Sy 2 -20.4 CT
2016-04-26, KP 2017-04-05 14 13 38.9 +07 39 33 NGC 5514 0.024 LINER -21.4 JKT
2016-05-06, JKT 2021-03-18 14 28 31.9 +27 24 31 NGC 5635 0.014 LINER -20.0 JKT
2016-05-03, KP 2016-05-03 14 47 33.5 -20 28 02 MCG -03.38.010 0.023 LINER
-20.0 CT 2016-08-05. KP 2016-07-02 15 06 50.7 -14 34 18 NGC 5849 0.024 LINER
-20.1 CT 2017-03-30, KP 2017-03-30 15 15 23.2 +55 31 05 NGC 5905 0.011 H II
-20.6 KP 2016-02-27 15 46 16.5 +02 24 56 NGC 5990 0.013 Sy 2 -20.4 KP
2016-05-05 16 32 32.1 +82 32 17 NGC 6251 0.024 Sy 2 -21.1 JKT 2016-05-06, KP
2016-06-17 16 41 27.8 +57 47 01 NGC 6211 0.020 Sy 2 -20.2 KP 2016-02-27 16 52
58.9 +02 24 01 NGC 6240 0.024 LINER -20.7 KP 2016-04-30 17 20 29.5 +50 22 37
NPM1G +50.0375 0.024 Sy 2 -20.8 JKT 2016-06-04, KP 2016-06-17 20 17 06.3 -12
05 51 MCG -02.51.008 0.020 LINER -21.2 CT 2016-08-03, KP 2019-10-01 20 27 46.0
-03 04 37 NGC 6915 0.019 LINER -22.0 CT 2016-08-05, KP 2016-07-02 20 34 31.4
-30 37 29 PGC 64989 0.019 Sy 1 -21.1 CT 2016-08-05, KP 2016-10-07 20 35 56.3
-25 16 48 NGC 6936 0.019 LINER -20.6 CT 2016-08-05, KP 2016-11-09 20 46 37.2
-23 37 49 MCG -04.49.001 0.020 LINER -20.0 CT 2016-08-05, KP 2016-10-09 20 47
34.1 +00 24 41 UGC 11630 0.012 Sy 2 -20.7 KP 2015-10-13, KP 2016-07-02 22 00
21.6 -13 08 50 IC 1417 0.018 Sy 2 -20.3 KP 2015-11-12 22 09 07.7 -27 48 34 NGC
7214 0.023 Sy 1.2 -20.3 KP 2015-11-02, KP 2016-07-02 22 12 31.5 +38 40 55 UGC
11950 0.020 LINER -20.2 JKT 2016-08-31, KP 2016-09-05 22 27 05.9 +36 21 41 UGC
12040 0.021 Sy 1.9 -20.1 JKT 2016-098-31, KP 2016-09-05 22 31 20.7 +39 21 30
UGC 12064 0.017 Sy -20.2 JKT 2016-08-31, KP 2016-10-07 22 36 03.5 +33 58 33
NGC 7319 0.022 Sy 2 -20.2 KP 2015-11-12 23 03 15.6 +08 52 26 NGC 7469 0.017 Sy
1.5 -20.9 JKT 2016-08-31, KP 2017-09-19 23 17 12.1 -06 54 42 NGC 7596 0.024 Sy
-20.0 JKT 2016-09-01, KP 2016-10-07 23 18 16.3 +06 35 09 NGC 7591 0.017 Sy
-20.8 JKT 2016-09-01, KP 2016-11-09 23 18 21.8 -04 24 56 NGC 7592E 0.024 H II
-20.1 V JKT 8/31/16 23 18 22.1 -04 24 59 NGC 7592W 0.024 Sy 2 -20.9 KP
2015-11-02 23 19 25.3 +05 54 21 IC 1481 0.020 LINER -20.1 JKT 2016-09-1, KP
2016-11-15 23 29 03.9 +03 32 00 NGC 7682 0.017 Sy 1 -20.0 JKT 2016-09-01, KP
2016-10-07 23 30 47.7 -13 29 08 IC 1495 0.021 Sy 2 -21.7 JKT 2016-09-27, KP
2016-11-25 23 41 47.3 -03 40 02 MCG -01.60.021 0.023 LINER -20.5 JKT
2016-09-27, KP 2016-11-09 23 56 03.9 -00 59 18 IC 1515 0.023 Sy 2 -20.6 JKT
2016-09-28, KP 2016-11-09

###  2.2 Imaging observations

Our imaging observations mostly spanned the period 2015 October - 2017
November, with a few later repeats for confirmation or improved image quality.
We used primarily the SARA 1-m telescope at Kitt Peak, Arizona for the
narrowband images, aiming for a single one-hour exposure wherever possible.
Since the image quality is generally better at the JKT than at the SARA
telescope on Kitt Peak, we re-observed confirmed EELRs with that setup for a
better view of their structures once a matching filter and autoguiding were
available there. The field sizes were 14.5 (Kitt Peak) or 11.6 (JKT) arcmin
square, with pixel scales 0.44 and 0.34″per pixel respectively. For a centered
target, the inscribed circle of complete coverage in Kitt Peak images projects
to radii 77–250 kpc at our sample’s minimum and maximum redshifts. As in the
Galaxy Zoo survey (Keel et al., 2012), our primary interest is in EELRs
projected more than 10 kpc from the AGN (distant EELRs), both to provide long
differential light-travel times to probe the history of the AGN luminosity,
and to examine gas outside the normal interstellar medium (ISM) of the host
galaxy.

Exposures in the F510 filter for redshifted [O III] emission were typically 1
hour, at Kitt Peak or the JKT. The narrowband Kitt Peak images were often
affected by internal stray light (probably due to an LED associated with the
fiber spectrograph, since fixed) which gave a diagonally sloping background
across the field of view. While the problem existed, it was sometimes dealt
with using similar exposures obtained with the telescope in the same position
on a cloudy night, with the CCD shutter open but mirror cover closed. A few
objects in the Toomre sample were sufficiently affected to produce flat-
fielding artifacts. In any case, median filtering with large kernel sizes
generally suppressed the background structure enough to seek features of the
sizes we sought. A few images were repeated once the problem was repaired.

Continuum subtraction used the Johnson $V$ band, sometimes at a different SARA
telescope than the narrowband image to use time more efficiently; for a few
galaxies in the Toomre sample, we used $V$ images obtained with the 1.1-m Hall
telescope at Lowell Observatory by Keel & Wu (1995). The goal was for the
broadband image to have at least twice the signal-to-noise ratio (SNR) on the
galaxy as in the narrowband image, to minimize loss of SNR in continuum
subtraction. The quality of this subtraction was generally limited by PSF
matching; seeing, guiding behavior, and focus shifts sometimes made the
subtraction unreliable very close to galaxy nuclei as well as star images.
Since our main goal was detecting large-scale emission features (targeting
clouds more than 10 kpc in projection from the nucleus), this was more of an
aesthetic than scientific problem. Using the conservative approach of
measuring the compromised regions around stars brighter then the galaxy
nuclei, the largest such region has a radius of 10″, with a sample median of
6″. Fig. 1 illustrates the depth of the data, comparing broad-band $V$ and
continuum-subtracted [O III] images of the well-known EELRs in NGC 5252 using
data matching our survey exposures.

Figure 1: Kitt Peak and JKT comparison images of the well-known EELR features
in NGC 5252, illustrating the depth of our survey imaging. From Kitt Peak, the
F510 image is the mean of 2 30-minute exposures, while the $V$ image is the
mean of 2 10-minute exposures; from the JKT, we have a single one-hour
narrowband exposure and 2 10-minute $V$ exposures. A $3\times 3$-pixel median
filter was used to smooth and reduce charged-particle artifacts in the
continuum-subtracted [O III] images. Images are shown in an offset-logarithmic
scale closely approximating the usual SDSS image displays. Effects of filter
mismatch are seen in the slight oversubtraction of the smooth galaxy light in
its inner regions, while the [O III] structure is a good match for that seen
with narrower filters (Tadhunter & Tsvetanov 1989, Tsvetanov et al. 1996)
Emission features are detected to a radius 51″(27 kpc). NGC 5252 itself was
not included in the TELPERION sample because its listed absolute magnitude in
VCV is slightly too faint (-19.6, while our sample limit was -20.0). North is
at the top and east to the left.

World-coordinate system (WCS) metadata for each image were determined by the
astrometry.net web service (Lang et al. 2010), separately for continuum and
narrowband images when they were from different telescopes or observing
sessions. The $V$ images were resampled to match the nominal pixel grids of
the narrowband images. A simple Gaussian matching was initially used to
account for PSF differences, with small pixel-level shifts sometimes needed
when one PSF had an asymmetric core. Relative scaling of the broadband image
started using count rates of foreground stars, and was iteratively changed for
better cancellation of galaxy light (sometimes requiring judgment as to what
parts should be best cancelled when colour gradients are significant). We
applied various kinds of smoothing - median over $3\times 3$ or $5\times 5$
pixels, Gaussian blurring - to seek fainter emission in the presence of
residual cosmic-ray effects. Emission from H II regions in spiral arms,
especially in the outer parts of some hosts, was frequently detected, and is
characterized by alignment along the arms and concentration to bright spots,
in contrast to the more diffuse or filamentary appearance of EELR emission.

Somewhat unexpectedly, having long times elapse between narrowband and
continuum images led to multiple asteroid trails which could mimic emission-
line features in difference images, similar to the apparent jet of NGC 3303 in
the Arp (1966) atlas which could not be reobserved and was eventually
identified as an unfortunately placed and timed asteroid (Kanipe, 2010). For
the record, we recorded asteroid (59585) in front of NGC 3822 on 2016-05-5,
(43336) near NGC 7591 on 2016-11-09, and (20865) near the EELR of NGC 5514
during a Kitt Peak reobservation on 2019-05-19. We caution other galaxy
observers.

Comparison with detections and nondetections in followup spectroscopy
indicates that our detection limit for diffuse [O III] $\lambda 5007$ emission
regions is at a typical surface brightness $1.0\times 10^{-16}$ ergs cm-2 s-1
arcsec-2. This is about four times deeper than the SDSS-based selection used
in the Galaxy Zoo survey reported by Keel et al. (2012), where the peak
surface brightnesses even when averaged over the tabulated $2\times
6.2$″regions are all greater than $3.5\times 10^{-16}$ ergs cm-2 s-1 arcsec-2.

Given the small incidence of distant EELRs, false positives will be much more
frequent than false negatives near the detection threshold. After initial
experience with spectroscopic followup. we first repeated the narrowband
imaging observations of candidate diffuse [O III] objects, usually with longer
exposures, and carried objects through for spectroscopy only when confirmed in
this way. This proved to be effective - all of the [O III] objects confirmed
by repeat imaging were also confirmed as emission-line regions (although not
necessarily AGN-ionized EELRs) when spectroscopically observed. This proved to
be a powerful strategy when the observations were spread over many observing
sessions, as ours were, allowing this kind of feedback and refinement.

###  2.3 Tunable-filter imaging

Several of the most interesting systems from this survey were observed in [O
III] and the adjacent continuum using tunable-filter systems. Data for IC 1481
and NGC 5514 used the tunable-filter imaging mode of the MaNGaL system
(Moiseev, Perepelitsyn, & Oparin, 2020) at the 2.5m
telescope444http://lnfm1.sai.msu.ru/kgo/main.php of the Caucasus Mountain
Observatory (Shatsky et al. 2020, Potanin et al. 2017) operated by the
Shternberg Astronomical Institute. These data improved both the typical image
quality and sensitivity over our SARA 1m images. These observations were done
with the scanning Fabry-Perot interferometer for a FWHM resolution near 13 Å.
For NGC 5514 (observed on 2018-04-11), on-band and off-band exposures were
each 1600 seconds; for IC 1481, observed on 2017-11-17, the on-band exposure
was 4800 seconds accompanied by 2400 seconds in the continuum band.

Similarly, NGC 7679 was observed at the Russian 6-m telescope in the imaging
mode of the SCORPIO-2 focal reducer (Afanasiev & Moiseev, 2011)on 2017-11-25.
We used two narrowband (FWHM$\approx 35$ Å) filters centered on redshifted
[OIII] $\lambda$5007 and adjacent continuum, with total exposures 1200 s in
the each filter. The pixel scale was 0.35’″and field of view was 6′.

###  2.4 Archival Hubble Space Telescope data

A Hubble Space Telescope image exists for NGC 235, a single 500s exposure from
snapshot program 5479 (Malkan, Gorjian, & Tam, 1998). After interactive
patching of the numerous cosmic-ray events in this region of the image and
smoothing, part of the EELR is (just) detected. The response curve of the
F606W filter on WFPC2 indicates that this detection is about equally sensitive
to the wavelengths of [O III] and H$\alpha$+[N II].

###  2.5 Long-slit spectroscopy

To explore the nature of extended emission-line structures, and identify which
ones are photoionized by AGN, we obtained followup optical spectroscopy. Most
of these observations were obtained with the SCORPIO (Afanasiev & Moiseev,
2005) and SCORPIO-2 (Afanasiev & Moiseev, 2011) multimode focal-reducer
systems at the 6-meter telescope (BTA) of the Special Astrophysical
Observatory, Russian Academy of Sciences, supplemented by several observations
using the new ADAM multimode instrument at the 1.6-meter telescope of Sayan
Observatory, Siberian Branch of the Russian Academy of Sciences (Afanasiev et
al., 2016). Each instrument uses reimaging optics and a volume-phase
holographic grating (Barden et al., 2000) for high efficiency and an
appropriate match between pixel and image scales. The spectral range and
spectral resolution were 3650–7750 Å and $\approx 10$ Å for the SCORPIO
observations of IC 1481, 3800– 8400 and $\approx 4$ Å for NGC 7679, and about
3600-7200 Å and 5 Å for the other SCORPIO-2 data. The ADAM spectra were
obtained in the range 3650-7300 Å with a typical resolution 10–15 Å depending
on the slit width.

The spectroscopic observations are listed in Table 4. In most cases, the slit
was oriented to pass through the galaxy nucleus and distant [O III] feature.
The companion south of NGC 2329 was observed; weather did not allow proper
intensity calibration for this spectrum as well as NGC 3798, although the
important line ratios for classification are scarcely affected. For all other
data, spectrophotometric calibration used standard stars observed each night.
For NGC 235, at declination $-23^{\circ}$ and necessarily observed at large
air mass, 2-pixel on-chip binning along the slit was used to increase the SNR
of the spectrum.

Table 4: Summary of long-slit spectra. Slit widths are in arcseconds, position angles are in degrees north through east, and exposure durations are in seconds. All slit locations went through the galaxy nuclei except for NGC 235, acquired best using the SW companion NGC 232, and the NGC 5514 cloud spectrum, which was offset to follow the long axis of the distant emission-line structure. Object | PA∘ | Date | Exp | Slit | Instrument
---|---|---|---|---|---
IC 1481 | 315 | 2017-08-16 | 2400 | 1.0″ | SCORPIO
MCG -01-25-049 | 130 | 2018-03-13 | 1800 | 1.0″ | SCORPIO-2
Mkn 373 | 17 | 2017-12-09 | 2700 | 1.0″ | SCORPIO-2
NGC 235 | 54 | 2020-11-10 | 2400 | 1.5″ | SCORPIO-2
NGC 1050 | 195 | 2018-01-25 | 2400 | 1.0″ | SCORPIO-2
NGC 2329 S | 0 | 2017-12-14 | 2400 | 1.0″ | SCORPIO-2
NGC 2617 | 155 | 2018-03-12 | 2700 | 1.0″ | SCORPIO-2
NGC 5514 nuc | 88 | 2018-03-13 | 3600 | 1.0″ | SCORPIO-2
NGC 5514 cloud | 10.5 | 2020-03-02 | 6000 | 1.0″ | SCORPIO-2
NGC 5635 | 55 | 2018-03-13 | 4500 | 1.0″ | SCORPIO-2
NGC 7679 | 90 | 2017-11-25 | 6000 | 0.5″ | SCORPIO-2
Mkn 373 | 17 | 2017-03-19 | 3600 | 3.0″ | ADAM
NGC 2617-1 | 334 | 2017-03-21 | 1200 | 2.0″ | ADAM
NGC 2617-2 | 333 | 2017-03-27 | 1200 | 2.0″ | ADAM
NGC 3798 | 264 | 2017-03-18 | 5400 | 3.0″ | ADAM
NGC 5514 | 88 | 2017-03-21 | 2400 | 3.0″ | ADAM
NGC 6251 | 170 | 2017-03-16 | 6000 | 2.0″ | ADAM

We augment our long-slit spectrum of the cloud in NGC 235 with the optical
spectrum of its AGN obtained by the BAT AGN Spectroscopic Survey (BASS,
distinct from the BASS portion of the Legacy Survey) collaboration (Koss et
al., 2017), and adopt their emission-line fit parameters.

Our long-slit spectra allowed a closer examination of some ambiguous AGN types
from the VCV catalog. NGC 2617 is a Sy 1, NGC 5635 a LINER, and NGC 1050 shows
nuclear star formation. A candidate emission cloud near NGC 6251 was not
detected spectroscopically, indirectly showing the surface-brightness limits
of our [O III] search. Most of the candidate EELR clouds are shown to be star-
forming regions; the BPT classification allows the possibility that the
emission 11″(3.3 kpc) NW of the nucleus in NGC 2617, coincident with a segment
of spiral arm, is AGN-ionized.

##  3 Results

###  3.1 Extended and distant emission-line clouds

We were able to obtain spectra of all plausible candidate distant EELRs, some
of which proved to be other kinds of emission-line features. Our primary
classification tools for ionization mechanisms were the traditional BPT
diagrams (Baldwin et al., 1981), incorporating revised region boundaries from
Kewley et al. (2001) and Kauffmann et al. (2003). This approach was further
augmented by the He II/H$\beta$ ratio as described by Knese et al. (2020), to
allow identification of AGN-ionized gas even at low metallicity, where the
strong-line ratios approach those of the starburst branch of the primary BPT
diagram (Groves, Heckman, & Kauffmann, 2006).

We identified two new distant AGN-ionized EELR clouds, near NGC 235 and NGC
5514, and recovered the previously-known cloud in NGC 7252 (Schweizer et al.,
2013). The feature in NGC 235 is marginally shown (in a single contour) by
Mulchaey, Wilson, & Tsvetanov (1996), but not discussed; they analyze the [O
III] blobs 10″SW of the nucleus. The EELR in NGC 7252 is centered about 5 kpc
from the nucleus in projection and barely extends outward to our minimum
search radius of 10 kpc, so we do not always include it in the sample
comparisons which follow.

Fig. 2 shows the cloud in the NGC 235 system, including detection of a portion
of the structure in an archival HST image. The emission-line cloud found in
NGC 5514 is especially distant from the nucleus - 135″(75 kpc) in projection.
The cloud location and structure are shown from three different data sets in
Fig. 3. In this case, the MaNGaL [O III] data suggest additional emission
closer to the AGN in the same direction as the distant cloud, projected at a
radius of 48″(27 kpc).

Figure 2: The emission-line cloud in NGC 235 (larger galaxy at top left). The
figure compares a $V$-band image to show the stellar structure of this
interacting system, a view of the cloud as seen in the F510 narrowband image,
and the archival HST WFPC2 F606W image which has been partially corrected for
cosmic-ray events using the interactive imedit tool within IRAF. Each image is
shown with on an offset-log intensity scale, similar to sinh mapping. The HST
image has been smoothed with a Gaussian of $\sigma=0.24$″for display, and the
F510 image has been smoothed with a $3\times 3$-pixel (1.2″) median filter.
North is at the top and east to the left. The cloud is marked by a circle of
radius 18 ″in the middle panel. The spectrograph slit used for the data in
Fig. 4 passed through the nucleus of NGC 232, at lower right, and the cloud,
along position angle $54^{\circ}$.  Figure 3: The distant emission-line cloud
in NGC 5514. The figure compares an offset-log-scaled $V$-band image to show
the stellar structure of this merger with three linearly-scaled views of the
cloud - from Kitt Peak (KP), the Jacobus Kapteyn Telescope (JKT) and the
MaNGaL system on the Caucasus 2.5m (MaNGaL). Continuum-subtracted data are
denoted as [O III]. The cloud location is shown on the two narrowband SARA
images, within circles of radius 18 ″.

Table 5 summarizes the spectroscopic results for the new objects. As in Knese
et al. (2020), errors in line flux were derived from Gaussian fits to groups
of emission lines, including fictitious lines placed in continuum regions,
constrained to have the same width as [O III] (in the blue) or H$\alpha$ (for
red lines). He II is weak, and while formally detected at levels
2.1-2.8$\sigma$. These values give a realistic estimate of the S/N ratio of
the detections, since the expected wavelength is independently known (in both
cases the peak appears within 1 Å of the expected wavelength). Therefore,
there is no additional uncertainty such as exists when a peak is sought within
a large range of noisy data (the “look-elsewhere effect"). As shown in Fig. 5,
these fall into the AGN-ionized region of the primary BPT line-ratio diagrams,
an assessment strengthened by the detections of He II emission at levels
characteristic of AGN photoionization. The distant cloud in NGC 5514 is close
to the maximum-starburst boundary in the first BPT diagram, but the strength
of He II is consistent with AGN rather than hot stars as photoionizing
sources. This interpretation fits with the spatial offset between the
emission-line cloud and starlight in the adjacent tidal tail, and lack of any
obvious star-forming complexes nearby.

Detection of these two distant EELR clouds is the primary result of this
study. We consider in section 5 the implications of their properties of the
luminosity history of these AGN, and in section 6 we update the statistics on
occurrence of distant EELRs in tidally distorted hosts, strengthening the
connection between the two phenomena.

Figure 4: BTA-SCORPIO spectra of the two newly identified distant EELRs from this survey. They are summed over regions $2\times 8.6$ ″2 for NGC 235 and $1\times 17.5$″2 for NGC 5514. Wavelengths are in the observed frame; NGC 235 was observed at large airmass, so the telluric B band near 6867 Å is deep and could not be well corrected, compromising measurement of the [S II] doublet. Figure 5: Strong-line BPT diagrams, showing the line ratios from the summed spectra of the two new EELRs as in Fig. 4. In the first diagram, the boundary between ionization by hot stars and AGN is shown (as given both by Kewley et al. 2001, with quoted systematic uncertainty shown by the dashed curves, and by Kauffmann et al. 2003). Gray symbols indicate the averaged spectra of EELR clouds found more than 10 kpc from the nuclei in the Galaxy Zoo survey (Keel et al., 2012), and the cloud near Mkn 1 (Knese et al., 2020). The two newly-found clouds are indicate by black symbols, larger for the more-distant cloud in NGC 5514, with uncertainty ranges from Table 5, some lost within the plotted symbols. Uninformative limits for NGC 235 are not shown. To reduce clutter, we do not plot the LINER/Seyfert boundary, often taken at log [O III]/H$\beta=0.30$, which passes below all these data points. [N II] denotes the 6584-Å line, [O I] indicates the 6300-Å line, and [S II] indicates the sum of the 6717 and 6731-Å lines. Table 5: EELR emission-line properties Property | NGC 235 | NGC 5514
---|---|---
Center RA (2000) | 0:42:49.71 | 14:13:29.88
Center Dec (2000) | -23:33:05.1 | +7:39:23.4
[O III] $\lambda 5007$ flux in slit (ergs cm-2 s-1) | $6.75\times 10^{-16}$ | $9.82\times 10^{-16}$
[O III] $\lambda 5007$/H$\beta$ | $8.06\pm 0.83$ | $4.03\pm 0.4$
He II $\lambda 4686$/H$\beta$ | $0.21\pm 0.10$ | $0.22\pm 0.08$
[N II] $\lambda 6583$/H$\alpha$ | $0.143\pm 0.026$ | $0.48\pm 0.02$
[O I] $\lambda 6300$/H$\alpha$ | $<0.042$ | $0.053\pm 0.006$
[S II] $\lambda\lambda 6716,6731$/H$\alpha$ | — | $0.46\pm 0.03$

##  4 Ionized gas within the AGN hosts - EELRs and ionization cones

Our images reveal four systems with extensive emission-line structures having
most or all of the detected emission smaller than our adopted cutoff for
distant clouds at 10 kpc projected radius, but still AGN-related and spanning
much of the host-galaxy ISM. Even with PSF-related artifacts at many nuclei,
these regions were not subtle detections in the survey imaging data, so they
are not just the tail of a continuum of EELR sizes and surface brightness. We
identify likely ionization cones in IC 1481 (UGC 12505, CGCG 406-064), recover
the bright off-nuclear feature in ESO 362-G08 noted by Mulchaey, Wilson, &
Tsvetanov (1996), and detect the extensive ionized regions in NGC 7679 and NGC
5514. For completeness, we compare these emission regions, those whose
combination of surface brightness and extent led to their detection in our
survey images outside the regions compromised by PSF mismatch (zones of median
radius 6″). Their [O III] morphologies are compared, along with the starlight
continuum, in Fig. 6. We also consider Mkn 938 (NGC 34), where our survey data
are sensitive enough to detect the ionized emission 18″NW of the brighter
nucleus (7.5 kpc), but not the emission along the tidal tail NNE, within the
uncertainties associated with continuum subtraction. The optical spectrum of
the nucleus (Kim et al., 1995) shows low-ionization LINER emission and a
poststarburst stellar population, while integral-field mapping (Afanasiev et
al., 2020) shows a powerful outflow and suggest a mix of starburst and AGN
contributions, but not AGN ionization farther out.

Figure 6: Montage of extended emission structures within AGN hosts, from the
indicated instruments. The left panels show continuum-subtracted [O III]
$\lambda 5007$ emission, while the right panels show a gray-scale mapping of
the adjacent continuum, with sparse contours of [O III] superimposed to show
their spatial relationship. The intensity scales have been adjusted to show
structure over wide dynamic ranges. Each shows emission over narrow enough
angular spans about the AGN to be candidate ionization cones, even around the
low-ionization nuclei of NGC 5514 and IC 1481. The S7 data for ESO 362-G08
have been sampled to a smoother grid of $3\times 3$ subpixels for display.
North is at the top as shown except for ESO 362-G08, where it is to the right.

####  4.0.1 ESO 362-G08

Due to its declination, we obtained no new spectroscopic data for ESO 362-G08,
but long-slit data crossing the bright off-nuclear clouds were analyzed by
Fraquelli, Storchi-Bergmann, & Binette (2000), and integral-field
spectroscopic data were released as part of the S7 survey (Thomas et al.,
2017). The extranuclear emission-line structure is dominated by a bright cloud
to the NE, with internal velocity structure. The S7 data also show a dimmer
cloud to the WNW; the two clouds might form parts of a set of ionization cones
if their boundaries are defined by the location of otherwise neutral gas. The
NE cloud has line ratios putting it firmly in the AGN region of the usual BPT
diagrams, as well as He II/H$\beta=0.47$.

####  4.0.2 NGC 7679

While the search results for distant clouds near NGC 7679 were reported in
Knese et al. (2020), we discuss its internal gas emission here since these
were not considered in the earlier paper and we have new relevant data. The
nucleus itself may be better described as a LINER than a Seyfert 2 based on
the usual BPT diagrams, in particular having [O III]/H$\beta<1$ (Fig. 7), but
He II $\lambda 4686$ is relatively strong not only at the nucleus but
throughout much of the extended emission region. Like IC 1481, the stellar
population shows post-starburst or quenching signatures, with the higher-order
Balmer lines prominent in absorption. The [O III] structure is elongated, but
if it includes ionization cones, there must be a substantial additional
contribution near the nucleus broadening its central area.

Figure 7: BTA SCORPIO-2 spectra of the low-ionization nuclei in IC 1481 and
NGC 7679. Each is summed by 2.5″along the slit. Important spectral features
are marked; the high-order Balmer lines, seen in absorption from a
poststarburst population of stars, are marked by the linked indicators at
lower left.

Our slit serendipitously crossed a background spiral galaxy, SDSS
J232848.57+033042.2, projected 286″due east of the nucleus, at redshift
$z=0.051$, whose structure is well resolved in an archival HST image from
program 15446.

####  4.0.3 IC 1481

IC 1481 is listed as showing extraplanar diffuse ionized gas based on the
CALIFA IFU survey by López-Cobá et al. (2019). Our BTA SCORPIO spectrum (Fig.
7) shows LINER emission at the nucleus, much like NGC 1052, with unusually
strong [N I] $\lambda 5199$ emission. The stellar population shows post-
starburst signatures, with strong absorption in the higher-order Balmer lines.
Higher ionization levels are seen off the nucleus. While the MaNGaL emission-
line image has better spatial sampling and image quality in [O III], the
CALIFA data are in good agreement with the overall emission-line structure,
and indicate that both H$\alpha$ and [O III] occur in similar regions (which
may mean that the structure we see is limited by the presence of neutral gas
rather than ionizing photons). The CALIFA radial-velocity measures show little
systematic motion across the emission structures.

The morphology of the emission-line structure can be interpreted as a pair of
ionization cones with axes east-west, crossing very close to the continuum and
H$\alpha$ peak, so IC 1481 may join NGC 1052 (Dopita et al., 2015) and NGC
2442 (da Silva et al., 2021) as LINERs with ionization cones. These objects
implicitly show that AGN with low radiative output can still be surrounding by
an obscuring torus, and may be fruitful in investigations of how the AGN
output and surrounding gas structure interact to produce low-ionization line
emission coupled with significant He II emission, constrained by the X-ray
spectra.

The MaNGAL data were obtained after our long-sit spectrum, so the spectrum was
aligned with the brightest off-nuclear knot to the northwest (including the
spectrum of scattered light from the 8th-magnitude K star HD 219905 to the SE)
. As shown in Fig. 6, IC 1481 shows merger or post-merger signatures; even the
faint features extending beyond the MaNGaL continuum field are detected in
both SARA and Legacy Survey images as well.

####  4.0.4 NGC 5514

Lípari et al. (2004) have used integral-field spectroscopy to analyze the
kinematics and ionization of the gas in the central regions of NGC 5514 in
great detail, finding multiple expanding (super)bubbles, and attributing the
bulk of the ionization to the accompanying shocks. Our long-slit spectrum
(Fig. 8) shows complex emission structure, with some very distant components
where the slit crosses tidal tails, and double line profiles in a pattern
suggesting an expanding shell on the western side. We examine this structure
through multicomponent Gaussian fits to emission lines and line blends, done
with the splot task in IRAF, in summed 3-pixel (1.05″) steps, after reducing
the effects of absorption features from starlight via subtracting a template
spectrum at the radial velocity derived from cross-correlation at each point
along the slit. These results are illustrated for a region with the largest
velocity separation in Fig. 10. The line profiles by themselves are double-
peaked when the separation is large compared to the line width, so what
happens at locations where Fig. 8 shows the lines to be just barely splitting
is poorly constrained. In assessing the gas kinematics, we omit lines where
components of adjacent lines (such as H$\alpha$ and one of the adjacent [N II]
features) nearly overlap at a particular slit location, since the measured
wavelength will reflect blending. H$\beta$, [O III] $\lambda 5007$, and [O I]
$\lambda 6300$ are free of this ambiguity. Fig. 9 compares mean values of the
emission-line velocities of each component with cross-correlation stellar
velocities along the slit, providing a complementary view to the maps shown by
Lípari et al. (2004). The blue- and redshifted components are separated by as
much as 650 kms-1, and the double line profiles are roughly symmetric about
the stellar velocities. The redder component departs from the stellar velocity
systematically at points to the west of about 9″east of the nucleus,
suggesting that this large expanding structure is seen substantially projected
across the core. The slopes of the emission-line velocities compared to the
stellar kinematics further suggest that this component is expanding away from
us. The emission-line ratios, and implied mix of ionization sources, change in
ways that correlate only poorly with location (Fig. 11).

Figure 8: Long-slit spectrum of the central regions of NGC 5514, along
position angle (PA) 88∘. This section at red wavelengths shows the patchy
elliptical traces of an expanding shell west of the nucleus. The velocity
scale applies strictly at H$\alpha$.  Figure 9: Spectroscopic properties of
NGC 5514 along our slit in position angle 88∘. Top, emission-line velocities,
averaging over lines detected and unconfused by B-band absorption or other
velocity components at each position, with the internal standard deviation
shown at each position (filled circles and error bars), compared to the cross-
correlation stellar velocities (shown as $\pm 1\sigma$ error ranges). For
measurement, data were summed along 3-pixel (1.05″) regions along the slit.
Lower panel, important emission-line ratios along the slit, plotted as the
decimal logarithm as used in the BPT diagrams. Virtually all these locations
fall within the “LINER" regions of the diagrams, interpreted by Lípari et al.
(2004) as showing a dominant role for shock ionization in most of the main
body of NGC 5514. We observe no increased ionization near the optically
brighter northwestern nucleus, which is at the origin of the angular scale
shown. Figure 10: Sample Gaussian decomposition of the blended H$\alpha$-[N
II] emission lines for a 1.05″region near the greatest velocity separation of
components, as used in Fig. 9. The components were constrained to have the
same width, and each velocity component of the weaker [N II]$\lambda 6548$
line was constrained to have ratio 1:3 in photon rate to the stronger $\lambda
6583$ line. The bluer H$\alpha$ and redder [N II] components nearly overlap at
this location, as do components of the redder [S II] doublet (making the [S
II] line ratio in the components almost unconstrained). The data are offset
upward for clarity; curves show individual model components and their sum. The
residuals from this fit are shown at the bottom, illustrating the limitations
of a Gaussian line profile in matching the line shapes in detail. Figure 11:
BPT diagrams of [O III]/H$\beta$ versus [N II]/H$\alpha$ and [O
I]/H$\alpha$for the long-slit data in Fig. 9, color-coded with location along
the slit according to the color bar at the top. The relative contribution of
shocks (or AGN emission) increases broadly going to the west side of the
system, with large local scatter. Regions denoting emission from star-
photoionized H II regions and AGN (or shocks) are indicated.As before, [N II]
denotes the 6583-Å line and [O I] indicates the 6300-Å line.

Table l6 compares the properties of these four extended emission regions,
measured from our data and the archival results from the S7 survey (Thomas et
al., 2017). We attempt to separate the [O III] component from the “normal"
narrow-line region (NLR) spatially, taking the NLR to be the central peak
similar in scale to the PSF. For these galaxies, the total [O III] luminosity
is dominated by the EELR, providing additional examples of the situation noted
by Hainline et al. (2014) and Hainline et al. (2016) where large regions of
the host galaxies need to be included in measurements of emission lines, such
as [O III], to derive an accurate total luminosity for the AGN. This holds
even for the relatively low AGN luminosities we deal with here. (These
luminosities include foreground Galactic extinction values following Schlafly
& Finkbeiner 2011). For ESO 362-G04, some of the cloud to the NE may fall
outside the S7 integral-field data and be excluded from the listed [O III]
flux. In contrast, much of the emission in both regions in NGC 5514 can be
attributed to shocks associated with outflows, and the central emission is
weak and simply summed within 3″of the continuum peak. Both [O III]
luminosities in NGC 5514 are therefore upper limits to the AGN contribution.

Table 6: EELR emission-line properties. Object | ESO 362-G04 | IC 1481 | NGC 5514 | NGC 7679
---|---|---|---|---
AGN type | Sy 2 | LINER | LINER | Sy 2
$z$ | 0.016 | 0.020 | 0.024 | 0.017
[O III] $\lambda 5007$ NLR flux (ergs cm-2 s-1) | $3.0\times 10^{-15}$ | $2.6\times 10^{-15}$ | $1.1\times 10^{-15}$ | $6.6\times 10^{-14}$
[O III] $\lambda 5007$ EELR flux (ergs cm-2 s-1) | $2.3\times 10^{-15}$ | $2.0\times 10^{-14}$ | $1.8\times 10^{-13}$ | $4.4\times 10^{-13}$
[O III] $\lambda 5007$ NLR luminosity (ergs s-1) | $2.2\times 10^{39}$ | $2.9\times 10^{39}$ | $1.8\times 10^{39}$ | $5.1\times 10^{40}$
[O III] $\lambda 5007$ EELR luminosity (ergs s-1) | $1.7\times 10^{39}$ | $3.7\times 10^{40}$ | $3.0\times 10^{41}$ | $3.4\times 10^{41}$
NLR/total [O III] | 0.56 | 0.07 | 0.006 | 0.11
EELR radii (″) | 3–17 | 3–23 | 3–22 | 1.5–11
Cone opening angle (∘) | 109 | 70 | 51 | 60

##  5 Energy balance and AGN luminosity history

The relevant quantities to examine the energy balance between the AGN as
observed directly, and photoionization requirements for the distant clouds,
are shown in Table 7. The recombination-balance luminosity lower limit follows
Keel et al. (2012), with the value for NGC 5514 adjusted for the fact that the
expression they used assumed a slit oriented radially to the AGN while our
slit position was almost perpendicular to that in order to include more of the
cloud. To account for this, estimating what we measure by integrating along a
radial slit, we multiply the mean H$\beta$ surface brightness within the cloud
per arcsecond2 by the radial extent 4.4″from our [O III] images. We take the
FIR luminosity as an upper bound on the absorbed AGN flux, since it is
integrated over essentially the whole galaxy at these distances and may
include a substantial contribution from ongoing star formation. We also
consider the mid-IR contribution to the obscured-AGN luminosity, derived from
WISE PSF magnitudes as in Keel et al. (2019). For the unobscured AGN ionizing
luminosity, we scale from the [O III] luminosity rather than broad lines since
these are type 2 AGN, following LaMassa et al. (2010) for a mean relation of
observed [O III] to mid-IR luminosity of 1/400, and mean ratio of mid/IR to
ionizing luminosity of 0.8 (following Elvis et al. 1994), so the unobscured
ionizing luminosity is 320 times the nuclear [O III] luminosity. Comparison of
the total AGN luminosity, an upper bound since not all the FIR emission is
from grains heated by ionizing radiation (with a substantial contribution by
young stars possible), to the minimum requirements to power the extended
clouds, shows that NGC 235 is consistent with a constant luminosity over the
extra light-travel time from nucleus to cloud and then to the observer, while
NGC 5514 has a shortfall of at least 2.5 times. Especially for NGC 5514, the
shortfall may be significant despite the approximate nature of the luminosity
analysis, since analysis of EELR images at higher resolution (Keel et al.,
2017b) shows that the highest-surface-brightness EELR clouds typically absorb
only a fraction $\approx 1/3$ of the incident ionizing radiation. Furthermore,
if much of the ionization near the core of NGC 5514 is by shocks rather than
photons from the AGN (Lípari et al., 2004), the implied AGN luminosity as
observed becomes an upper limit.

NGC 235 is part of a multiple system including NGC 232, a comparably luminous
spiral in which López-Cobá et al. (2017) have detected a radial, high-
ionization emission line-structure extending 3 kpc in projection along
position angle $163^{\circ}$, likely related to an AGN on this galaxy. The
EELR we have found is seen roughly midway between NGC 235 and NGC 232; the
nucleus of NGC 232 has lower ionization than NGC 235, very close to the
LINER/Seyfert boundary from López-Cobá et al. (2017), so its estimated
ionizing output is correspondingly lower. We therefore attribute the
ionization on the cloud to the brighter AGN in NGC 235.

As noted above and by Lípari et al. (2004), the optical emission from gas in
NGC 5514 has low ionization, and not line ratios and the complex kinematics
are consistent with widespread shock ionization. in fact, it is unclear
whether our spectral data allow us to isolate an AGN component in this system,
motivating us to look more broadly for estimates of AGN luminosity as observed
directly. Conservatively, the lack of changes in ionization-sensitive line
ratios such as [O III]/H$\beta$ and [O I]/H$\alpha$ between the nucleus and
its surroundings (Fig. 9) suggests that less than half the emission projected
at the nucleus could come from a higher-ionization typical AGN component. No
high-sensitivity pointed X-ray observations have been obtained; the typical
flux level $2\times 10^{-13}$ ergs cm-2 s-1 for the ROSAT survey (Boller et
al., 2016) would translate to ionizing luminosity $\approx 3\times 10^{41}$
ergs s-1 at this distance. Any spectral signature of the AGN must fall with
the “obscured" category, based on far-IR data. An important role for dust
heating by star formation is likely in dusty merging systems of this kind.

Table 7: Quantities related to the energy balance between AGN output, as seen directly, and photoionization requirements to power the distant ionized clouds. Object | NGC 235 | NGC 5514
---|---|---
$z$ | 0.0222 | 0.0235
Nucleus [O III] flux, ergs cm-2 s-1 | $1.18\times 10^{-13}$ | $9.4\times 10^{-16}$
Nucleus [O III] luminosity, ergs s-1 | $1.3\times 10^{41}$ | $1.1\times 10^{39}$
Unobscured ionizing luminosity, ergs s-1 | $4.2\times 10^{43}$ | $3.5\times 10^{41}$
MIR flux, erg cm-2 s-1 | $1.0\times 10^{-10}$ | $6.0\times 10^{-12}$
MIR luminosity, erg s-1 | $2.4\times 10^{40}$ | $1.2\times 10^{39}$
FIR flux, erg cm-2 s-1 | $1.4\times 10^{-6}$ | $7.2\times 10^{-7}$
FIR luminosity erg s-1 | $2.4\times 10^{44}$ | $1.4\times 10^{44}$
Obscured+unobscured ionizing luminosity, ergs s-1 | $2.8\times 10^{44}$ | $1.4\times 10^{44}$
Cloud H$\beta$ flux, erg cm-2 s-1 | $1.5\times 10^{-16}$ | $5.4\times 10^{-17}$
Projected separation, arcsec | 57 | 135
Projected separation, kpc | 26 | 75
Recombination-balance AGN luminosity, erg s-1 | $>2.4\times 10^{44}$ | $>3.9\times 10^{44}$

##  6 Distant EELRs and tidal debris

The kinematically quiescent distant EELRs studied in our earlier work (Lintott
et al. 2009, Keel et al. 2012, Knese et al. 2020) occur overwhelmingly in
systems which are strongly interacting or merger remnants, as are the two new
distant EELRs identified in this survey. As a step toward using these results
to estimate the occurrence of these large-scale ionized clouds around AGN, we
have examined our broadband images and the Legacy Survey data (Dey et al.,
2019) in the VCV sample for evidence of tidal effects - near-radial tails,
tangential shells, and external loops or arcs (Table 8). “Data source"
indicates whether the listed tidal structure was detected on our SARA $V$
images or seen only on Legacy Survey $g$ images (when both sources are listed,
they apply to different structures in order). The surface-brightness
thresholds are bimodal, since not all sample galaxies are included in the
Legacy Survey (and the Legacy Survey differs in exposure properties
essentially between northern and southern regions). Typical detection
thresholds of the structures noted here are $V=25.6$ ″-2 for SARA, very
roughly three times deeper or $g\approx 27$ ″-2 for the Legacy Survey. We see
tidal structures - tails, shells, or arcs - in 26 of the 111 VCV sample
galaxies (23%); our categories are illustrated with SARA $V$ images in Fig.
12. Very broadly, we detect distant EELR emission in 10% of disturbed AGN
hosts. One occurs in the Toomre sample (albeit marginally reaching our 10-kpc
radial limits), which was preselected for tidal disturbances.

All the EELRs detected in this survey are sections of tidal debris. This
extends the results from Keel et al. (2012), where at least 18 of 19 EELRs
were in clearly interacting or merging galaxies (Mkn 78 was the possible
exception).

Figure 12: Sample images of kinds of tidal features in Table 8, from SARA $V$-band images. Scale bars show 1 arc minute in each case. The intensity mapping was tailored for each image, to show both bright structures and the faint tidal features. Table 8: Tidal features in sample galaxies from SARA and Legacy Survey (LS) data. Galaxy | Structure | Data source
---|---|---
ESO 548-G81 | Tails | LS
ESO 575-IG016 | Distorted; tail | SARA
IC 1481 | Postmerger, tails | SARA/LS
IC 1833 | Patchy shells | LS
Mkn 938 | Postmerger, tail to 84″ | SARA
NGC 235 | tail+fan | SARA
MCG -01.05.047 | End of disc warped; NNW compn has tail | LS
MCG -01.34.008 | External loops | LS
MCG -01.60.021 | Tails | SARA
MCG -03.05.007 | Outer asymm arm or tail | LS
NGC 788 | Shells | LS
NGC 3303A | Merging, tails | SARA
NGC 5318 | Faint tails | LS
NGC 5395 | Tail | LS
NGC 5514 | Merging/tails | SARA
NGC 5635 | Faint tails | SARA
NGC 6211 | Faint tail | LS
NGC 6240 | merging, tails | SARA
NGC 7214 | Tails | SARA
NGC 7319 | Tail | SARA
NGC 7469 | Compn w/tail; tail/arc | SARA/LS
NGC 7592E | Merger, tail | SARA
NGC 7592W | Merger, tail | SARA
UGC 7064 | Tail | LS
NGC 7679 | merger, tails | SARA/LS
NGC 7682 | Arcs | LS

We use starlight features as tracers of tidal distortion strong enough to
bring substantial neutral gas outside the main ISM of a galaxy, and often well
outside its inner-disc plane with attendant absorption as well. While H I
tails are typically roughly coincident with stellar tails, they may extend
much farther and have some angular offset (Hibbard et al., 2001). Unlike the
sample in Knese et al. (2020), H I mapping has not yet been done for most of
the galaxies in this sample.

##  7 Conclusions

From an [O III] imaging survey of 17 merging systems and 109 additional
putative AGN hosts, we find three AGN-photoionized distant emission-line
regions, projected more than 10 kpc from the galaxy nucleus. The cloud in the
merger remnant NGC 7252 was previously known (Schweizer et al., 2013) and only
partly passes our 10-kpc threshold. We report new detections, with
spectroscopic confirmation, of these distant EELR clouds in the strongly
interacting systems NGC 235 and NGC 5514. Emission-line ratios indicate that
they are photoionized by the AGN. Together with earlier surveys, this
strengthens that pattern of kinetically quiescent distant EELRs occurring
almost solely in interacting or merging systems. Of systems with tidal
distortion in available images, roughly 10% show these distant ionized clouds.
Energy balance indicates that the AGN in NGC 5514 has likely faded by at least
a factor 3 over the light-travel time difference of order 250,000 years; the
AGN in NGC 235 may have faded but the uncertainties are too large to make a
strong statement.

Our survey images also show four systems with unusually large and luminous
EELRs within the host galaxies. These range from 1–170 times as bright as the
usual NLR in [O III]. We find likely ionization cones in IC 1481, unusual for
a LINER nucleus, and elongated emission on comparable scales around the
similarly low-ionization nucleus of NGC 7679. Both these galaxies show post-
starburst stellar populations.

This survey contributes to our understanding of the occurrence of distant AGN-
ionized clouds, and of the timescales of luminous AGN episodes. Use of a
narrowband imaging survey reaches lower surface brightness levels than colour
selection from SDSS images (as in the Galaxy Zoo survey of Keel et al. 2012),
sampling clouds which may be farther from the ionizing source and have higher
space density. A similar survey specifically of merging systems is ongoing, as
is colour selection using Legacy Survey data carried out by Galaxy Zoo
participants. Together, these samples will further improve constraints on the
radiation escaping AGN in direction and time.

## Acknowledgements

The narrowband filters were obtained thanks to a University of Alabama College
of Arts and Sciences Dean’s Leadership Board Faculty Fellowship. Acquisition
of new imagers for the SARA Observatory was supported by the National Science
Foundation through grant 0922981 to East Tennessee State University, and
retrofitting of the Jacobus Kapteyn Telescope through grant 1337566 to Texas
A&M University – Commerce. We thank the BASS team for public release of their
optical spectral data and fits. We thank Dr. Sebastian Sanchez for access to
data obtained as part of the eCALIFA extension of their IFU survey, and Dr.
Adam Thomas for facilitating access to the S7 data release.

This work is partly based on observations obtained with the 6-m telescope of
the Special Astrophysical Observatory of the Russian Academy of Sciences
carried out with the financial support of the Ministry of Science and Higher
Education of the Russian Federation (including agreement No. 05.619.21.0016,
project ID RFMEFI61919X0016). The work of the 2.5-m telescope is supported by
the Program of development of M.V. Lomonosov Moscow State University. The
analysis of the ionized gas properties according to ADAM, SCORPIO, SCORPIO-2
and MaNGaL data performed by the SAO RAS team was supported by the grant of
Russian Science Foundation project 17-12-01335 “Ionized gas in galaxy discs
and beyond the optical radius". The work of the 1.6-m telescope was performed
with budgetary funding of Basic Research program II.16; the telescope is
equipment of Center for Common Use “Angara" (http://ckp-rf.ru/ckp/3056/).

The authors, and the Legacy Surveys project, are honored to be permitted to
conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with
particular significance to the Tohono O’odham Nation. This research has made
use of the NASA/IPAC Extragalactic Database (NED), which is operated by the
Jet Propulsion Laboratory, Caltech, under contract with the National
Aeronautics and Space Administration. This research has made use of the VizieR
catalogue access tool, CDS, Strasbourg, France (DOI : 10.26093/cds/vizier).
The original description of the VizieR service was published in 2000, A&AS
143, 23.

The Legacy Surveys consist of three individual and complementary projects: the
Dark Energy Camera Legacy Survey (DECaLS; Proposal ID #2014B-0404; PIs: David
Schlegel and Arjun Dey), the Beijing-Arizona Sky Survey (BASS; NOAO Prop. ID
#2015A-0801; PIs: Zhou Xu and Xiaohui Fan), and the Mayall $z$-band Legacy
Survey (MzLS; Prop. ID #2016A-0453; PI: Arjun Dey). DECaLS, BASS and MzLS
together include data obtained, respectively, at the Blanco telescope, Cerro
Tololo Inter-American Observatory, NSF’s NOIRLab; the Bok telescope, Steward
Observatory, University of Arizona; and the Mayall telescope, Kitt Peak
National Observatory, NOIRLab. NOIRLab is operated by the Association of
Universities for Research in Astronomy (AURA) under a cooperative agreement
with the National Science Foundation.

This project used data obtained with the Dark Energy Camera (DECam), which was
constructed by the Dark Energy Survey (DES) collaboration. Funding for the DES
Projects has been provided by the U.S. Department of Energy, the U.S. National
Science Foundation, the Ministry of Science and Education of Spain, the
Science and Technology Facilities Council of the United Kingdom, the Higher
Education Funding Council for England, the National Center for Supercomputing
Applications at the University of Illinois at Urbana-Champaign, the Kavli
Institute of Cosmological Physics at the University of Chicago, Center for
Cosmology and Astro-Particle Physics at the Ohio State University, the
Mitchell Institute for Fundamental Physics and Astronomy at Texas A& M
University, Financiadora de Estudos e Projetos, Fundacao Carlos Chagas Filho
de Amparo, Financiadora de Estudos e Projetos, Fundacao Carlos Chagas Filho de
Amparo a Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de
Desenvolvimento Cientifico e Tecnologico and the Ministerio da Ciencia,
Tecnologia e Inovacao, the Deutsche Forschungsgemeinschaft and the
Collaborating Institutions in the Dark Energy Survey. The Collaborating
Institutions are Argonne National Laboratory, the University of California at
Santa Cruz, the University of Cambridge, Centro de Investigaciones
Energeticas, Medioambientales y Tecnologicas-Madrid, the University of
Chicago, University College London, the DES-Brazil Consortium, the University
of Edinburgh, the Eidgenossische Technische Hochschule (ETH) Zurich, Fermi
National Accelerator Laboratory, the University of Illinois at Urbana-
Champaign, the Institut de Ciencies de l’Espai (IEEC/CSIC), the Institut de
Fisica de Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig
Maximilians Universitat Munchen and the associated Excellence Cluster
Universe, the University of Michigan, NSF’s NOIRLab, the University of
Nottingham, the Ohio State University, the University of Pennsylvania, the
University of Portsmouth, SLAC National Accelerator Laboratory, Stanford
University, the University of Sussex, and Texas A& M University.

BASS is a key project of the Telescope Access Program (TAP), which has been
funded by the National Astronomical Observatories of China, the Chinese
Academy of Sciences (the Strategic Priority Research Program “The Emergence of
Cosmological Structures" Grant #XDB09000000), and the Special Fund for
Astronomy from the Ministry of Finance. The BASS is also supported by the
External Cooperation Program of Chinese Academy of Sciences (Grant #
114A11KYSB20160057), and Chinese National Natural Science Foundation (Grant #
11433005).

The Legacy Survey team makes use of data products from the Near-Earth Object
Wide-field Infrared Survey Explorer (NEOWISE), which is a project of the Jet
Propulsion Laboratory/California Institute of Technology. NEOWISE is funded by
the National Aeronautics and Space Administration.

The Legacy Surveys imaging of the DESI footprint is supported by the Director,
Office of Science, Office of High Energy Physics of the U.S. Department of
Energy under Contract No. DE-AC02-05CH1123, by the National Energy Research
Scientific Computing Center, a DOE Office of Science User Facility under the
same contract; and by the U.S. National Science Foundation, Division of
Astronomical Sciences under Contract No. AST-0950945 to NOAO.

## Data Availability

The new data described here have been deposited with zenodo.org under DOI
10.5281/zenodo.5248363.

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