Christopher Thom

The Large, Oxygen-Rich Halos of Star-Forming Galaxies Are a Major Reservoir of Galactic Metals

Observations with the Hubble Space Telescope show that halos of ionized gas are common around star-forming galaxies. 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; we found 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. Our data indicate that 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.

The COS-Halos Survey: Physical Conditions and Baryonic Mass in the Low-Redshift Circumgalactic Medium

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 (nHii/nH 99%) by the extragalactic ultraviolet background. Scaling by estimates for the virial radius, Rvir, 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 nH = (10 −4.2±0.25 )(R/Rvir) −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 Hi column densities, we estimate a lower limit to the cool gas mass M cool CGM > 6.5 × 10

Discovery of HE 1523-0901: A Strongly r-Process Enhanced Metal-Poor Star with Detected Uranium

We present age estimates for the newly discovered, very r-process-enhanced metal-poor star HE 1523-0901 ([Fe/H] = -2.95) based on the radioactive decay of Th and U. The bright (V = 11.1) giant was found among a sample of bright metal-poor stars selected from the Hamburg/ESO Survey. From an abundance analysis of a high-resolution (R = 75,000) VLT/UVES spectrum, we find HE 1523-0901 to be strongly overabundant in r-process elements ([r/Fe] = 1.8). The abundances of heavy neutron-capture elements (Z > 56) measured in HE 1523-0901 match the scaled solar r-process pattern extremely well. We detect the strongest optical U line at 3859.57 A. For the first time, we are able to employ several different chronometers, such as the U/Th, U/Ir, Th/Eu, and Th/Os ratios to measure the age of a star. The weighted average age of HE 1523-0901 is 13.2 Gyr. Several sources of uncertainties are assessed in detail.

Bright Metal-poor Stars from the Hamburg/ESO Survey. I. Selection and Follow-up Observations from 329 Fields

We present a sample of 1777 bright (9 1.0) metal-poor ([Fe/H] 20%) and higher values with increasing distance from the Galactic plane. Although the numbers of stars at low metallicity are falling rapidly at the lowest metallicities, there is evidence that the fraction of carbon-enhanced metal-poor stars is increasing rapidly as a function of declining metallicity. For ~60 objects, high-resolution data have already been obtained; one of these, HE 1327-2326, is the new record holder for the most iron-deficient star known.

THE BIMODAL METALLICITY DISTRIBUTION OF THE COOL CIRCUMGALACTIC MEDIUM AT z ≲ 1*

We assess the metal content of the cool (~104?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.

An Accurate Distance to High-Velocity Cloud Complex C

We report an accurate distance of -->d = 10 ? 2.5 kpc to the high-velocity cloud Complex C. Using high signal-to-noise ratio Keck HIRES spectra of two horizontal-branch stars, we have detected Ca II K absorption lines from the cloud. Significant nondetections toward a further three stars yield robust lower distance limits. The resulting H I mass of Complex C is -->MH I = 4.9+2.9?2.2 ? 106 M?; a total mass of -->Mtot = 8.2+ 4.6?2.6 ? 106 M? is implied, after corrections for helium and ionization. At 10 kpc, Complex C has physical dimensions -->3 ? 15 kpc, and if it is as thick as it is wide, then the average density is -->log n ?2.5. We estimate the contribution of Complex C to the mass influx may be as high as ~0.14 M? yr?1.

The Hidden Mass and Large Spatial Extent of a Post-Starburst Galaxy Outflow

A galaxy that has experienced a recent burst of star formation has an extended halo of hot, ionized gas surrounding it. Outflowing winds of multiphase plasma have been proposed to regulate the buildup of galaxies, but key aspects of these outflows have not been probed with observations. By using ultraviolet absorption spectroscopy, we show that “warm-hot” plasma at 105.5 kelvin contains 10 to 150 times more mass than the cold gas in a post-starburst galaxy wind. This wind extends to distances > 68 kiloparsecs, and at least some portion of it will escape. Moreover, the kinematical correlation of the cold and warm-hot phases indicates that the warm-hot plasma is related to the interaction of the cold matter with a hotter (unseen) phase at >>106 kelvin. Such multiphase winds can remove substantial masses and alter the evolution of post-starburst galaxies.

The COS-Dwarfs Survey: The Carbon Reservoir around Sub-L* Galaxies

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 Rvir) of the host galaxies. The C IV absorption strength falls off radially as a power law and beyond ≈ 0.5 Rvir, no C IV absorption is detected above our sensitivity limit of ≈ 50-100 mA. We find a tentative correlation between detected C IV absorption strength and star formation, paralleling the strong correlation see n 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 Wr ≥ 100 mA 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. Subject headings: galaxies: evolution,halos— general—galaxies: quasars: absorption lines— intergalactic medium

NOT DEAD YET: COOL CIRCUMGALACTIC GAS IN THE HALOS OF EARLY-TYPE GALAXIES*

We report new observations of circumgalactic gas in the halos of early-type galaxies (ETGs) obtained by the COS-Halos Survey with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. We find that detections of H I surrounding ETGs are typically as common and strong as around star-forming galaxies, implying that the total mass of circumgalactic material is comparable in the two populations. For ETGs, the covering fraction for H I absorption above 1016 cm?2 is ~40%-50% within ~150 kpc. Line widths and kinematics of the detected material show it to be cold (T 105 K) in comparison to the virial temperature of the host halos. The implied masses of cool, photoionized circumgalactic medium baryons may be up to 109-1011 M ?. Contrary to some theoretical expectations, strong halo H I absorbers do not disappear as part of the quenching of star formation. Even passive galaxies retain significant reservoirs of halo baryons that could replenish the interstellar gas reservoir and eventually form stars. This halo gas may feed the diffuse and molecular gas that is frequently observed inside ETGs.

A STIS Survey for O VI Absorption Systems at 0.12 < z ≲ 0.5. I. The Statistical Properties of Ionized Gas*

We have conducted a systematic survey for intervening O VI absorbers in available echelle spectra of 16 QSOs at -->zQSO = 0.17–0.57. These spectra were obtained using HST STIS with the E140M grating. Our search uncovered a total of 27 foreground O VI absorbers with rest-frame absorption equivalent width -->Wr(1031) 25 mA. Ten of these QSOs exhibit strong O VI absorbers in their vicinity. Our O VI survey does not require the known presence of -->Ly α , and the echelle resolution allows us to identify the O VI absorption doublet based on their common line centroid and known flux ratio. We estimate the total redshift survey path, -->Δ z, using a series of Monte Carlo simulations, and find that -->Δ z = 1.66,2.18, and 2.42 for absorbers of strength -->Wr = 30,50, and 80 mA, respectively, leading to a number density of d(W ≥ 50 mA)/dz = 6.7 ± 1.7