The Cos-Halos Survey: An Empirical Description of Metal-Line Absorption in the Low-Redshift Circumgalactic Medium

Abstract

We present the equivalent width and column density measurements for low and intermediate ionization states of the circumgalactic medium (CGM) surrounding 44 low-z, L ≈ L * galaxies drawn from the COS-Halos survey. These measurements are derived from far-UV transitions observed in HST/COS and Keck/HIRES spectra of background quasars within an impact parameter R < 160 kpc to the targeted galaxies. The data show significant metal-line absorption for 33 of the 44 galaxies, including quiescent systems, revealing the common occurance of a cool (T ≈ 10 4−5 K), metal-enriched CGM. The detection rates and column densities derived for these metal lines decrease with increasing impact parameter, a trend we interpret as a declining metal surface density profile for the CGM. A comparison of the relative column densities of adjacent ionization states indicates the gas is predominantly ionized. The large surface density in metals demands a large reservoir of metals and gas in the cool CGM (very conservatively, M cool CGM > 10 9 M ⊙ ), which likely traces a distinct density and/or temperature regime from the highly-ionized CGM traced by O+5 absorption. The large dispersion in absorption strengths (including non-detections) suggests the cool CGM traces a wide range of densities or a mix of local ionizing conditions. Lastly, the kinematics inferred from the metal-line profiles are consistent with the cool CGM being bound to the dark matters halos hosting the galaxies; this gas may serve as fuel for future star-formation. Future work will leverage this dataset to provide estimates on the mass, metallicity, dynamics, and origin of the cool CGM in low-z, L * galaxies.

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

“…Thus, the detection rate of Si III in the outer halo is 30±10%. This is smaller than results for the inner CGM, as found by (COS-Halos survey; Werk et al 2013), Liang & Chen (2014) and Richter et al (2016) Liang & Chen (2014). Both the numbers are broadly consistent, given that the error ranges in the estimate do have an overlap.…”

“…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).…”

Mining circumgalactic baryons in the low-redshift universe

“…Chen et al 2001b;Adelberger et al 2005;Steidel et al 2010), the Mg II λλ 2796, 2803 doublet (e.g. Bowen et al 1995;Chen & Tinker 2008;Kacprzak et al 2008;Barton & Cooke 2009;Chen et al 2010a,b;Gauthier et al 2010;Bordoloi et al 2011;Werk et al 2013), and the O VI λλ 1031, 1036 doublet (Chen & Mulchaey 2009;Wakker & Savage 2009;Prochaska et al 2011;Tumlinson et al 2011;Johnson et al 2013). A general finding from different studies is that most galaxies are surrounded by chemically enriched gas out to ≈ 100−300 kpc in projected distance.…”

“…This program, known as the "COS-Halos" survey, was designed to map the multi-phase CGM using O VI and other metal-line diagnostics Werk et al 2013). The low-redshift cut at z = 0.15 was dictated by the requirement of detecting the O VI doublet using COS and the FUV channel.…”