Galaxies with background QSOs - I. A search for strong galactic Hα lines

A search for emission lines in foreground galaxies in quasar spectra (z_gal < z_QSO) of the Sloan Digital Sky Survey data release 5 reveals 23 examples of quasars shining through low redshift, foreground galaxies at small impact parameters (<10 kpc). About 74 000 quasar spectra were examined by searching for narrow Hα emission lines at z < 0.38, at a flux level greater than 5 × 10^-17 erg cm^-2 s^-1, and then confirming that other expected emission lines of the H II regions in the galaxy are detected. The galaxies were deblended from the quasar images to get colours and morphologies. For cases that allow the galaxy and the quasar to be deblended, the galaxies are blue (0.95 < (u-r) < 1.95). Extinction and reddening through the galaxies are determined from the (g-i) colour excess of the quasars. These reddening values are compared with the flux ratio of Hα to Hβ, which reflect the extinction for an undetermined fraction of the sightline through each galaxy. No trends were found relating E(B-V)_(g-i), impact parameter (b), and (u-r) for the galaxies or between E(B-V) derived from (g-i) and that derived from Hα/Hβ. Comparison with previous studies of quasar absorption systems indicates that our sample is more reddened, suggesting disc-dominated absorber galaxies. Measurement or limits on galactic, interstellar Ca II and Na I absorption lines are given from the quasar spectrum. No trends were found relating the Ca II equivalent width W (Ca II) or the Na I equivalent width W (Na I) to b, but a correlation of r_s=-0.77 (α= 0.05) was found relating W (Ca II) and E(B-V)_(g-i).