The Cos-Halos Survey: Rationale, Design, and a Census of Circumgalactic Neutral Hydrogen

Abstract

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 CGM exhibits strong H I, averaging 1Å 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 10 5 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.15 Four systems outside the saturation region possess lower limits because their highest Lyman series lines are screened by foreground Lyman limit systems or other absorption at a different redshift, so the highest Lyman line for which we can obtain a measurement is not accurately reflected by their redshift alone.

The Properties of the Circumgalactic Medium in Red and Blue Galaxies: Results From the Cos-Gass+cos-Halos Surveys

“…However, the variation in CGM properties during this time is expected to be minimal (Chen 2012). Also, the COS-Halos sample was selected to be all centrals (with a couple of non-centrals Tumlinson et al 2013, Section 2.5). This is not one of the criteria for COS-GASS, although the mass range of the galaxies ensured that most of the COS-GASS galaxies (34/45) are centrals (based on the group catalog by Yang et al 2005Yang et al , 2007.…”

“…For more information on the properties of the target galaxies, including their redshifts, stellar masses, SFRs 13 , sSFR, galaxy colors, and impact parameter of the sightlines, we refer the reader to Table 1 presented in this paper (for the COS-GASS sample) and Table 2 from the published work by Tumlinson et al (2013).…”

“…More information on the data reduction can be found in the previous publication of COS-GASS . This procedure is identical to that followed by Tumlinson et al (2013) and Werk et al (2013) for the COSHalos program.…”

“…Therefore, a fair comparison is to compare the Notes. a Details on the COS-Halos survey can be found in the published work by Tumlinson et al (2013) and Werk et al (2013). b Using prescription from Kravtsov et al (2014) and Mandelbaum et al (2016).…”

Star formation sustained by gas accretion

“…In a systematic search for such systems, Tumlinson et al (2013) find absorption in all starforming galaxies, with a detection rate as large as 75 % for the early types and 100% for late types. The sample contains all Hubble types in the near universe (z < 0.35) with intermediate masses 9.5 ≤ log(M /M ) ≤ 11.5, and the lines-of-sight sample impact parameters d < 150 kpc.…”