Joseph D. Meiring

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: Rationale, Design, and a Census of Circumgalactic Neutral Hydrogen

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{sub 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{sub *}/M{sub ☉} = 9.5-11.5. We find that the circumgalactic medium exhibits strong H I, averaging ≅ 1 A 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{sup –1} of the galaxies. This material generally exists well below the halo virial temperatures at T ∼< 10{supmorexa0» 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.«xa0less

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 First Observations of Low-redshift Damped Lyα Systems with the Cosmic Origins Spectrograph

We report on the first Cosmic Origins Spectrograph observations of damped Ly{alpha} systems (DLAs) and sub-damped Ly{alpha} systems (sub-DLAs) discovered in a new survey of the gaseous halos of low-redshift galaxies. From observations of 37 sightlines, we have discovered three DLAs and four sub-DLAs. We measure the neutral gas density {Omega}{sub HI}, and redshift density d N/d z, of DLA and sub-DLA systems at z < 0.35. We find d N/dz = 0.25{sup +0.24}-{sub 0.14} and {Omega}{sub HI} = 1.4{sup +1.3}{sub -0.7} x 10{sup -3} for DLAs, and d N/d z = 0.08{sup +0.19}{sub -0.06} with {Omega}{sub HI} = 4.2{sup +9.6}{sub -3.5} x 10{sup -5} for sub-DLAs over a redshift path {Delta}z = 11.9. To demonstrate the scientific potential of such systems, we present a detailed analysis of the DLA at z{sub abs} = 0.1140 in the spectrum of SDSS J1009+0713. Profile fits to the absorption lines determine log N(H I) = 20.68 {+-} 0.10 with a metallicity determined from the undepleted element sulfur of [S/H] = -0.62 {+-} 0.18. The abundance pattern of this DLA is similar to that of higher z DLAs, showing mild depletion of the refractory elements Fe and Ti with [S/Fe] = +0.24 {+-} 0.22morexa0» and [S/Ti] = +0.28 {+-} 0.15. Nitrogen is underabundant in this system with [N/H] = -1.40 {+-} 0.14, placing this DLA below the plateau of the [N/{alpha}] measurements in the local universe at similar metallicities. This DLA has a simple kinematic structure with only two components required to fit the profiles and a kinematic width of {Delta}v{sub 90} = 52 km s{sup -1}. Imaging of the QSO field with the Hubble Space Telescope/Wide Field Camera 3 reveals a spiral galaxy at very small impact parameter to the QSO and several galaxies within 10, or 20 comoving kpc at the redshift of the DLA. Follow-up spectra with the Low Resolution Imaging Spectrometer on the Keck telescope reveal that none of the nearby galaxies are at the redshift of the DLA. The spiral galaxy is identified as the host galaxy of the QSO based on the near perfect alignment of the nucleus and disk of the galaxy as well as spectra of an H II region showing emission lines at the QSO redshift. A small feature appears 0.70 from the nucleus of the QSO after point-spread function subtraction, providing another candidate for the host galaxy of the DLA system. Even with these supporting data, we are unable to unambiguously identify the host galaxy of the DLA, exemplifying some of the difficulties in determining DLA hosts even at low redshift.«xa0less

EVIDENCE FOR COLD ACCRETION: PRIMITIVE GAS FLOWING ONTO A GALAXY AT z ∼ 0.274*

We present UV and optical observations from the Cosmic Origins Spectrograph on the Hubble Space Telescope and Keck of a z = 0.27395 Lyman limit system (LLS) seen in absorption against the QSO PG1630+377. We detect H I absorption with log N(H I) = 17.06 ± 0.05 as well as Mg II, C III, Si III, and O VI in this system. The column densities are readily explained if this is a multi-phase system, with the intermediate and low ions arising in a very low metallicity ([Mg/H] = –1.71 ± 0.06) photoionized gas. We identify via Keck spectroscopy and Large Binocular Telescope imaging a 0.3 L * star-forming galaxy projected 37 kpc from the QSO at nearly identical redshift (z = 0.27406 and Δv = –26 km s–1) with near solar metallicity ([O/H] = –0.20 ± 0.15). The presence of very low metallicity gas in the proximity of a near-solar metallicity, sub-L * galaxy strongly suggests that the LLS probes gas infalling onto the galaxy. A search of the literature reveals that such low-metallicity LLSs are not uncommon. We found that 50% (4/8) of the well-studied z 1 LLSs have metallicities similar to the present system and show sub-L * galaxies with ρ < 100 kpc in those fields where redshifts have been surveyed. We argue that the properties of these primitive LLSs and their host galaxies are consistent with those of cold mode accretion streams seen in galaxy simulations.

QSO ABSORPTION SYSTEMS DETECTED IN Ne VIII: HIGH-METALLICITY CLOUDS WITH A LARGE EFFECTIVE CROSS SECTION*

Using high-resolution, high signal-to-noise ultraviolet spectra of the z em = 0.9754 quasar PG1148+549 obtained with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope, we study the physical conditions andxa0abundances of Ne VIII+O VI absorption line systems at z abs = 0.68381, 0.70152, 0.72478. In addition to Ne VIII and O VI, absorption lines from multiple ionization stages of oxygen (O II, O III, O IV) are detected and are well aligned with the more highly ionized species. We show that these absorbers are multiphase systems including hot gas (T 105.7xa0K) that produces Ne VIII and O VI, and the gas metallicity of the cool phase ranges from Z = 0.3 Z ☉ to supersolar. The cool (104xa0K) phases have densities nH 10–4xa0cm–3 and small sizes ( 0.1 L* galaxy and a large effective absorption cross section ( 100xa0kpc), and indeed, we find a star-forming ~L galaxy at the redshift of the z abs = 0.72478 system, at an impact parameter of 217xa0kpc. Multiphase absorbers like these Ne VIII systems are likely to be an important reservoir of baryons and metals in the circumgalactic media of galaxies.

High-velocity clouds as streams of ionized and neutral gas in the halo of the Milky Way

High-velocity clouds (HVCs), fast-moving ionized and neutral gas clouds found at high galactic latitudes, may play an important role in the evolution of the Milky Way. The extent of this role depends sensitively on their distances and total sky covering factor. We search for HVC absorption in Hubble Space Telescope high-resolution ultraviolet (UV) spectra of a carefully selected sample of 133 active galactic nuclei (AGN) using a range of atomic species in different ionization stages (e.g. Ou2009i, Cu2009ii, Cu2009iv, Siu2009ii, Siu2009iii, Siu2009iv). This allows us to identify neutral, weakly ionized or highly ionized HVCs over several decades in Hu2009i column densities. The sky covering factor of UV-selected HVCs with |vLSR| ≥ 90u2009kmu2009s−1 is about 68u2009per cent for the Galactic sky at . About 74u2009per cent of the HVC directions have u2009cm−2 and 46u2009per cent have u2009cm−2. We show that our survey is essentially complete, i.e. an undetected population of HVCs with extremely low H (Hu2009i + Hu2009ii) column density is unlikely to be important for the HVC mass budget. We confirm that the predominantly ionized HVCs contain at least as much mass as the traditional Hu2009i HVCs and show that large Hu2009i HVC complexes have generally ionized envelopes extending far from the Hu2009i contours. There are also large regions of the Galactic sky that are covered with ionized high-velocity gas with little Hu2009i emission nearby. We show that the covering factors of HVCs with u2009kmu2009s−1 drawn from the AGN and stellar (with stars at d > 3u2009kpc) samples are similar. This confirms that most of the HVCs are within 5–15u2009kpc of the sun. The HVCs with |vLSR |≳170u2009kmu2009s−1 are largely associated with the Magellanic Stream at b 20° as well as other large known Hu2009i complexes. We conclude that there is no evidence in the Local Group that any galaxy shows a population of HVCs extending much farther away than 50u2009kpc from its host, except possibly for those tracing remnants of galaxy interaction.

MULTIPHASE GAS IN GALAXY HALOS: THE O VI LYMAN-LIMIT SYSTEM TOWARD J1009+0713

We have serendipitously detected a strong O VI-bearing Lyman-limit system (LLS) at z{sub abs} = 0.3558 toward the quasi-stellar object (QSO) J1009+0713 (z{sub em} = 0.456) in our survey of low-redshift galaxy halos with the Hubble Space Telescopes (HST) Cosmic Origins Spectrograph. Its total rest-frame equivalent width of W{sub r} = 835 {+-} 49 mA and column density of log N(O VI) = 15.0 are the highest for an intervening absorber yet detected in any low-redshift QSO sightline, with absorption spanning at least four major kinematic component groups over 400 km s{sup -1} in its rest frame. HST/Wide Field Camera 3 images of the galaxy field show that the absorber is associated with two galaxies lying at 14 and 46 kpc from the QSO line of sight. The absorber is kinematically complex and there are no less than nine individual Mg II components spanning 200 km s{sup -1} in our Keck/HIRES optical data. The bulk of the absorbing gas traced by H I resides in two strong, blended component groups that possess a total log N(H I) {approx_equal} 18-18.8, but most of the O VI is associated with two outlying components with log N(H I) = 14.8 and 16.5. Themorexa0» ion ratios and column densities of C, N, O, Mg, Si, S, and Fe, except the O VI, can be accommodated into a simple photoionization model in which diffuse, low-metallicity halo gas is exposed to a photoionizing field from stars in the nearby galaxies that propagates into the halo at 10% efficiency. In this model, the clouds have neutral fractions of {approx}1%-10% and thus total hydrogen column densities of log N(H) {approx_equal} 19.5. Direct measurement of the gas metallicity is precluded by saturation of the main components of H I, but we constrain the metallicity firmly within the range 0.1-1 Z{sub sun}, and photoionization modeling indirectly indicates a subsolar metallicity of 0.05-0.5 Z{sub sun}. This highly ionized, multiphase, possibly low-metallicity halo gas resembles gas with similar properties in the Milky Way halo and other low-redshift LLS, suggesting that at least some other galaxies have their star formation fueled by metal-poor gas accreting from the intergalactic medium and ionized by the stars in the host galaxy. As observed in the Milky Way high-velocity clouds, the strong detected O VI is not consistent with the photoionization scenario but is consistent with general picture in which O VI arises in interface material surrounding the photoionized clouds or in a hotter, diffuse component of the halo. The appearance of strong O VI and nine Mg II components in this system, and our review of similar systems in the literature, offer some support to this interface picture of high-velocity O VI: the total strength of the O VI shows a positive correlation with the number of detected components in the low-ionization gas.«xa0less

The First Observations of Low-redshift Damped Lyα Systems with the Cosmic Origins Spectrograph: Chemical Abundances and Affiliated Galaxies

We present Cosmic Origins Spectrograph (COS) measurements of metal abundances in eight 0.083 < zabs < 0.321 damped Lyman-α (DLA) and sub-damped Lyα absorption systems serendipitously discovered in the COS-Halos survey. We find that these systems show a large range in metallicities, with 1.10 < [Z/H] < 0.31, similar to the spread found at higher redshifts. These low-redshift systems on average have subsolar metallicities, but do show a rise in metallicity over cosmic time when compared to higher-redshift systems. We find the average sub-DLA metallicity is higher than the average DLA metallicity at all redshifts. Nitrogen is underabundant with respect to α-group elements in all but perhaps one of the absorbers. In some cases, [N/α] is significantly below the lowest nitrogen measurements in nearby galaxies. Systems for which depletion patterns can be studied show little, if any, depletion, which is characteristic of Milky Way halo-type gas. We also identify affiliated galaxies for 3 of the sub-DLAs using spectra obtained from Keck/LRIS. None of these sub-DLAs arise in the stellar disks of luminous galaxies; instead, these absorbers may exist in galaxy halos at impact parameters ranging from 38 to 92 kpc. Multiple galaxies are present near two of the sub-DLAs, and galaxy interactions may play a role in the dispersal of the gas. Many of these low-redshift absorbers exhibit simple kinematics, but one sub-DLA has a complicated mix of at least 13 components spread over 150 km s −1 . We find three galaxies near this sub-DLA, which also suggests that galaxy interactions roil the gas. This study reinforces the view that DLAs have a variety of origins, and low-redshift studies are crucial for understanding absorber-galaxy connections. Subject headings: galaxies: abundances — galaxies: ISM — quasars : absorption lines.

THE COS-HALOS SURVEY: KECK LRIS AND MAGELLAN MagE OPTICAL SPECTROSCOPY

We present high signal-to-noise optical spectra for 67 low-redshift (0.1 < z < 0.4) galaxies that lie within close projected distances (5 kpc < rho < 150 kpc) of 38 background UV-bright QSOs. The Keck LRIS and Magellan MagE data presented here are part of a survey that aims to construct a statistically sampled map of the physical state and metallicity of gaseous galaxy halos using the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). We provide a detailed description of the optical data reduction and subsequent spectral analysis that allow us to derive the physical properties of this uniquely data-rich sample of galaxies. The galaxy sample is divided into 38 pre-selected L ~ L*, z ~ 0.2 target galaxies and 29 bonus galaxies that lie in close proximity to the QSO sightlines. We report galaxy spectroscopic redshifts accurate to +/- 30 km s-1, impact parameters, rest-frame colors, stellar masses, total star formation rates, and gas-phase interstellar medium oxygen abundances. When we compare the distribution of these galaxy characteristics to those of the general low-redshift population, we find good agreement. The L ~ L* galaxies in this sample span a diverse range of color (1.0 < u-r < 3.0), stellar mass (10^9.5 < M/M_sun < 10^11.5), and SFRs (0.01 - 20 M_sun yr-1). These optical data, along with the COS UV spectroscopy, comprise the backbone of our efforts to understand how halo gas properties may correlate with their host galaxy properties, and ultimately to uncover the processes that drive gas outflow and/or are influenced by gas inflow.