Intervening Metal Systems in GRB and QSO sight-lines: The Mgii and Civ Question

Prochter et al. 2006 recently found that the number density of strong intervening 0.5<z<2 MgII absorbers detected in gamma-ray burst (GRB) afterglow spectra is nearly 4 times larger than in QSO spectra. We have conducted a similar study using CIV absorbers. Our CIV sample, consisting of a total of 20 systems, is drawn from 3 high resolution and high to moderate S/N VLT/UVES spectra of 3 long-duration GRB afterglows, covering the redshift interval 1.6< z<3.1. The column density distribution and number density of this sample do not show any statistical difference with the same quantities measured in QSO spectra. We discuss several possibilities for the discrepancy between CIV and MgII absorbers and conclude that a higher dust extinction in the MgII QSO samples studied up to now would give the most straightforward solution. However, this effect is only important for the strong MgII absorbers. Regardless of the reasons for this discrepancy, this result confirms once more that GRBs can be used to detect a side of the universe that was unknown before, not necessarily connected with GRBs themselves, providing an alternative and fundamental investigative tool of the cosmic evolution of the universe.Comment: 21 pages, 4 figures, ApJ accepted, Revised after Referee Repor

The VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems: Physical conditions in the neutral gas

We study the physical conditions in DLAs, using a sample of 33 systems toward 26 QSOs acquired for a recently completed survey of H_2 by Ledoux et al. (2003). We use the column densities of H_2 in different rotational levels, together with those of C I, C I*, C I**, C II* and singly ionized atomic species to discuss the kinetic temperature, the density of hydrogen and the electronic density in the gas together with the ambient UV radiation field. Detailed comparisons are made between the observed properties in DLAs, the interstellar medium (ISM) of the Galaxy, LMC and SMC. The mean kinetic temperature of the gas corresponding to DLA subcomponents in which H_2 absorption line is detected, derived from the ortho-to-para ratio (153+/-78 K), is higher than that measured in the ISM (77+/-17 K) and the Magellanic clouds (82+/-21 K). Typical pressure in these components (corresponding to T = 100-300 K and n = 10-200 cm^-3), measured using C I fine-structure excitation, are higher than what is measured along ISM sightlines. This is consistent with the corresponding higher values for N(H_2,J=2)/N(H_2,J=0) seen in DLAs. From the column densities of the high-J rotational levels, we derive that the typical radiation field in the H_2 bearing components is of the order of or slightly higher than the mean UV field in the Galactic ISM. Determination of electron density in the gas with H_2 and C I show the ionization rate is similar to that of a cold neutral medium (CNM) in a moderate radiation field. This, together with the fact that we see H_2 in 13-20% of the DLAs, can be used to conclude that DLAs at z>1.9 could contribute as much as 50% star formation rate density seen in Lyman break galaxies (LBGs). (Abridged)Comment: 21 pages, 16 figures, accepted for publication in MNRA

The high A_V Quasar Survey: A z=2.027 metal-rich damped Lyman-alpha absorber towards a red quasar at z=3.21

To fully exploit the potential of quasars as probes of cosmic chemical evolution and the internal gas dynamics of galaxies it is important to understand the selection effects behind the quasar samples and in particular if the selection criteria exclude foreground galaxies with certain properties (most importantly a high dust content). Here we present spectroscopic follow-up from the 10.4-m GTC telescope of a dust-reddened quasar, eHAQ0111+0641, from the extended High A_V Quasar (HAQ) survey. We find that the z=3.21 quasar has a foreground Damped Lyman-alpha Absorber (DLA) at z=2.027 along the line of sight. The DLA has very strong metal lines due to a moderately high metallicity (with an inferred lower limit of 25% of the solar metallicity), but a very large gas column density along the line-of-sight in its host galaxy. This discovery is further evidence that there is a dust bias affecting the census of metals, caused by the combined effect of dust obscuration and reddening, in existing samples of z>2 DLAs. The case of eHAQ0111+0641 illustrates that dust bias is not only caused by dust obscuration, but also dust reddening.Comment: 7 pages, 4 figures. Accepted for publication in A&A. A few typos have been correcte

A clumpy and anisotropic galaxy halo at z=1 from gravitational-arc tomography

Every star-forming galaxy has a halo of metal-enriched gas extending out to at least 100 kpc, as revealed by the absorption lines this gas imprints on the spectra of background quasars. However, quasars are sparse and typically probe only one narrow pencil beam through the intervening galaxy. Close quasar pairs and gravitationally lensed quasars have been used to circumvent this inherently one-dimensional technique, but these objects are rare and the structure of the circum-galactic medium remains poorly constrained. As a result, our understanding of the physical processes that drive the re-cycling of baryons across the lifetime of a galaxy is limited. Here we report integral-field (tomographic) spectroscopy of an extended background source -a bright giant gravitational arc. We can thus coherently map the spatial and kinematic distribution of Mg II absorption -a standard tracer of enriched gas- in an intervening galaxy system at redshift 0.98 (i.e., ~8 Gyr ago). Our gravitational-arc tomography unveils a clumpy medium in which the absorption-strength decreases with increasing impact parameter, in good agreement with the statistics towards quasars; furthermore, we find strong evidence that the gas is not distributed isotropically. Interestingly, we detect little kinematic variation over a projected area of ~600 kpc squared, with all line-of-sight velocities confined to within a few tens of km/s of each other. These results suggest that the detected absorption originates from entrained recycled material, rather than in a galactic outflow.Comment: Published online in Nature on 31 January 201

Statistics and characteristics of MgII absorbers along GRB lines of sight observed with VLT-UVES

We analyse the properties of MgII absorption systems detected along the sightlines toward GRBs using a sample of 10 GRB afterglow spectra obtained with VLT-UVES over the past six years. The S/N ratio is sufficiently high that we can extend previous studies to smaller equivalent widths (typically Wr>0.3A). Over a pathlength of Delta(z)~14 the number of weak absorbers detected is similar along GRB and QSO lines of sight, while the number of strong systems is larger along GRB lines of sight with a 2-sigma significance. Using intermediate and low resolution observations reported in the literature, we increase the absorption length for strong systems to Delta(z)=31.5 (about twice the path length of previous studies) and find that the number density of strong MgII systems is a factor of 2.1+/-0.6 higher (about 3-sigma significance) toward GRBs as compared to QSOs, about twice smaller however than previously reported. We divide the sample in three redshift bins and we find that the number density of strong MgII is larger in the low redshift bins. We investigate in detail the properties of strong MgII systems observed with UVES. Both the estimated dust extinction in strong GRB MgII systems and the equivalent width distribution are consistent with what is observed for standard QSO systems. We find also that the number density of (sub)-DLAs per unit redshift in the UVES sample is probably twice larger than what is expected from QSO sightlines which confirms the peculiarity of GRB lines of sight. These results indicate that neither a dust extinction bias nor different beam sizes of the sources are viable explanations for the excess. It is still possible that the current sample of GRB lines of sight is biased by a subtle gravitational lensing effect. More data and larger samples are needed to test this hypothesis. (abridged)Comment: 12 pages, 10 figures; accepted for publication in A&A (12 June 2009

Comprehensive Study of a z = 2.35 DLA Galaxy: Mass, Metallicity, Age, Morphology and SFR from HST and VLT

We present a detailed study of the emission from a z = 2.35 galaxy that causes damped Lyman-alpha absorption in the spectrum of the background QSO, SDSS J 2222-0946. We present the results of extensive analyses of the stellar continuum covering the rest frame optical-UV regime based on broad-band HST imaging, and of spectroscopy from VLT/X-Shooter of the strong emission lines: Ly-alpha, [OII], [OIII], [NII], H-alpha and H-beta. We compare the metallicity from the absorption lines in the QSO spectrum with the oxygen abundance inferred from the strong-line methods (R23 and N2). The two emission-line methods yield consistent results: [O/H] = -0.30+/-0.13. Based on the absorption lines in the QSO spectrum a metallicity of -0.49+/-0.05 is inferred at an impact parameter of 6.3 kpc from the centre of the galaxy with a column density of hydrogen of log(N_HI)=20.65+/-0.05. The star formation rates of the galaxy from the UV continuum and H-alpha line can be reconciled assuming an amount of reddening of E(B-V) = 0.06+/-0.01, giving an inferred SFR of 13+/-1 M_sun / yr (assuming a Chabrier IMF). From the HST imaging, the galaxy associated with the absorption is found to be a compact (re=1.12 kpc) object with a disc-like, elongated (axis ratio 0.17) structure indicating that the galaxy is seen close to edge on. Moreover, the absorbing gas is located almost perpendicularly above the disc of the galaxy suggesting that the gas causing the absorption is not co-rotating with the disc. We investigate the stellar and dynamical masses from SED-fitting and emission-line widths, respectively, and find consistent results of 2x10^9 M_sun. We suggest that the galaxy is a young proto-disc with evidence for a galactic outflow of enriched gas. This galaxy hints at how star-forming galaxies may be linked to the elusive population of damped Lyman-alpha absorbers.Comment: 11 pages, 7 figures. Accepted for publication in MNRA

Three intervening galaxy absorbers towards GRB060418: faint and dusty?

We present an analysis of three strong, intervening Mg II absorption systems (z_abs = 0.603, 0.656, 1.107) towards the optical afterglow of gamma-ray burst (GRB) 060418. From high resolution UVES spectra we measure metal column densities and find that the highest redshift absorber exhibits a large amount of dust depletion compared with DLAs seen in QSO spectra. The intervening z_abs = 1.107 absorber is also unusual in exhibiting a clear 2175 A bump, the first time this feature has been definitively detected in a GRB spectrum. The GRB afterglow spectrum is best fit with a two component extinction curve: an SMC extinction law at z=1.49 (the redshift of the host) with E(B-V) = 0.07+-0.01 and a Galactic extinction curve at z ~ 1.1 with E(B-V) = 0.08+-0.01. We also present a moderately deep NTT R-band image of the GRB060418 field and spectroscopy of four galaxies within 1 arcminute. None of these objects has a redshift that matches any of the intervening absorbers, and we conclude that the galaxies responsible for the two intervening Mg II absorbers at z ~ 0.6 have luminosities ~<0.3 L^star.Comment: Accepted for publication in MNRAS (Letters), 5 pages. Updated with more accurate host positio

A new precise determination of the primordial abundance of deuterium

We present the detection of D I lines in the metal poor ([O/H] = −1.73 ± 0.02 ) sub-Damped Lyman-α system (log N(H I) = 19.310 ± 0.005 ) at zabs = 3.42 towards the quasar J1332+0052. From simultaneous fitting the Lyman series of H I and D I as well as metal lines observed at high signal-to-noise and high spectral resolution with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope and High Resolution Echelle Spectrometer on W.M. Keck Observatory, we measure 105×D I/H I= 2.388 ± 0.112 (stat + syst). Given the expected negligible ionisation corrections and the lack of deuterium astration at this metallicity, this ratio provides a direct measurement of the primordial deuterium abundance. Together with previous similar measurements, we pin down the primordial deuterium abundance to 105×(D/H)p= 2.541 ± 0.025, in remarkable agreement with the Standard Model value (to within 1.1σ). The work is supported by RSF grant No 23-12-00166