Galaxies with background qsos - II. An automated search for multiple galaxy emission lines

Lorrie A. Straka, Zakary L. Whichard, Varsha P. Kulkarni, Michael Bishof, David Bowen, Pushpa Khare, Donald G. York

Research output: Contribution to journalArticlepeer-review

7 Scopus citations


We have improved upon our previous search technique of systematically searching quasistellar object (QSO) spectra for narrow galactic Hα emission, which indicates a foreground galaxy within the Sloan Digital Sky Survey (SDSS) spectral fibre. We now search for Hα plus eight other galactic emission lines in the same manner. We have scanned the SDSS data release 7 (DR7) QSO catalogue spectra searching for these emission lines. Here, we present our sample which focuses on the redshift range z < 0.401 where galactic Hα is detectable in the SDSS spectra. This has revealed 27 unique galaxies on top of QSOs(GOTOQs). We have deblended the QSOs from the respective galaxies and determined the photometric properties of these systems.We find upon deblending that most of the galaxies are primarily blue, late-type galaxies with colours in the range -0.71 < (u - r) < 2.07. We find a slight anticorrelation between reddening and impact parameter [E(B - V)(g - i) versus b]. The galaxies have average star formation rates of 0.01-1 M⊙ yr-1, with an average of 0.6 M⊙ yr-1. They range in z from 0 to 0.4 and in stellar luminosity from about 0.01 L* to 3.0 L*. They are foreground to QSOs of brightness 17.4-20.4 mag (r band) with the impact parameters of 1-10 kpc. They represent a fair sample of typical galaxies for which it should be possible to determine accurately various quantities (e.g. abundances, dust extinction and Faraday rotation) using follow-up analysis of the background QSOs. Where present, Ca II λ3934 (K) and Na I λ5892 (D2) absorption lines were also measured in the QSO spectrum.We find 15 systems with Ca II K absorption and 6 with Na I D2 absorption. We find no trends relating the equivalent widths of these lines to impact parameter or reddening of the background QSO. Eight of our fields show significant reddening [E(B - V)(g - i) > 0.20], which are targeted for follow-up studies of interstellar clouds.We find three systems (Q0059-0009, Q1033+2059 and Q2356+0029) with detectable galactic spectral emission, but no visible galaxy in imaging and no detectable absorption features.We speculate on the nature of these galaxies, which are strong candidates for dark galaxies and dwarf haloes.

Original languageEnglish (US)
Pages (from-to)3200-3223
Number of pages24
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Dec 2013

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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