Metal abundances at z<1.5: new measurements in sub-Damped Lyman-alpha Absorbers

Damped Lyman-alpha systems (DLAs) and sub-DLAs seen toward background quasars provide the most detailed probes of elemental abundances. Somewhat paradoxically these measurements are more difficult at lower redshifts due to the atmospheric cut-off, and so a few years ago our group began a programme to study abundances at z < 1.5 in quasar absorbers. In this paper, we present new UVES observations of six additional quasar absorption line systems at z < 1.5, five of which are sub-DLAs. We find solar or above solar metallicity, as measured by the abundance of zinc, assumed not to be affected by dust, in two sub-DLAs: one, towards Q0138-0005 with [Zn/H]=+0.28 +/- 0.16; the other towards Q2335+1501 with [Zn/H]=+0.07 +/- 0.34. Relatively high metallicity was observed in another system: Q0123-0058 with [Zn/H]=-0.45 +/- 0.20. Only for the one DLA in our sample, in Q0449-1645, do we find a low metallicity, [Zn/H]=-0.96 +/- 0.08. We also note that in some of these systems large relative abundance variations from component to component are observed in Si, Mn, Cr and Zn.Comment: 7 figures and 10 tables. Accepted for publication in MNRA

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

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.Comment: This paper is part of a set of three papers on circumgalactic gas observed with the Cosmic Origins Spectrograph on HST, to be published in Science, together with related papers by Tripp et al. and Lehner & Howk, in the November 18, 2011 edition. This version has not undergone final copyediting. Please see Science online for the final printed versio

The Role of Sub-damped Lyman-alpha Absorbers in the Cosmic Evolution of Metals

Observations of low mean metallicity of damped Lyman-alpha (DLA) quasar absorbers at all redshifts studied appear to contradict the predictions for the global mean interstellar metallicity in galaxies from cosmic chemical evolution models. On the other hand, a number of metal-rich sub-DLA systems have been identified recently, and the fraction of metal-rich sub-DLAs appears to be considerably larger than that of metal-rich DLAs, especially at z < 1.5. In view of this, here we investigate the evolution of metallicity in sub-DLAs. We find that the mean Zn metallicity of the observed sub-DLAs may be higher than that of the observed DLAs, especially at low redshifts, reaching a near-solar level at z <~ 1. This trend does not appear to be an artifact of sample selection, the use of Zn, the use of N_{HI}-weighting, or observational sensitivity. While a bias against very low metallicity could be present in the sub-DLA sample in some situations, this cannot explain the difference between the DLA and sub-DLA metallicities at low z. The primary reason for the difference between the DLAs and sub-DLAs appears to be the dearth of metal-rich DLAs. We estimate the sub-DLA contribution to the total metal budget using measures of their metallicity and comoving gas density. These calculations suggest that at z <~ 1, the contribution of sub-DLAs to the total metal budget may be several times that of DLAs. At higher redshifts also, there are indications that the sub-DLAs may contribute significantly to the cosmic metal budget.Comment: 9 pages, 2 figures, Accepted for Publication in the Astrophysical Journa

Complete homochirality induced by the nonlinear autocatalysis and recycling

A nonlinear autocatalysis of a chiral substance is shown to achieve homochirality in a closed system, if the back-reaction is included. Asymmetry in the concentration of two enantiomers or the enantiometric excess increases due to the nonlinear autocatalysis. Furthermore, when the back-reaction is taken into account, the reactant supplied by the decomposition of the enantiomers is recycled to produce more and more the dominant one, and eventually the homochirality is established.Comment: 4 pages, 2 figure

Metal-rich Damped/sub-Damped Lyman-alpha Quasar Absorbers at z<1

Damped Lyman-alpha absorbers (DLAs), seen in absorption against a background quasar, provide the most detailed probes available of element abundances in the Universe over > 90 % of its age. DLAs can be used to observationally measure the global mean metallicity in the Universe and its evolution with time. Paradoxically, these observations are more difficult at lower redshifts, where the absorber rest-frame UV spectra are cut-off due to the atmospheric absorption. We present here high-resolution VLT/UVES observations of several elements contained in three DLAs and one sub-DLA with 0.6<z_abs<0.9. We detect Mg I, Mg II, Fe II, Zn II, Cr II, Mn II, Ti II and Ca II. Our observations more than double the high-resolution sample of [Zn/H] at z<1. We also report the discovery of three metal-rich systems, whereas most previous measurements show low N(HI)-weighted mean metallicity projecting to about 1/6th solar level at z=0. We derive [Zn/H]=-0.11+/-0.04 at z_abs=0.725, [Zn/H]=-0.54+/-0.20 at z_abs=0.740 and [Zn/H]=-0.49+/-0.22 at z_abs=0.652, plus one additional upper limit ([Zn/H]<-0.36 at z_abs=0.842). These measurements confirm the existence of quasar absorbers with relatively high metallicities based on abundance estimates free from the effect of dust depletion. Possible implications of these results for the metallicity of neutral gas phase in the past ~ 8 Gyr are presented and compared with models.Comment: 13 pages, 8 figures. Accepted for publication in MNRA

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(HI)=17.06\pm0.05 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 \pm 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, \Delta v = -26 \kms) with near solar metallicity ([O/ H]=-0.20 \pm 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 rho < 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.Comment: Accepted for publication in the Astrophysical Journa

TB Control in Humans and Animals in South Africa: A Perspective on Problems and Successes

Mycobacterium tuberculosis (M. tb) remains one of the most globally serious infectious agents for human morbidity and mortality, but with significant differences in prevalence across the globe. In many countries, the incidence is now low and declining, but control and eradication remain a distant view. Similarly, the prevalence of bovine TB caused by Mycobacterium bovis (M. bovis), varies significantly across regions, although unlike for M. tuberculosis, data are sparse. The reduction in incidence and prevalence and control of both human and bovine TB is difficult and costly, yet some countries have managed to do this with some success. This perspective will consider some of the critical control steps we now know to be important for the control of TB from M. tuberculosis in humans living in South Africa, where the incidence of TB is the highest currently experienced. Despite the high incidence of human TB, South Africa has been able to reduce this incidence remarkably in the past few years, despite limited resources and high HIV prevalence. We draw from our experience to ascertain whether we may learn useful lessons from control efforts for both diseases in order to suggest effective control measures for bovine TB

Super-Solar Metallicity in Weak Mg II Absorption Systems at z ~ 1.7

Through photoionization modeling, constraints on the physical conditions of three z ~ 1.7 single-cloud weak Mg II systems (W_r(2796) < 0.3A) are derived. Constraints are provided by high resolution R = 45,000, high signal-to-noise spectra of the three quasars HE0141-3932, HE0429-4091, and HE2243-6031 which we have obtained from the ESO archive of VLT/UVES. Results are as follows: (1) The single-cloud weak Mg II absorption in the three z ~ 1.7 systems is produced by clouds with ionization parameters of -3.8 < logU < -2.0 and sizes of 1-100 pc. (2) In addition to the low-ionization phase Mg II clouds, all systems need an additional 1-3 high-ionization phase C IV clouds within 100 km/s of the Mg II component. The ionization parameters of the C IV phases range from -1.9 < logU < -1.0, with sizes of tens of parsecs to kiloparsecs. (3) Two of the three single-cloud weak Mg II absorbers have near-solar or super-solar metallicities, if we assume a solar abundance pattern. Although such large metallicities have been found for z < 1 weak Mg II absorbers, these are the first high metallicities derived for such systems at higher redshifts. (4) Two of the three weak Mg II systems also need additional low-metallicity, broad Lya absorption lines, offset in velocity from the metal-line absorption, in order to reproduce the full Lya profile. (5) Metallicity in single-cloud weak Mg II systems are more than an order of magnitude larger than those in Damped Lya systems at z ~ 1.7. In fact, there appears to be a gradual decrease in metallicity with increasing N(HI), from these, the most metal-rich Lya forest clouds, to Lyman limit systems, to sub-DLAs, and finally to the DLAs.Comment: 25 pages, 7 figures, accepted by the Astrophysical Journa

Multiphase Gas In Galaxy Halos: The OVI Lyman-limit System toward J1009+0713

We have serendipitously detected a strong O VI-bearing Lyman limit system at z_abs = 0.3558 toward the QSO J1009+0713 (z_em = 0.456) in our survey of low-redshift galaxy halos with the Hubble Space Telescope's Cosmic Origins Spectrograph. Its rest-frame equivalent width of W_r = 835 +/- 49 mA is the highest for an intervening absorber yet detected in any low-redshift QSO sightline, with absorption spanning 400 km s^-1 in its rest frame. HST/WFC3 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 bulk of the absorbing gas traced by H I resides in two strong, blended component groups that possess a total logN(HI) = 18 - 18.8. The 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. We constrain the metallicity firmly within the range 0.1 - 1 Zsun, and photoionization modeling indirectly indicates a subsolar metallicity of 0.05 - 0.5 Zsun. The appearance of strong O VI and nine Mg II components and our review of similar systems in the literature support the "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, however the total O VI column densities still far exceed the values expected from interface models for the number of detected clouds.Comment: 20 pages, 11 figures, accepted for publication in Ap

The COS Absorption Survey of Baryon Harbors (CASBaH): Warm-hot Circumgalactic Gas Reservoirs Traced by Ne VIII Absorption

We survey the highly ionized circumgalactic media (CGM) of 29 blindly selected galaxies at 0.49 < z_(gal) < 1.44 based on high-S/N ultraviolet spectra of z > 1 QSOs and the galaxy database from the COS Absorption Survey of Baryon Harbors (CASBaH). We detect the Ne VIII doublet in nine of the galaxies, and for gas with N(Ne VIII) > 10^13.3 cm^-2 (> 10^13.5 cm^-2), we derive a Ne VIII covering fraction f_c = 75 +15/-25% (44 +22/-20%) within impact parameter (rho) < 200 kpc of M_* = 10^(9.5-11.5) Msol galaxies and f_c = 70 +16/-22% (f_c = 42 +20/-17%) within rho < 1.5 virial radii. We estimate the mass in Ne VIII-traced gas to be M_gas(Ne VIII) > 10^9.5 Msol (Z/Zsol)^-1, or 6-20% of the expected baryonic mass if the Ne VIII absorbers have solar metallicity. Ionizing Ne VII to Ne VIII requires 207 eV, and photons with this energy are scarce in the CGM. However, for the median halo mass and redshift of our sample, the virial temperature is close to the peak temperature for the Ne VIII ion, and the Ne VIII-bearing gas is plausibly collisionally ionized near this temperature. Moreover, we find that photoionized Ne VIII requires cool and low-density clouds that would be highly underpressured (by approximately two orders of magnitude) relative to the putative, ambient virialized medium, complicating scenarios where such clouds could survive. Thus, more complex (e.g., non-equilibrium) models may be required; this first statistical sample of Ne VIII absorber/galaxy systems will provide stringent constraints for future CGM studies.Comment: Published in ApJL, Volume 877, Issue 2, Article L2