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 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 report new observations of circumgalactic gas from the COS-Dwarfs survey, a systematic investigation of the gaseous halos around 43 low-mass z ≤ 0.1 galaxies using background QSOs observed with the Cosmic Origins Spectrograph. From the projected 1D and 2D distribution of C IV absorption, we find that C IV is detected out to ≈ 100 kpc (corresponding roughly to ≈ 0.5 R vir ) of the host galaxies. The C IV absorption strength falls off radially as a power law and beyond ≈ 0.5 R vir , no C IV absorption is detected above our sensitivity limit of ≈ 50-100 mÅ. We find a tentative correlation between detected C IV absorption strength and star formation, paralleling the strong correlation seen in highly ionized oxygen for L∼L * galaxies by the COS-Halos survey. The data imply a large carbon reservoir in the CGM of these galaxies, corresponding to a minimum carbon mass of 1.2 × 10 6 M ⊙ out to ∼ 110 kpc. This mass is comparable to the carbon mass in the ISM and exceeds the carbon mass currently in the stars of these galaxies. The C IV absorption seen around these sub-L * galaxies can account for almost two-thirds of all W r ≥ 100 mÅ C IV absorption detected at low z. Comparing the C IV covering fraction with hydrodynamical simulations, we find that an energy-driven wind model is consistent with the observations whereas a wind model of constant velocity fails to reproduce the CGM or the galaxy properties.

We examine how HI and metal absorption lines within low-redshift galaxy halos trace the dynamical state of circumgalactic gas, using cosmological hydrodynamic simulations that include a well-vetted heuristic model for galactic outflows. We categorize inflowing, outflowing, and ambient gas based on its history and fate as tracked in our simulation. Following our earlier work showing that the ionisation level of absorbers was a primary factor in determining the physical conditions of absorbing gas, we show here that it is also a governing factor for its dynamical state. Low-ionisation metal absorbers (e.g. Mg ii) tend to arise in gas that will fall onto galaxies within several Gyr, while high-ionisation metal absorbers (e.g. O vi) generally trace material that was deposited by outflows many Gyr ago. Inflowing gas is dominated by enriched material that was previously ejected in an outflow, hence accretion at low redshifts is typically substantially enriched. Recycling wind material is preferentially found closer to galaxies, and is more dominant in lower-mass halos since high-mass halos have more hot gas that is able to support itself against infall. Low-mass halos also tend to re-eject more of their accreted material, owing to our outflow prescription that employs higher mass loading factors for lower-mass galaxies. Typical HI absorbers trace unenriched ambient material that is not participating in the baryon cycle, but stronger HI absorbers arise in cool, enriched inflowing gas. Instantaneous radial velocity measures of absorbers are generally poor at distinguishing between inflowing and outflowing gas, except in the case of very recent outflows. These results suggest that probing halo gas using a range of absorbers can provide detailed information about the amount and physical conditions of material that is participating in the baryon cycle.