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\nintermediate ionization states of the circumgalactic medium (CGM) surrounding\n44 low-z, L ~ L* galaxies drawn from the COS-Halos survey. These measurements\nare derived from far-UV transitions observed in HST/COS and Keck/HIRES spectra\nof background quasars within an impact parameter R < 160 kpc to the targeted\ngalaxies. The data show significant metal-line absorption for 33 of the 44\ngalaxies, including quiescent systems, revealing the common occurance of a cool\n(T ~ 10^{4 - 5} K), metal-enriched CGM. The detection rates and column\ndensities derived for these metal lines decrease with increasing impact\nparameter, a trend we interpret as a declining metal surface density profile\nfor the CGM. A comparison of the relative column densities of adjacent\nionization states indicates the gas is predominantly ionized. The large surface\ndensity in metals demands a large reservoir of metals and gas in the cool CGM\n(very conservatively, M_ CGMcool > 10^9 MSun), which likely traces a distinct\ndensity and/or temperature regime from the highly-ionized CGM traced by OVI\nabsorption. The large dispersion in absorption strengths (including\nnon-detections) suggests the cool CGM traces a wide range of densities or a mix\nof local ionizing conditions. Lastly, the kinematics inferred from the\nmetal-line profiles are consistent with the cool CGM being bound to the dark\nmatters halos hosting the galaxies; this gas may serve as fuel for future\nstar-formation. Future work will leverage this dataset to provide estimates on\nthe mass, metallicity, dynamics, and origin of the cool CGM in low-z, L*\ngalaxies.\n