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 L ∼ L * galaxies at z ∼ 0.2. These data are well described by simple photoionization models, with the gas highly ionized (n HII /n H 99%) by the extragalactic ultraviolet background (EUVB). 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 cool CGM > 6.5 × 10 10 M ⊙ for the volume within R < R vir . Allowing for an additional warm-hot, OVI-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.

The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of intergalactic gas, but its mass, heavy element enrichment, and relation to galaxy properties are poorly constrained by observations. In a survey of the outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we detected ubiquitous, large (150 kiloparsec) halos of ionized oxygen surrounding star-forming galaxies, but we find much less ionized oxygen around galaxies with little or no star formation. This ionized CGM contains a substantial mass of heavy elements and gas, perhaps far exceeding the reservoirs of gas in the galaxies themselves. It is a basic component of nearly all star-forming galaxies that is removed or transformed during the quenching of star formation and the transition to passive evolution.Galaxies grow by accreting gas from the intergalactic medium (IGM) and converting it to stars. Stellar winds and explosions release gas enriched with heavy elements (or metals, 1), some of which is ejected in galactic-scale outflows (2). The circumgalactic medium (CGM), 1 arXiv:1111.3980v1 [astro-ph.CO] 16 Nov 2011 loosely defined as gas surrounding galaxies within their own halos of dark matter (out to 100-300 kiloparsec), lies at the nexus of accretion and outflows, but the structure of the CGM and its relation to galaxy properties are still uncertain. Galactic outflows are observed at both low (2-4) and high (5-7) redshift, but it is unclear how far they propagate, what level of heavyelement enrichment they possess, and whether the gas escapes the halo or eventually returns to fuel later star formation. Models of galaxy evolution require significant outflows to explain observed galaxy masses and chemical abundances and to account for metals observed in the more diffuse IGM (8, 9). The CGM may also reflect the theoretically-predicted transition from filamentary streams of cold gas that feed low mass galaxies to hot, quasi-static envelopes that surround high mass galaxies (10, 11). Both outflow and accretion through the CGM may be intimately connected to the observed dichotomy between blue, star-forming, disk-dominated galaxies and red, passively evolving, elliptical galaxies with little or no star formation (12). However, the low density of the CGM makes it extremely difficult to probe directly, thus models of its structure and influences are typically constrained indirectly by its effects on the visible portions of galaxies, not usually by observations of the gas itself.We have undertaken a large program with the new Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope to directly map the CGM using the technique of absorptionline spectroscopy, in which a diffuse gas is detected by its absorption of light from a background source. Our background sources are UV-bright quasi-stellar objects (QSOs), which are the luminous active nuclei of galaxies lying far behind the galaxies of interest. We focus on the ultraviolet 1032,1038Å doublet of O VI (O +5 ), the most accessible tracer of ...

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.

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.

We assess the metal content of the cool (∼10 4 K) circumgalactic medium (CGM) about galaxies at z 1 using an H I-selected sample of 28 Lyman limit systems (LLS, defined here as absorbers with 16.2 log N H I 18.5) observed in absorption against background QSOs by the Cosmic Origins Spectrograph on-board the Hubble Space Telescope. The N H I selection avoids metallicity biases inherent in many previous studies of the low-redshift CGM. We compare the column densities of weakly ionized metal species (e.g., O II, Si II, Mg II) to N H I in the strongest H I component of each absorber. We find that the metallicity distribution of the LLS (and hence the cool CGM) is bimodal with metal-poor and metal-rich branches peaking at [X/H] −1.6 and −0.3 (or about 2.5% and 50% solar metallicities). The cool CGM probed by these LLS is predominantly ionized. The metal-rich branch of the population likely traces winds, recycled outflows, and tidally stripped gas; the metal-poor branch has properties consistent with cold accretion streams thought to be a major source of fresh gas for star forming galaxies. Both branches have a nearly equal number of absorbers. Our results thus demonstrate there is a significant mass of previously-undiscovered cold metal-poor gas and confirm the presence of metal enriched gas in the CGM of z 1 galaxies.