Quasars behind the disk of M31 galaxy

Abstract

We aim to increase the limited number of quasars behind M31, 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 were 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 archives) and summarize all available information about bona-fide quasars with reliable redshift, bringing their number to 124 within the mu_B=26 mag/arcsec^2 isophote. We carried out a comparison of redshifts from different sources and excluded some objects with redshifts derived from low-resolution spectra. We derive the reddening for them from the color excess with respect to dereddened counterparts with similar redshifts in the field. Comparisons of our reddenings with M31 reddening maps found no significant correlations. Most QSOs behind M31 show low reddening and do not probe high-extinctions underlining the need to identify fainter quasars behind nearby galaxies, especially behind higher extinction regions -- probably due to a bias towards following up brighter and less extinct candidates. Finally, the redshifts derived from low-resolution spectra must be treated with caution, because they can contain significant errors.

Figures (9)

• Figure 1: Map of the 24 bona-fide QSOs (red squares) with prominent emission lines and 1 BL Lac object (red dot) known from the literature before Gaia DR3 2023AA...674A...41A. The confirmed QSOs from our target list are also shown: 3 Spitzer-selected (cyan), 2 selected from optical colors and UV excess (yellow) and 18 selected by other means (see Table \ref{['tab:obslog']}; black). All objects are located within the $\mu_B$=26$^m$/$\Box^{\prime\prime}$ isophote (black solid line). The $\mu_B$=25$^m$/$\Box^{\prime\prime}$ isophote (dotted line), the approximate boundaries of the Spitzer PSC catalog (dashed line) and the footprints of LGGS (red) and PHAT (blue) catalogs are also plotted.
• Figure 2: Illustration of our MIR color selection (see Sec. \ref{['sec:selection']} for details). Top: WISE color--color (left) and color-magnitude diagram (right). The symbols in all panels are the same as in Fig. \ref{['fig:qso_before_GDR3.png']}. Small gray dots show the entire WISE photometry within the Spitzer footprint. Small black solid dots are the sample of bright WISE sources used to adjust the coordinates (see Appx. \ref{['app:mid_ir_coords']} for details). The red dashed lines show our adopted quasar locus defined in Eqs. \ref{['eq:MIR_selection']} after 2012MNRAS.426.3271M. The sources falling inside are plotted with dark gray in all panels. Middle and bottom: Similar diagrams, but with Spitzer 3.6, 4.6 and 8 or 24 $\mu$m bands. The dashed vertical line in the bottom plot shows our additional color constraint. Summarizing, our targets for follow up spectroscopy were selected among dark gray points that meet this additional criterion.
• Figure 3: Spectra of the observed objects sorted by redshift, shown at rest frame wavelength. Some more prominent features are marked with vertical dashed lines and are labeled. The SDSS composite quasar spectrum 2001AJ....122..549V is also plotted at the bottom of each panel.
• Figure 4: Cross-comparison of redshifts from different sources: $z_1$ -- 2023AA...674A...1G; $z_2$ -- 2024ApJ...964...69S; $z_3$ -- 2023ApJ...944....1D; $z_4$ -- all other sources (listed in Table \ref{['tab:redshifts']}), including this work redshift preference when available. Left: $z_2$, $z_3$ and $z_4$ versus $z_1$. Right: $z_3$ and $z_4$ versus $z_2$. The published measurement errors are shown, except for 2023ApJ...944....1D who do not list them -- in this case for plotting purposes we adopted errors equal to 0.005 dex. The bracketed numbers in the legends show how many objects constitute the overlapping subset between each pair of samples. The black dotted line has a slope of one.
• Figure 5: Intrinsic colors of our Quaia reference quasar sample, obtained after correcting the Pan-STARRS Survey DR1 $grizy$ photometry 2016arXiv161205560C for the Milky Way extinction according to the 2011ApJ...737..103S. The color of the points marks the distance of the quasars from the M 31 center $\rho$ in units of $\rho_{25}$ isophotal radii. The distance bins' widths were set to ensure these bins contain similar number of quasars. The solid lines connect the medians colors within 0.5 dex wide redshift bins. The vertical error bars are the standard deviations of the colors of all quasars within each redshift bin, regardless of the distance from the M 31 center, and they were calculated as 1.48$\times$MAD (median absolute deviation), to suppress the influence of outliers.