J/MNRAS/483/4847 AGN photoionization of gas in companion galaxies (Keel+, 2019)
AGN photoionization of gas in companion galaxies as a probe of AGN radiation in
time and direction.
Keel W.C., Bennert V.N., Pancoast A., Harris C.E., Nierenberg A.,
Chojnowski S.D., Moiseev A.V., Oparin D.V., Lintott C.J., Schawinski K.,
Mitchell G., Cornen C.
<Mon. Not. R. Astron. Soc., 483, 4847-4865 (2019)>
=2019MNRAS.483.4847K 2019MNRAS.483.4847K (SIMBAD/NED BibCode)
ADC_Keywords: Active gal. nuclei ; Interstellar medium ; Galaxies, Seyfert
Keywords: galaxies: active - galaxies: ISM - galaxies: Seyfert
Abstract:
We consider active galactic nucleus (AGN) photoionization of gas in
companion galaxies (cross-ionization) as a way to sample the intensity
of AGN radiation in both direction and time, independent of the gas
properties of the AGN host galaxies. From an initial set of 212
AGN+companion systems, identified with the help of Galaxy Zoo
participants, we obtained long-slit optical spectra of 32 pairs that
were a priori likely to show cross-ionization based on projected
separation or angular extent of the companion. From emission-line
ratios, 10 of these systems are candidates for cross-ionization,
roughly the fraction expected if most AGNs have ionization cones with
70° opening angles. Among these, Was 49 remains the strongest
nearby candidate. NGC 5278/9 and UGC 6081 are dual-AGN systems with
tidal debris, complicating identification of cross-ionization. The two
weak AGNs in the NGC 5278/9 system ionize gas filaments to a projected
radius 14kpc from each galaxy. In UGC 6081, an irregular
high-ionization emission region encompasses both AGNs, extending more
than 15kpc from each. The observed AGN companion galaxies with and
without signs of external AGN photoionization have similar
distributions in estimated incident AGN flux, suggesting that geometry
of escaping radiation or long-term variability controls this facet of
the AGN environment. This parallels conclusions for luminous QSOs
based on the proximity effect among Lyman α absorbers. In some
galaxies, mismatch between spectroscopic classifications in the common
BPT diagram and the intensity of weaker HeII and [NeV] emission lines
highlights the limits of common classifications in low-metallicity
environments.
Description:
We compiled a finding list of AGNs with companion galaxies based on
our redshift and geometric criteria, largely through the efforts of
volunteer participants in the Galaxy Zoo project (Lintott et al.
2008MNRAS.389.1179L 2008MNRAS.389.1179L). A post on the project forum setting out the
desired kinds of galaxy pairs led to responses beginning both from
objects seen in the normal course of classification for Galaxy Zoo and
from SQL queries of the SDSS photometric and spectroscopic catalogs.
This initial query selected object pairs projected within 15arcsec
with both redshifts in SDSS DR8, where one had an AGN spectroscopic
class and the other had a non-AGN galaxy spectroscopic class. The
cut-off in projected separation corresponds to 17.5kpc at the median
sample redshift z=0.060. To this we added additional pairs found by
Galaxy Zoo volunteers that satisfied the same criteria except for not
having both redshifts available from the SDSS, or fulfilling the
linear separation criterion at lower redshifts. In some cases the
missing redshift had been measured and was available from sources
referenced in NED, and in others, tidal structure made physical
association between the galaxies virtually certain (which was
confirmed by our spectroscopy). Similarly, we also included nearby
systems passing these tests, previously known outside the SDSS imaging
region (i.e. Keel 1996ApJS..106...27K 1996ApJS..106...27K, Cat. J/ApJS/106/27 building on
the catalogue by Lipovetsky, Neizvestny & Neizvestnaya
1988SoSAO..55....5L 1988SoSAO..55....5L, Cat. VII/173): NGC 2992, NGC 6786, Kaz 63, and
Kaz 199.
This set of pairs was further refined by our inspection of spectra to
confirm the clear presence of a spectroscopic AGN. We required a
Seyfert nucleus; originally we started compiling LINER AGN as well,
but as their inferred ionizing luminosities all fell below our
threshold for observation, we ceased collecting them for this program.
When only the AGN redshift is known, tidal distortion was taken as
secondary evidence that two galaxies are physically associated. These
factors led us to a finding list of 212 AGN/companion pairs (table
A1), incorporating Galaxy Zoo forum postings between 2012 January 16
and 2014 March 15.
We obtained spectra of 32 candidate pairs, some multiple times for
confirmation or at different position angles, using the Kast double
spectrograph (Miller J. S., Stone R. P. S., 1992, Lick Obs. Technical
Report 66. Lick Observatory, Santa Cruz, CA) at the 3-m Shane
telescope of Lick Observatory during 13 nights from 2013 to 2015. For
each session, the D46 dichroic beamsplitter separated light into blue
and red optical paths, with a nominal split centred at 4600Å. The
wavelength settings were roughly 3400-4600Å in the blue side and
4600-7400Å on the red side. The slit width was 2.0arcsec. Flux
calibration used observations of 1-3 standard stars per night. Clouds
prohibited observing standards on 2015 April 23, so we used the
response curve derived for April 26, and scaled line fluxes in the
case of SDSS 1354+1327 to match earlier data where the slits crossed
at the nucleus.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 108 212 Finding list: Low redshift AGN/companion galaxy
pairs
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Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 19 A19 --- ID Object identification by coordinate
designation (1)
21 A1 --- f_ID [*] Flag on ID (2)
23- 41 I19 --- objID ? SDSS DR12 ObjID (Alam et al.
2015ApJS..219...12A 2015ApJS..219...12A, Cat. V/147)
43- 45 F3.1 --- Type AGN type (3)
47- 52 F6.4 --- zAGN AGN redshift
54- 59 F6.4 --- zcompn ? Redshift of the companion
61- 63 I3 arcsec r Projected distance from the AGN to the
centre of the companion
65 A1 --- l_theta Limit flag on theta
67- 69 A3 deg theta Projected angle subtended by the
companion galaxy around the AGN
71- 77 F7.2 10-19W/m2/nm Fcont ? Local continuum flux Fcont, as
tabulated in SDSS DR7
79- 84 F6.2 0.1nm [OIII]EW ? OIII λ5007 line equivalent width
86- 91 F6.1 --- Fion ? Predicted ionizing flux from the AGN at
the projected distance of the companion
galaxy (4)
93-108 A16 --- Notes Notes
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Note (1): Initial J indicating designations used in the Sloan surveys
(Alam et al. 2015ApJS..219...12A 2015ApJS..219...12A, Cat. V/147)
Note (2): Flag as follows:
* = In a few cases we used additional sources for emission-line data:
Petrov, Kovachev & Mineva (1985Ap&SS.116..333P 1985Ap&SS.116..333P) for NGC 6786,
Heckman et al. (1984AJ.....89..958H 1984AJ.....89..958H) for the companion of PG 1048+342,
and Keel et al. (1985AJ.....90..708K 1985AJ.....90..708K) for NGC 5278/9.
Note (3): AGN type as follows:
0 = BL Lac objects
1 = Broad-line objects (as in Sy 1)
2 = Narrow-line objects
Note (4): Fion is given in arbitrary units, since the quantity has systematic
uncertainties as well as projection effects and was used only to rank
objects for spectroscopic observation
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History:
From electronic version of the journal
(End) Ana Fiallos [CDS] 02-Aug-2022