The cos-halos survey: Rationale, design, and a census of circumgalactic neutral hydrogen

Jason Tumlinson, Christopher Thom, Jessica K. Werk, J. Xavier Prochaska, Todd M. Tripp, Neal Katz, Romeel Davé, Benjamin D. Oppenheimer, Joseph D. Meiring, Amanda Brady Ford, John M. O'Meara, Molly S. Peeples, Kenneth R. Sembach, David H. Weinberg

Research output: Contribution to journalArticlepeer-review

214 Scopus citations


We present the design and methods of the COS-Halos survey, a systematic investigation of the gaseous halos of 44 z = 0.15-0.35 galaxies using background QSOs observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. This survey has yielded 39 spectra of z em ≃ 0.5 QSOs with S/N ∼10-15 per resolution element. The QSO sightlines pass within 150 physical kpc of the galaxies, which span early and late types over stellar mass log M */M = 9.5-11.5. We find that the circumgalactic medium exhibits strong H I, averaging ≃ 1 Å in Lyα equivalent width out to 150 kpc, with 100% covering fraction for star-forming galaxies and 75% covering for passive galaxies. We find good agreement in column densities between this survey and previous studies over similar range of impact parameter. There is weak evidence for a difference between early- and late-type galaxies in the strength and distribution of H I. Kinematics indicate that the detected material is bound to the host galaxy, such that ≳ 90% of the detected column density is confined within ±200 km s-1 of the galaxies. This material generally exists well below the halo virial temperatures at T ≲ 105 K. We evaluate a number of possible origin scenarios for the detected material, and in the end favor a simple model in which the bulk of the detected H I arises in a bound, cool, low-density photoionized diffuse medium that is generic to all L* galaxies and may harbor a total gaseous mass comparable to galactic stellar masses.

Original languageEnglish (US)
Article number59
JournalAstrophysical Journal
Issue number1
StatePublished - Nov 1 2013
Externally publishedYes


  • galaxies: formation
  • galaxies: halos
  • intergalactic medium
  • quasars: absorption lines

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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