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Register a new userThe VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems
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We have searched for molecular hydrogen in damped Lyman-alpha (DLA) and sub-DLA systems at z>1.8 using UVES at the VLT. Out of the 33 systems in our sample, 8 have firm and 2 have tentative detections of associated H2 absorption lines. Considering that 3 detections were already known from past searches, H2 is detected in 13 to 20 percent of the newly-surveyed systems. We report new detections of molecular hydrogen at z=2.087 and 2.595 toward, respectively, Q 1444+014 and Q 0405-443, and also reanalyse the system at z=3.025 toward Q 0347-383. We find that there is a correlation between metallicity and depletion factor in both our sample and also the global population of DLA systems (60 systems in total). The DLA and sub-DLA systems where H2 is detected are usually amongst those having the highest metallicities and the largest depletion factors. Moreover, the individual components where H2 is detected have depletion factors systematically larger than other components in the profiles. In two different systems, one of the H2-detected components even has [Zn/Fe]>=1.4. These are the largest depletion factors ever seen in DLA systems. All this clearly demonstrates the presence of dust in a large fraction of the DLA systems. The mean H2 molecular fraction is generally small in DLA systems and similar to what is observed in the Magellanic Clouds. From 58 to 75 percent of the DLA systems have log f<-6. This can be explained if the formation rate of H2 onto dust grains is reduced in those systems, probably because the gas is warm (T>1000 K) and/or the ionizing flux is enhanced relative to what is observed in our Galaxy.
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We report results from our mini-survey for molecular hydrogen in eight high redshift damped Lyman-alpha (DLA) systems using the ESO Ultra-violet and Visible Spectrograph on the VLT. In addition, we investigate two systems using ESO public data. We include in the sample the only system where H2 was previously detected and studied at high-spectral resolution. Altogether our sample consists of eleven absorbers with 1.85<z_abs<3.4. We confirm the presence of H2 in the z_abs = 2.3377, metal-poor ([Si/H] = -1.20) system toward PKS 1232+082. The derived molecular fraction, f = 2N(H2)/(2N(H2)+N(HI)) = 4 10-4, is two orders of magnitude less than what has been claimed previously from low-resolution data. The physical conditions within the cloud can be constrained directly from observation. The kinetic temperature and particle density are in the ranges, respectively, 100<T<300 K and 30<nH<50 cm-3. In addition, UV pumping is of the same order of magnitude than in our Galaxy. The upper limits on the molecular fraction derived in nine of the systems are in the range 1.2 10-7 - 1.6 10-5. The molecular abundance in a few DLA systems (and in particular in the two systems where H2 is detected) is consistent with what is seen in the Magellanic clouds. But most of the DLA measurements are well below these values. This is probably partly due to small amounts of dust and/or high UV flux. We argue however that the lack of molecules is a direct consequence of high kinetic temperature (T>3000 K) implying a low formation rate of H2 onto dust grains. Therefore, most of the DLA systems arise in warm and diffuse neutral gas. (Abridged)
We have used the Very Long Baseline Array to image 18 quasars with foreground damped Lyman-$alpha$ systems (DLAs) at 327, 610 or 1420 MHz, to measure the covering factor $f$ of each DLA at or near its redshifted HI 21cm line frequency. Including six systems from the literature, we find that none of 24 DLAs at $0.09 < z < 3.45$ has an exceptionally low covering factor, with $f sim 0.45 - 1$ for the 14 DLAs at $z > 1.5$, $f sim 0.41 - 1$ for the 10 systems at $z < 1$, and consistent covering factor distributions in the two sub-samples. The observed paucity of detections of HI 21cm absorption in high-$z$ DLAs thus cannot be explained by low covering factors and is instead likely to arise due to a larger fraction of warm HI in these absorbers.
We report results from a programme aimed at investigating the temperature of neutral gas in high-redshift damped Lyman-$alpha$ absorbers (DLAs). This involved (1) HI 21cm absorption studies of a large DLA sample, (2) VLBI studies to measure the low-frequency quasar core fractions, and (3) optical/ultraviolet spectroscopy to determine DLA metallicities and velocity widths. Including literature data, our sample consists of 37 DLAs with estimates of the spin temperature $T_s$ and the covering factor. We find a strong $4sigma$) difference between the $T_s$ distributions in high-z (z>2.4) and low-z (z<2.4) DLA samples. The high-z sample contains more systems with high $T_s$ values, $gtrsim 1000$ K. The $T_s$ distributions in DLAs and the Galaxy are also clearly (~$6sigma$) different, with more high-$T_s$ sightlines in DLAs than in the Milky Way. The high $T_s$ values in the high-z DLAs of our sample arise due to low fractions of the cold neutral medium. For 29 DLAs with metallicity [Z/H] estimates, we confirm the presence of an anti-correlation between $T_s$ and [Z/H], at $3.5sigma$ significance via a non-parametric Kendall-tau test. This result was obtained with the assumption that the DLA covering factor is equal to the core fraction. Monte Carlo simulations show that the significance of the result is only marginally decreased if the covering factor and the core fraction are uncorrelated, or if there is a random error in the inferred covering factor. We also find evidence for redshift evolution in DLA $T_s$ values even for the z>1 sub-sample. Since z>1 DLAs have angular diameter distances comparable to or larger than those of the background quasars, they have similar efficiency in covering the quasars. Low covering factors in high-z DLAs thus cannot account for the observed redshift evolution in spin temperatures. (Abstract abridged.)
We present a detailed analysis of three extremely strong intervening DLAs (log N(HI)>=21.7) observed towards quasars with VLT/UVES. We measure overall metallicities of [Zn/H]~-1.2, -1.3 and -0.7 at respectively zabs=2.34 towards SDSS J2140-0321 (log N(HI) = 22.4+/-0.1), zabs=3.35 towards SDSS J1456+1609 (log N(HI) = 21.7+/-0.1) and zabs=2.25 towards SDSS J0154+1935 (log N(HI) = 21.75+/-0.15). We detect H2 towards J2140-0321 (log N(H2) = 20.13+/-0.07) and J1456+1609 (log N(H2) = 17.10+/-0.09) and argue for a tentative detection towards J0154+1935. Absorption from the excited fine-structure levels of OI, CI and SiII are detected in the system towards J2140-0321, that has the largest HI column density detected so far in an intervening DLA. This is the first detection of OI fine-structure lines in a QSO-DLA, that also provides us a rare possibility to study the chemical abundances of less abundant atoms like Co and Ge. Simple single phase photo-ionisation models fail to reproduce all the observed quantities. Instead, we suggest that the cloud has a stratified structure: H2 and CI likely originate from both a dense (log nH~2.5-3) cold (80K) and warm (250K) phase containing a fraction of the total HI while a warmer (T>1000 K) phase probably contributes significantly to the high excitation of OI fine-structure levels. The observed CI/H2 column density ratio is surprisingly low compared to model predictions and we do not detect CO molecules: this suggests a possible underabundance of C by 0.7 dex compared to other alpha elements. The absorber could be a photo-dissociation region close to a bright star (or a star cluster) where higher temperature occurs in the illuminated region. Direct detection of on-going star formation through e.g. NIR emission lines in the surrounding of the gas would enable a detailed physical modelling of the system.
We have obtained high signal:to:noise optical spectroscopy at 5AA resolution of 27 quasars from the APM z$>$4 quasar survey. The spectra have been analyzed to create new samples of high redshift Lyman-limit and damped Lyman-$alpha$ absorbers. These data have been combined with published data sets in a study of the redshift evolution and the column density distribution function for absorbers with $log$N(HI)$ge17.5$, over the redshift range 0.01 $<$ z $<$ 5. The main results are: begin{itemize} item Lyman limit systems: The data are well fit by a power law $N(z) = N_0(1 + z)^{gamma}$ for the number density per unit redshift. For the first time intrinsic evolution is detected in the product of the absorption cross-section and comoving spatial number density for an $Omega = 1$ Universe. We find $gamma = 1.55$ ($gamma = 0.5$ for no evolution) and $N_0 = 0.27$ with $>$99.7% confidence limits for $gamma$ of 0.82 & 2.37. item Damped lya systems: The APM QSOs provide a substantial increase in the redshift path available for damped surveys for $z>3$. Eleven candidate and three confirmed damped Ly$alpha$ absorption systems, have been identified in the APM QSO spectra covering the redshift range $2.8le z le 4.4$ (11 with $z>3.5$). Combining the APM survey confirmed and candidate damped lya absorbers with previous surveys, we find evidence for a turnover at z$sim$3 or a flattening at z$sim$2 in the cosmological mass density of neutral gas, $Omega_g$. end{itemize} The Lyman limit survey results are published in Storrie-Lombardi, et~al., 1994, ApJ, 427, L13. Here we describe the results for the DLA population of absorbers.
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