Redshift and reddening of 125 QSOs behind M31

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.