The cos-halos survey: Physical conditions and baryonic mass in the low-redshift circumgalactic medium

Jessica K. Werk, J. Xavier Prochaska, Jason Tumlinson, Molly S. Peeples, Todd M. Tripp, Andrew J. Fox, Nicolas Lehner, Christopher Thom, John M. O'Meara, Amanda Brady Ford, Rongmon Bordoloi, Neal Katz, Nicolas Tejos, Benjamin D. Oppenheimer, Romeel Davé, David H. Weinberg

Research output: Contribution to journalArticlepeer-review

465 Scopus citations


We analyze the physical conditions of the cool, photoionized (T 10 4 K) circumgalactic medium (CGM) using the COS-Halos suite of gas column density measurements for 44 gaseous halos within 160 kpc of LL* galaxies at z0.2. These data are well described by simple photoionization models, with the gas highly ionized (n H II /n H ≳ 99%) by the extragalactic ultraviolet background. Scaling by estimates for the virial radius, R vir, we show that the ionization state (tracked by the dimensionless ionization parameter, U) increases with distance from the host galaxy. The ionization parameters imply a decreasing volume density profile n H = (10-4.2 ± 0.25)(R/R vir) -0.8 ± 0.3. Our derived gas volume densities are several orders of magnitude lower than predictions from standard two-phase models with a cool medium in pressure equilibrium with a hot, coronal medium expected in virialized halos at this mass scale. Applying the ionization corrections to the H I column densities, we estimate a lower limit to the cool gas mass M for the volume within R < R vir. Allowing for an additional warm-hot, O VI-traced phase, the CGM accounts for at least half of the baryons purported to be missing from dark matter halos at the 10 12 M scale.

Original languageEnglish (US)
Article number8
JournalAstrophysical Journal
Issue number1
StatePublished - Sep 1 2014


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

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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