J/A+A/708/A20 Redshift and reddening of 125 QSOs behind M31 (Nedialkov+, 2026)
Quasars behind the disk of M31 galaxy.
Nedialkov P., Williams B.F., Ivanov V.D., Valcheva A., Solovyeva Y.,
Vinokurov A., Malygin E, Oparin D., Sholukhova O.
<Astron. Astrophys. 708, A20 (2026)>
=2026A&A...708A..20N 2026A&A...708A..20N (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; QSOs ; Active gal. nuclei ;
Spectroscopy ; Redshifts ; Photometry
Keywords: techniques: spectroscopic - galaxies: distances and redshifts -
galaxies: ISM - Local Group - quasars: emission lines -
quasars: general
Abstract:
Our aim is to increase the limited number of quasars behind M 31,
necessary for probing the chemical content of the gas and for proper
motion reference, with reliable and homogeneous redshift measurements
from emission lines.
We carried out spectroscopic follow-up of 32 quasar candidates.
We confirm 23 quasars. Two are new discoveries (J004029.727+403705.68
and J004215.489+412031.52) and the rest have been reported elsewhere,
but with somewhat deficient analysis; 16 spectra are published for the
first time. We report new homogeneous redshifts for 34 quasars (from
40 spectra, adding 17 from the archives) and summarize all available
information about bona fide quasars with reliable redshift, bringing
their number to 124 within the µB=26m/arcsec2 isophote. We
carried out a comparison of redshifts from different sources and
excluded some objects with redshifts derived from low-resolution
spectra. We derived the reddening for them from the color excess with
respect to dereddened counterparts with similar redshifts in the
field. Comparisons of our reddenings with M 31 reddening maps found no
significant correlations.
Most QSOs behind M 31 show low reddening and do not probe high
extinctions, probably due to a bias toward following up brighter and
less extinct candidates, which underlines the need to identify fainter
quasars behind nearby galaxies, especially behind higher extinction
regions. Finally, the redshifts derived from low-resolution spectra
must be treated with caution because they can contain significant
errors.
Description:
We increase the limited number of quasars behind M31 and provide
reliable and homogeneous redshift measurements for them from emission
lines. We carried out spectroscopic follow up of 32 quasar candidates
to verify their nature and to derive their redshifts. We confirm 23
quasars, two of which (J004029.727+403705.68 and
J004215.489+412031.52) are new discoveries; the rest were reported
elsewhere in the literature, and 16 spectra among 23 are published for
the first time; we measured new homogeneous redshifts for 34 quasars
(from 40 spectra, adding 17 archival spectra to our observations) and
summarize the available archival and literature information about
quasars with measured redshift, bringing their number to 184 within
the D26 isophote. For 124 bona-fide QSOs with a reliable redshift, we
derive reddening from the color excess over their counterparts with
similar redshifts in the field.
The catalog contains 124 spectroscopically confirmed quasars and 1 BL
Lac behind the disk of M31, within the muB=26mag per sq. arcsec
isophote. The sample is assembled from object with new spectroscopy
reported in the paper and from previous publications, including
Story-Fisher et al. (2024ApJ...964...69S 2024ApJ...964...69S), Dey et al.
(2023ApJ...944....1D 2023ApJ...944....1D) and others - the complete list is given at the
end. The redshifts were measured from multiple emission lines, listed
in the table.
Most of the new spectra ∼24, marked with A in column spectrum ID -
were obtained with the Dual Imaging Spectrograph (DIS) at the 3.5m
telescope at the Apache Point Observatory with the B400 and R300
gratings in the blue (3425-5900Å) and red (5180-9860Å) channel,
respectively. The slit was 1.5 arcsec wide, delivering a resolving
power of R∼700 and 850 at the centers of the wavelength ranges for the
blue and red arms, respectively. Spectra of the remaining 8 candidates
were obtained at the 6m telescope at the Special Astrophysical
Observatory (SAO) of the Russian Academy of Sciences - marked with B
in column spectrum ID. Two spectrographs were used: SCORPIO-1 with
grisms VPHG1200B (3600-5400Å and R∼800) and VPHG1200R,
(5700-7400Å, R∼1300), observing in both cases trough 1.2 arcsec
wide slit; SCORPIO-2 with grism VPHG1200@540 (3650-7250Å, R∼1000)
and 1.0 arcsec wide slit. The integration times varied, depending on
the targets' apparent magnitudes, telescope apertures and the
weather conditions, mostly often 2 or 3 exposures of 30 minute were
collected. The instruments and set up employed to obtain the archival
spectra are described in the respective publications.
The presence of broad lines in our spectra indicates that 23 out of
the 32 observed objects are quasars; No. 6 and No. 11 in table1.dat
are new and the rest are confirmations, although for 16 of these our
spectra are the first to be published. Among 13 X-ray selected
candidates (Williams et al., 2014MNRAS...443.2499W) 12 turn out to be
quasars; the UV excess in LGGS-PHAT selected 4 candidates yielded 2
quasars; the SPITZER-LGGS selected 8 candidates also yielded 3
quasars; 6 of 7 Yuhan Yao (https://speakerdeck.com/yaoyuhan/) objects
appear to be quasars. We also uniformly measured redshifts for 34
unique quasars from 40 spectra, adding 17 archival to our data.
The absorption AV was measured for each of the 124 quasars (the BL
Lac was omitted because because of the contradicting redshift
measurements in the literature and because it may not exhibit typical
quasar colors) comparing their apparent colors from Pan-STARRS Survey
DR1 (Chambers et al., 2016ArXiv161205560C 2016ArXiv161205560C) with the colors of quasars
with similar redshifts (within ±0.1dex) in the field, after
correcting the latter for Milky Way absorption according to the map of
Schlafly & Finkbeiner (2011ApJ...737..103S 2011ApJ...737..103S). The final AV for each
quasar was error-averaged from the measurements for four colors:
(g-r), (g-i), (g-z) and (g-y). The total-to-selective extinction ratio
RV=3.1 was adopted (Cardelli et al., 1989ApJ...345..245C 1989ApJ...345..245C).
Some additional parameters of interest are also included: Gaia G band
magnitudes Gmag and its error e_Gmag, class and the probability of the
object being a quasar PQSO (in %) from the Discrete Source
Classifier-Combmode from Gaia DR3 (Gaia Collaboration et al.,
2023A&A...674A..41A 2023A&A...674A..41A); a missing Class is marked as "uncl" from
unclassified, and if there is no entry for an object the Class is set
to "NA" from not applicable.
The spectra of two quasars, J004029.727+403705.68 and
J004215.489+412031.52, reported here for the first time have been
added.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tableb1.dat 364 131 Redshift and reddening of 125 QSOs behind M31 disk
tablec1.dat 14 5312 Spectra of QSOs J004029.727+403705.68 and
J004215.487+412031.52
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Byte-by-byte Description of file: tableb1.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- Seq QSO consecutive number (1)
5- 23 A19 --- Name Gaia DR3 unique source identifier or
Pan-STARRS unique source identifier (2)
25- 26 I2 h RAh Right Ascension (J2000)
28- 29 I2 min RAm Right Ascension (J2000)
31- 36 F6.3 s RAs Right Ascension (J2000)
38 A1 --- DE- Declination sign (J2000)
39- 40 I2 deg DEd Declination (J2000)
42- 43 I2 arcmin DEm Declination (J2000)
45- 49 F5.2 arcsec DEs Declination (J2000)
52- 56 F5.3 --- rho Galactocentric distance (3)
59- 64 F6.3 mag Gmag The mean G-band magnitude
66- 70 F5.3 mag e_Gmag The 1σ error in Gmag
73- 76 A4 --- Class Name of best class (Cat. I/356/qsocand)
79- 81 I3 % PQSO ?=- Probability of being a quasar
84- 90 F7.4 --- z1 ?=-9.9999 First measured redshift z1
92- 98 F7.4 --- e_z1 ?=-9.9999 The 1σ error in z1
100-102 A3 --- r_z1 The z1 redshift reference (4)
105-111 F7.4 --- z2 ?=-9.9999 Second measured redshift z2
113-119 F7.4 --- e_z2 ?=-9.9999 The 1σ error in z2
121-123 A3 --- r_z2 The z2 redshift reference (4)
126-132 F7.4 --- z3 ?=-9.9999 Third measured redshift z3
134-140 F7.4 --- e_z3 ?=-9.9999 The 1σ error in z3
142-144 A3 --- r_z3 The z3 redshift reference (4)
147-153 F7.4 --- z4 ?=-9.9999 Fourth measured redshift z4
155-161 F7.4 --- e_z4 ?=-9.9999 The 1σ error in z4
163-165 A3 --- r_z4 The z4 redshift reference (4)
168-174 F7.4 --- z5 ?=-9.9999 Fifth measured redshift z5
176-182 F7.4 --- e_z5 ?=-9.9999 The 1σ error in z5
184-186 A3 --- r_z5 The z5 redshift reference (4)
189-195 F7.4 --- z6 ?=-9.9999 Sixth measured redshift z6
197-203 F7.4 --- e_z6 ?=-9.9999 The 1σ error in z6
205-207 A3 --- r_z6 The z6 redshift reference (4)
210-223 A14 --- Eml1 Detected emission line l1
225-238 A14 --- Eml2 Detected emission line l2
240-253 A14 --- Eml3 Detected emission line l3
255-268 A14 --- Eml4 Detected emission line l4
270-283 A14 --- Eml5 Detected emission line l5
285-298 A14 --- Eml6 Detected emission line l6
300-313 A14 --- Eml7 Detected emission line l7
315-321 F7.4 --- zad ?=-9.9999 Adopted redshift zad
323-329 F7.4 --- e_zad ?=-9.9999 The 1σ error in zad
332-337 F6.3 mag AV ?=-9.9999 measured reddening AV
339-344 F6.3 mag e_AV ?=-9.999 The 1σ error in AV
348-350 I3 --- robs [1/32]?=- Observing log table ID
353-364 A12 --- rspec ID in the plot (Fig. 3)
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Note (1): QSO Nos. 41, 45, 46, 95, 108 and 116 have double entry lines since
their redshift analysis is based on 2 independent spectra
Note (2): If QSO not found in Gaia DR3 Part 1. Main source catalog
(Cat. I/355//gaiadr3) their unique ID comes from Pan-STARRS Survey DR1
(Cat. II/349/ps1) marked with 'a'
Note (3): Deprojected distance within the plane of M31 in units of the half
standard D25 diameter (de Vaucouleur et al., 1991rc3..book.....D 1991rc3..book.....D,
Cat. VII/155).
Assumes D25=190.5, disk plane inclination to the line of sight 17.1 deg,
and a position angle of the major axis of 35.0°.
Note (4): References for redshift defined as follows:
1 = Gaia collaboration, 2023A&A...674A...1G 2023A&A...674A...1G
2 = Story-Fisher et al., 2024ApJ...964...69S 2024ApJ...964...69S
3 = Dey et al., 2023ApJ...944....1D 2023ApJ...944....1D
4 = Huo et al., 2013AJ....145..159H 2013AJ....145..159H
5 = Neugent, Massey & Georgy, 2012ApJ...759...11N 2012ApJ...759...11N
6 = this work
7 = Dong et al., 2018AJ....155..189D 2018AJ....155..189D
8 = Liao et al., 2019RAA....19...29L 2019RAA....19...29L
9 = Massey, Neugent, & Levesque, 2019AJ....157..227M 2019AJ....157..227M
10 = Dahari & De Robertis, 1988ApJS...67..249D 1988ApJS...67..249D
11 = Meusinger et al., 2010A&A...512A...1M 2010A&A...512A...1M
12 = Dorn-Wallenstein, Levesque & Ruan, 2017ApJ...850...86D 2017ApJ...850...86D
13 = Nedialkov et al., 2018ATel12250....1N 2018ATel12250....1N
14 = Rao et al., 2013MNRAS.432..866R 2013MNRAS.432..866R
15 = Paiano et al., 2017ApJ...851..135P 2017ApJ...851..135P
Multiple references are given when we have reanalyzed spectra from the
literature to remeasure the redshifts.
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Byte-by-byte Description of file: tablec1.dat
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Bytes Format Units Label Explanations
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1- 4 I4 0.1nm lambda Wavelength
6- 12 F7.4 --- Flux Flux in arbitrary units
14 I1 --- Index [1/2] Spectrum identifier (1)
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Note (1): 1 for the spectrum of QSOs J004029.727+403705.68 and
2 for the spectrum and J004215.487+412031.52.
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Acknowledgements:
Petko Nedialkov, japet(at)phys.uni-sofia.bg
License: CC-BY-4.0 [see https://spdx.org/licenses/]
(End) Patricia Vannier [CDS] 13-Mar-2026