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

Donald G. York, Lorrie A. Straka, Michael Bishof, Seth Kuttruff, David Bowen, Varsha P. Kulkarni, Mark Subbarao, Gordon Richards, Daniel Vanden Berk, Patrick B. Hall, Timothy Heckman, Pushpa Khare, Jean Quashnock, Lara Ghering, Sean Johnson

Research output: Contribution to journalArticlepeer-review

17 Scopus citations


A search for emission lines in foreground galaxies in quasar spectra (zgal < zQSO) of the Sloan Digital Sky Survey data release 5 reveals 23 examples of quasars shining through low redshift, foreground galaxies at small impact parameters (<10kpc). About 74000 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-2s-1, and then confirming that other expected emission lines of the Hii 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 Caii and Nai absorption lines are given from the quasar spectrum. No trends were found relating the Caii equivalent width W (Caii) or the Nai equivalent width W (Nai) to b, but a correlation of rs=-0.77 (α= 0.05) was found relating W (Caii) and E(B-V)(g-i).

Original languageEnglish (US)
Pages (from-to)3692-3708
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Jul 2012

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Cosmology: observations
  • Galaxies: evolution
  • Galaxies: photometry
  • Quasars: absorption lines


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