Quasars behind the disk of M31 galaxy

¹ Department of Astronomy, Faculty of Physics, Sofia University “St. Kliment Ohridski”, 5 J. Bourchier blvd, Sofia 1164, Bulgaria
² Department of Astronomy, Box 351580, University of Washington, Seattle, WA 98195, USA
³ European Southern Observatory, Karl-Schwarszchild-Str. 2, 85748 Garching bei München, Germany
⁴ Special Astrophysical Observatory, Nizhny Arkhyz 369167, Russia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 June 2025
Accepted: 16 February 2026

Abstract

Aims. 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.

Methods. We carried out spectroscopic follow-up of 32 quasar candidates.

Results. 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 µ_B = 26^m/□″ 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.

Conclusions. 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.