Andrew J. Fox

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

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 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.

Highly Ionized Gas Surrounding High-Velocity Cloud Complex C*

We present Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope observations of high-, intermediate-, and low-ion absorption in high-velocity cloud (HVC) Complex C along the lines of sight toward five active galaxies. Our purpose is to investigate the idea that Complex C is surrounded by an envelope of highly ionized material, arising from the interaction between the cloud and a hot surrounding medium. We measure column densities of high-velocity high-ion absorption and compare the kinematics of low-, intermediate-, and high-ionization gas along the five sight lines. We find that in all five cases, the H I and O VI high-velocity components are centered within 20 km s-1 of one another, with an average displacement of = 3 ± 12 km s-1. In those directions where the H I emission extends to more negative velocities (the so-called high-velocity ridge), so does the O VI absorption. The kinematics of Si II is also similar to that of O VI, with = 0 ± 15 km s-1. We compare our high-ion column density ratios to the predictions of various models, adjusted to account for both recent updates to the solar elemental abundances and relative elemental abundance ratios in Complex C. Along the PG 1259+593 sight line, we measure N(Si )/N(O ) = 0.10 ± 0.02, N(C )/N(O ) = 0.35, and N(N )/N(O ) < 0.07 (3 σ). These ratios are inconsistent with collisional ionization equilibrium at one kinetic temperature. Photoionization by the extragalactic background is ruled out as the source of the high ions since the path lengths required would make HVCs unreasonably large; photoionization by radiation from the disk of the Galaxy also appears unlikely since the emerging photons are not energetic enough to produce O VI. By themselves, ionic ratios are insufficient to discriminate between various ionization models, but by considering the absorption kinematics as well, we consider the most likely origin for the highly ionized high-velocity gas to be at the conductive or turbulent interfaces between the neutral/warm ionized components of Complex C and a surrounding hot medium. The presence of interfaces on the surface of HVCs provides indirect evidence for the existence of a hot medium in which the HVCs are immersed. This medium could be a hot (T 106 K) extended Galactic corona or hot gas in the Local Group.

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

Optical atmospheric extinction over Cerro Paranal

Aims. The present study was conducted to determine the optical extinction curve for Cerro Paranal under typical clear-sky observing conditions, with the purpose of providing the community with a function to be used to correct the observed spectra, with an accuracy of 0.01 mag airmass −1 . Additionally, this work was meant to analyze the variability of the various components, to derive the main atmospheric parameters, and to set a term of reference for future studies, especially in view of the construction of the Extremely Large Telescope on the nearby Cerro Armazones. Methods. The extinction curve of Paranal was obtained through low-resolution spectroscopy of 8 spectrophotometric standard stars observed with FORS1 mounted at the 8.2 m Very Large Telescope, covering a spectral range 3300–8000 A. A total of 600 spectra were collected on more than 40 nights distributed over six months, from October 2008 to March 2009. The average extinction curve was derived using a global fit algorithm, which allowed us to simultaneously combine all the available data. The main atmospheric parameters were retrieved using the LBLRTM radiative transfer code, which was also utilised to study the impact of variability of the main molecular bands of O2 ,O 3 ,a nd H 2O, and to estimate their column densities. Results. In general, the extinction curve of Paranal appears to conform to those derived for other astronomical sites in the Atacama desert, like La Silla and Cerro Tololo. However, a systematic deficit with respect to the extinction curve derived for Cerro Tololo before the El Chichon eruption is detected below 4000 A. We attribute this downturn to a non standard aerosol composition, probably revealing the presence of volcanic pollutants above the Atacama desert. An analysis of all spectroscopic extinction curves obtained since 1974 shows that the aerosol composition has been evolving during the last 35 years. The persistence of traces of non meteorologic haze suggests the effect of volcanic eruptions, like those of El Chichon and Pinatubo, lasts several decades. The usage of the standard CTIO and La Silla extinction curves implemented in IRAF and MIDAS produce systematic over/under-estimates of the absolute flux.

Physical conditions in high-redshift GRB-DLA absorbers observed with VLT/UVES: Implications for molecular hydrogen searches

Aims. We aim to understand the nature of the absorbing neutral gas in the galaxies hosting high-redshift long-duration gamma-ray bursts (GRBs) and to determine their physical conditions. Methods. A detailed analysis of high-quality VLT/UVES spectra of the optical afterglow of GRB 050730 and other Swift-era bursts is presented. Results. We report the detection of a significant number of previously unidentified allowed transition lines of Fe + , involving the fine structure of the ground term ( 6 D7/2, 6 D5/2, 6 D3/2, 6 D1/2) and that of other excited levels ( 4 F9/2, 4 F7/2, 4 F5/2, 4 F3/2, 4 D7/2, 4 D5/2), from the zabs = 3.969, log N(H 0 ) = 22.10, damped Lyman-α (DLA) system located in the host galaxy of GRB 050730. No molecular hydrogen (H2) is detected down to a molecular fraction of log f < −8.0. We derive accurate metal abundances for Fe + ,S + ,N 0 , Ni + , and, for the first time in this system, Si + and Ar 0 . The absorption lines are best-fit as a single narrow velocity component at zabs = 3.96857. The time-dependent evolution of the observed Fe + energy-level populations is modelled by assuming the excitation

A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds

We present a Far Ultraviolet Spectroscopic Explorer survey of highly ionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with (S/N)1030 > 8. We search the spectra for high-velocity (100 km s-1 180?. Eleven of these highly ionized HVCs are positive-velocity wings (broad O VI features extending asymmetrically to velocities of up to 300 km s-1). We find that 81% (30 of 37) of highly ionized HVCs have clear accompanying C III absorption, and 76% (29 of 38) have accompanying H I absorption in the Lyman series. We present the first (O VI selected) sample of C III and H I absorption line HVCs and find b(C ) = 30 ? 8 km s-1, log Na(C ) ranges from 14.4, b(H ) = 22 ? 5 km s-1, and log Na(H I) ranges from 16.9. The lower average width of the high-velocity H I absorbers implies the H I lines arise in a separate, lower temperature phase than the O VI. The ratio Na(C III)/Na(O VI) is generally constant with velocity in highly ionized HVCs, suggesting that at least some C III resides in the same gas as the O VI. Collisional ionization equilibrium models with solar abundances can explain the O VI/C III ratios for temperatures near 1.7 ? 105 K; nonequilibrium models with the O VI frozen in at lower temperatures are also possible. Photoionization models are not viable since they underpredict O VI by several orders of magnitude. The presence of associated C III and H I strongly suggests the highly ionized HVCs are not formed in the hotter plasma that gives rise to O VII and O VIII X-ray absorption. We find that the shape of the O VI positive-velocity wing profiles is well reproduced by a radiatively cooling, vertical outflow moving with ballistic dynamics, with T0 = 106 K, n0 ? 2 ? 10-3 cm-3, and v0 ? 250 km s-1. However, the outflow has to be patchy and out of ionization equilibrium to explain the sky distribution and the simultaneous presence of O VI, C III, and H I. We found that a spherical outflow can produce high-velocity O VI components (as opposed to the wings), showing that the possible range of outflow model results is too broad to conclusively identify whether or not an outflow has left its signature in the data. An alternative model, supported by the similar multiphase structure and similar O VI properties of highly ionized and 21 cm HVCs, is one where the highly ionized HVCs represent the low N(H I) tail of the HVC population, with the O VI formed at the interfaces around the embedded H I cores. Although we cannot rule out the possibility that some highly ionized HVCs exist in the Local Group or beyond, we favor a Galactic origin. This is based on the recent evidence that both H I HVCs and the million-degree gas detected in X-ray absorption are Galactic phenomena. Since the highly ionized HVCs appear to trace the interface between these two Galactic phases, it follows that highly ionized HVCs are Galactic themselves. However, the nondetection of high-velocity O VI in halo star spectra implies that any Galactic high-velocity O VI exists at z distances beyond a few kpc.

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. O i, C ii, C iv, Si ii, Si iii, Si iv). This allows us to identify neutral, weakly ionized or highly ionized HVCs over several decades in H i column densities. The sky covering factor of UV-selected HVCs with |vLSR| ≥ 90 km s−1 is about 68 per cent for the Galactic sky at . About 74 per cent of the HVC directions have  cm−2 and 46 per cent have  cm−2. We show that our survey is essentially complete, i.e. an undetected population of HVCs with extremely low H (H i + H ii) 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 H i HVCs and show that large H i HVC complexes have generally ionized envelopes extending far from the H i contours. There are also large regions of the Galactic sky that are covered with ionized high-velocity gas with little H i emission nearby. We show that the covering factors of HVCs with  km s−1 drawn from the AGN and stellar (with stars at d > 3 kpc) samples are similar. This confirms that most of the HVCs are within 5–15 kpc of the sun. The HVCs with |vLSR |≳170 km s−1 are largely associated with the Magellanic Stream at b 20° as well as other large known H i 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 50 kpc from its host, except possibly for those tracing remnants of galaxy interaction.

Hot halos around high redshift protogalaxies : Observations of O VI and N V absorption in damped Lyman-α systems

Aims. We present a study of the highly ionized gas (plasma) associated with damped Lyman-α (DLA) systems at z = 2.1−3.1. Methods. We search for O vi absorption and corresponding Si iv ,C iv ,a nd Nv in a Very Large Telescope/Ultraviolet-Visible Echelle Spectrograph (VLT/UVES) sample of 35 DLA systems with data covering O vi at S /N > 10. We then use optical depth profile comparisons and ionization modelling to investigate the properties, phase structure, and origin of the plasma. Results. We report twelve DLAs (nine intervening and three at 19. 5t o>21.1, and logN Warmii > 19. 4t o>20.9. On average, the hot and warm phases thus contain >40% and >20% of the baryonic mass of the neutral phase in DLAs, respectively. Conclusions. If the temperature in the O vi phase is ≈10 6 Ka nd sofOvi = Ovi/O � 0. 2, the plasma can make as ignif icant contribution to the metal budget at high redshift. Additional searches for O vi in Lyman Limit Systems (QSO absorbers with 17.0 < NHi < 20.3) will be necessary to determine the total quantity of baryons and metals hidden in hot halos at z ≈ 2.