Todd M. Tripp

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

Intervening O VI Quasar Absorption Systems at Low Redshift: A Significant Baryon Reservoir*

Far-UV echelle spectroscopy of the radio-quiet QSO H1821+643 (zem=0.297), obtained with the Space Telescope Imaging Spectrograph (STIS) at approximately 7 km s-1 resolution, reveals four definite O vi absorption-line systems and one probable O vi absorber at 0.15<zabs<0.27. The four definite O vi absorbers are located near galaxies and are highly displaced from the quasar in redshift; these are likely intervening systems unrelated to the background QSO. In the case of the strong O vi system at zabs=0.22497, multiple components are detected in Si iii and O vi as well as H i Lyman series lines, and the differing component velocity centroids and b-values firmly establish that this is a multiphase absorption system. A weak O vi absorber is detected at zabs=0.22637, i.e., offset by approximately 340 km s-1 from the zabs=0.22497 system. Lyalpha absorption is detected at zabs=0.22613, but no Lyalpha absorption is significantly detected at 0.22637. Other weak O vi absorbers at zabs=0.24531 and 0.26659 and the probable O vi system at 0.21326 have widely diverse O vi/H i column density ratios with N(O vi)/N(H i) ranging from </=0.14+/-0.03 to 5.2+/-1.2. The number density of O vi absorbers with rest equivalent width greater than 30 mÅ in the H1821+643 spectrum is remarkably high, dN&solm0;dz approximately 48, which implies with a high (90%) confidence that it is greater than 17 in the low-redshift intergalactic medium. We conservatively estimate that the cosmological mass density of the O vi systems is Omegab(Ovi&parr0; greater, similar0.0008 h-175. With an assumed metallicity of 1/10 solar and a conservative assumption that the fraction of oxygen in the O vi ionization stage is 0.2, we obtain Omegab(Ovi&parr0; greater, similar0.004 h-175. This is comparable to the combined cosmological mass density of stars and cool gas in galaxies and X-ray-emitting gas in galaxy clusters at low redshift.

A High-Resolution Survey of Low-Redshift QSO Absorption Lines: Statistics and Physical Conditions of O VI Absorbers*

Using high-resolution ultraviolet spectra of 16 low-z QSOs obtained with the E140M echelle mode of the Space Telescope Imaging Spectrograph, we study the physical conditions and statistics of O VI absorption in the intergalactic medium (IGM) at -->z zabs zQSO) O VI systems comprising 77 individual components, and we find 14 proximate systems ( -->zabs ? zQSO) containing 34 components. For intervening systems (components) with rest-frame equivalent width -->Wr > 30 m?, the number of O VI absorbers per unit redshift -->dN/dz = 15.6?2.4+2.9 -->(21.0?2.8+3.2) , and this decreases to -->dN/dz = 0.9?0.5+1.0 -->(0.3?0.3+0.7) for -->Wr > 300 m?. The number per redshift increases steeply as zabs approaches zQSO; we find that -->dN/dz is 3-10 times higher within 2500 km s?1 of zQSO. The most striking difference between intervening and proximate systems is that some proximate absorbers have substantially lower H I/O VI ratios. The lower ratios in proximate systems could be partially due to ionization effects, but these proximate absorbers must also have significantly higher metallicities. We find that 37% of the intervening O VI absorbers have velocity centroids that are well aligned with corresponding H I absorption. If the O VI and the H I trace the same gas, the relatively small differences in line widths imply that the absorbers are cool, with -->T T ? 105.5 K if the metallicity is high enough to cause the associated broad Ly? absorption to be too weak to detect. We show that 53% of the intervening O VI systems are complex multiphase absorbers that can accommodate both lower metallicity collisionally ionized gas with -->T > 105 K and cold photoionzed gas.

The Distribution of Thermal Pressures in the Interstellar Medium from a Survey of C I Fine-Structure Excitation*

It is generally recognized that the interstellar medium has a vast range of densities and temperatures. While these two properties are usually anticorrelated with each other, there are nevertheless variations in their product, i.e., the thermal gas pressure divided by the Boltzmann constant k. In neutral gas, the relative populations of neutral carbon atoms in the excited fine-structure states can give a direct measure of a local thermal pressure. A picture of the distribution function for thermal pressures in H I regions is now arising from a survey of interstellar C I absorption features in the UV spectra of 21 early-type stars, observed with a wavelength resolving power of 200,000 by the STIS instrument on the Hubble Space Telescope. Most of the gas is within the range 1000 10^5 cm^{-3}K. While the fraction of gas at these elevated pressures is quite small, it seems nearly ubiquitous. This phenomenon may arise from small-scale, short-lived density enhancements that are produced by converging flows of material in supersonic turbulence.

Multiphase High-Velocity Clouds toward HE 0226–4110 and PG 0953+414*

We study the physical conditions, elemental abundances, and kinematics of the high-velocity clouds (HVCs) along the sight lines toward active galaxies HE 0226-4110 and PG 0953+414 using Hubble Space Telescope Space Telescope Imaging Spectrograph and Far Ultraviolet Spectroscopic Explorer data. No 21 cm H I emission is detected in these clouds, but our observations reveal multiple components of HVC absorption in lines of H I, C II, C III, C IV, O VI, Si II, Si III, and Si IV in both directions. We investigate whether photoionization by the extragalactic background radiation or by escaping Milky Way radiation can explain the observed ionization pattern. We find that photoionization is a good explanation for the C II, C III, Si II, and Si III features but not for the O VI or C IV associated with the HVCs, suggesting that two principal phases exist: a warm (T ≈ 104 K), photoionized phase and a hotter (T = 1-3 × 105 K), collisionally ionized phase; the broader line widths of the high ions are consistent with this multiphase hypothesis. The warm HVCs toward HE 0226-4110 have high levels of ionization (97%-99%) and metallicities ([Z/H] between -0.9 and -0.4) close to those in the Magellanic Stream, which lies 11° away on the sky at similar velocities. These HVCs may well be stripped fragments of the Stream that have been ionized by the pervading radiation field; they have thermal pressures that would place them close to equilibrium in a fully ionized 106 K Galactic corona with nH = 4-9 × 10-5 cm-3 at 50 kpc. The warm HVCs seen at -146 and 125 km s-1 toward PG 0953+414 have [Z/H] = -0.6 ± 0.2 and -0.8 ± 0.2, respectively, suggesting they are not formed from purely Galactic material. A minisurvey of the hot, collisionally ionized HVC components seen here and in five other sight lines finds that in 11/12 cases, the high ions have kinematics and ionic ratios that are consistent with an origin in conductive interfaces, where energy flows into the HVCs from a hot surrounding medium and produces O VI- and C IV-bearing boundary layers. However, the broad absorption wing on the O VI profile toward PG 0953+414 is not completely explained by the interface scenario. This feature may be tracing the outflow of hot gas into the Milky Way halo as part of a Galactic fountain or wind.

The Relationship between Galaxies and Low-Redshift Weak Lyα Absorbers in the Directions of H1821+643 and PG 1116+215* **

To study the nature of low-z Lyα absorbers in the spectra of QSOs, we have obtained high signal-to-noise ratio (S/N) UV spectra of H1821+643 (zem = 0.297) and PG 1116+215 (zem = 0.177) with the Goddard High-Resolution Spectrograph on the Hubble Space Telescope. The spectra have minimum S/Ns of ~70-100 and 3 σ limiting equivalent widths of 50-75 mA at a resolution of ~150 km s-1. Excluding lines within 3000 km s-1 of zem, we detect 26 Lyα lines with Wr > 50 mA toward H1821+643 and 13 toward PG 1116+215 (comparable to the 13 Lyα lines observed toward 3C 273 by Morris et al.), which implies a density of 102 ± 16 lines per unit redshift for Wr > 50 mA and zabs −175 Mpc with velocity separations of 350 km s-1 or less. Monte Carlo simulations show that if the Lyα lines are randomly distributed, the probability of observing this many close pairs on the two sight lines is 3.6 × 10-5. We find that all galaxies with projected distances ρ ≤ 600 h -->−175 kpc from the QSO sight lines have associated Lyα absorbers within 1000 km s-1, and the majority of these galaxies have absorbers within 350 km s-1. We also find that the Lyα equivalent width is anticorrelated with the projected distance of the nearest galaxy out to at least ρ ≈ 600 h -->−175 kpc. For ρ > 600h -->−175 kpc, we find galaxies that do not have associated Lyα lines, but nevertheless the anticorrelation persists if we select galaxies with ρ 2 h -->−175 Mpc that are within 500 or 1000 km s-1 of a Lyα absorber. This anticorrelation has a high significance but should be interpreted cautiously because there are potential selection biases that could lead to an artificial correlation. Statistical tests also show that the Lyα absorbers are not randomly distributed with respect to the galaxies. Splitting the sample into roughly equal sets with Wr > 100 mA and Wr < 100 mA shows that the weakest absorbers are not randomly distributed either. Comparison of the nearest neighbor distances of the weaker and stronger absorbers suggests that the weakest absorbers are less closely associated with galaxies, but the difference is not yet statistically significant. We find several galaxy groups that do not have clearly associated Lyα absorbers. However, given the projected distance of the nearest galaxy, we do not necessarily expect to find detectable Lyα lines in these groups based on the equivalent width-projected distance anticorrelation. Furthermore, we find several counterexamples of comparable galaxy groups that do have associated Lyα lines. As in previous studies, we find some Lyα absorbers in regions apparently devoid of galaxies, although this may be caused by the limited spatial extent and/or limited depth of the redshift survey. The equivalent width distributions of the absorbers apparently in voids and nonvoid absorbers are statistically indistinguishable, but the sample is small. We discuss the nature of the Lyα absorbers in light of the new data. The observations are consistent with the hypothesis that many of the low-redshift Lyα absorption lines with rest equivalent widths in the range from 50 to ~500 mA trace the overall gas distributions in the large-scale structures of galaxies rather than the gaseous halos of individual galaxies. Other phenomena may also cause Lyα absorption lines.

Far Ultraviolet Spectroscopic Explorer and Space Telescope Imaging Spectrograph Observations of Intervening O VI Absorption Line Systems in the Spectrum of PG 0953+415* ** ***

We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS) observations of the intergalactic medium toward the bright QSO PG 0953+415 (zem = 0.239). The FUSE spectra extend from 905 to 1187 A and have a resolution of 25 km s-1, while the STIS spectra cover 1150-1730 A and have a resolution of 7 km s-1. Additional STIS observations at 30 km s-1 are obtained in selected wavelength ranges. An O VI system at z = 0.06807 is detected in H I Lyα, Lyβ, Lyγ, O VI λλ1031.93, 1037.62, N V λλ1238.82, 1242.80, C IV λλ1548.20, 1550.77, and C III λ977.02. The observed column densities can be modeled as a low-density intervening gas with a metallicity of 0.4 times solar in photoionization equilibrium with the ionizing extragalactic background radiation. The best fit is achieved with an ionization parameter, log U = -1.35, which implies nH ~ 10-5 cm-3 and a path length of ~80 kpc through the absorbing gas. H I Lyα absorption at z = 0.14232 spans a velocity range of 410 km s-1 with the strongest components near 0 and 80 km s-1 in the z = 0.14232 rest frame. In this system, O VI λλ1031.93, 1037.62 absorption is strong near 0 km s-1 and not detected at 80 km s-1. C III λ977.02 absorption is marginally detected at 80 km s-1 but is not detected at 0 km s-1. The observations place constraints on the properties of the z = 0.14232 system but do not discriminate between collisional ionization in hot gas versus photoionization in a very low density medium with an ionization parameter log U > -0.74. The z = 0.06807 and 0.14232 O VI systems occur at redshifts where there are peaks in the number density of intervening galaxies along the line of sight determined from WIYN redshift measurements of galaxies in the ~1° field centered on PG 0953+415. We combine our observations of PG 0953+415 with those for other QSOs to update the estimate of the low-redshift number density of intervening O VI systems. Over a total unobscured redshift path of Δz = 0.43, we detect six O VI systems with rest-frame equivalent widths of the O VI λ1031.93 line exceeding 50 mA, yielding dN/dz = 14 for z = 0.09. This implies a low-redshift value of the baryonic contribution to the closure density of the O VI systems of Ωb(O VI) > 0.002 h, assuming that the average metallicity in the O VI systems is 0.1 solar.

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.

Abundances of Deuterium, Nitrogen, and Oxygen in the Local Interstellar Medium: Overview of First Results from the FUSE Mission

Observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) have been used to determine the column densities of D i ,N i, and O i along seven sight lines that probe the local interstellar medium (LISM) at distances from 37 to 179 pc. Five of the sight lines are within the Local Bubble, and two penetrate the surrounding H i wall. Reliable values of N(H i) were determined for five of the sight lines from Hubble Space Telescope (HST) data, International Ultraviolet Explorer (IUE) data, and published Extreme Ultraviolet Explorer (EUVE) measurements. The weighted mean of D i/H i for these five sight lines is ð1:52 � 0:08 Þ� 10 � 5 (1 � uncertainty in the mean). It is likely that the D i/H i ratio in the Local Bubble has a single value. The D i/O i ratio for the five sight lines within the Local Bubble is ð3:76 � 0:20 Þ� 10 � 2 .I t is likely that O i column densities can serve as a proxy for H i in the Local Bubble. The weighted mean for O i/H i for the seven FUSE sight lines is ð3:03 � 0:21 Þ� 10 � 4 , comparable to the weighted mean ð3:43 � 0:15 Þ� 10 � 4 reported for 13 sight lines probing larger distances and higher column densities. The FUSE weighted mean of N i/H i for five sight lines is half that reported by Meyer and colleagues for seven sight lines with larger distances and higher column densities. This result combined with the variability of O i/N i (six sight lines) indicates that at the low column densities found in the LISM, nitrogen ionization balance is important. Thus, unlike O i ,N i cannot be used as a proxy for H i or as a metallicity indicator in the LISM. Subject headings: cosmology: observations — Galaxy: abundances — ISM: abundances — ISM: evolution — ultraviolet: ISM