The COS-Halos Survey: An Empirical Description of the Metal-Line
  Absorption in the Low-Redshift Circumgalactic Medium
  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_ CGMcool > 10^9 MSun), which likely traces a distinct
density and/or temperature regime from the highly-ionized CGM traced by OVI
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.
