67 research outputs found
The ESO UVES Advanced Data Products Quasar Sample - I. Dataset and New N_HI Measurements of Damped Absorbers
We present here a dataset of quasars observed with the Ultraviolet Visual
Echelle Spectrograph (UVES) on the VLT and available in the ESO UVES Advanced
Data Products archive. The sample is made up of a total of 250 high resolution
quasar spectra with emission redshifts ranging from 0.191 < z_em <6.311. The
total UVES exposure time of this dataset is 1560 hours. Thanks to the high
resolution of UVES spectra, it is possible to unambiguously measure the column
density of absorbers with damping wings, down to N_HI > 10^{19} cm^{-2}, which
constitutes the sub-damped Lya absorber (sub-DLA) threshold. Within the
wavelength coverage of our UVES data, we find 150 damped Lya systems
(DLAs)/sub-DLAs in the range 1.5 < z_abs < 4.7. Of these 150, 93 are DLAs and
57 are sub-DLAs. An extensive search in the literature indicates that 6 of
these DLAs and 13 of these sub-DLAs have their N_HI measured for the first
time. Among them, 10 are new identifications as DLAs/sub-DLAs. For each of
these systems, we obtain an accurate measurement of the HI column density and
the absorber's redshift in the range 1.7 < z_abs < 4.2 by implementing a Voigt
profile-fitting algorithm. These absorbers are further confirmed thanks to the
detection of associated metal lines and/or lines from members of the Lyman
series. In our data, a few quasars' lines-of-sight are rich. An interesting
example is towards QSO J0133+0400 (z_em = 4.154) with six DLAs and sub-DLAs
reported.Comment: 16 pages, 24 figures, 3 table
Cosmological Evolution of the Universe Neutral Gas Mass Measured by Quasar Absorption Systems
The cosmological evolution of neutral hydrogen is an efficient way of tracing
structure formation with redshift. It indicates the rate of evolution of gas
into stars and hence the gas consumption and rate star formation history of the
Universe. In measuring HI, quasar absorbers have proven to be an ideal tool and
we use observations from a recent survey for high-redshift quasar absorption
systems together with data gathered from the literature to measure the
cosmological comoving mass density of neutral gas. This paper assumes
Omega_M=0.3, Omega_lambda=0.7 and h=0.65.Comment: 3 pages, 2 figures. To appear in the proceedings of the "Cosmic
Evolution" conference, held at l'Institut d'Astrophysique de Paris, November
13-17, 200
A Homogeneous Sample of Sub-DLAs IV: Global Metallicity Evolution
An accurate method to measure the abundance of high-redshift galaxies
consists in the observation of absorbers along the line of sight toward a
background quasar. Here, we present abundance measurements of 13 z>3 sub-Damped
Lyman-alpha Systems (quasar absorbers with HI column density 19 < log N(HI) <
20.3 cm^-2) based on the high resolution observations with VLT UVES
spectrograph. These observations more than double the metallicity information
for sub-DLAs previously available at z>3. This new data, combined with other
sub-DLA measurements from the literature, confirm the stronger metallicity
redshift evolution than for the classical Damped Lyman-alpha absorbers.
Besides, these observations are used to compute for the first time the fraction
of gas ionised from photo-ionisation modelling in a sample of sub-DLAs. Based
on these results, we calculate that sub-DLAs contribute no more than 6% of the
expected amount of metals at z~2.5. We therefore conclude that even if sub-DLAs
are found to be more metal-rich than classical DLAs, they are insufficient to
close the so-called ``missing metals problem''.Comment: 30 figures, 24 tables. Accepted for publication in MNRA
The ESO UVES Advanced Data Products Quasar Sample - II. Cosmological Evolution of the Neutral Gas Mass Density
Quasar foreground damped absorbers, associated with HI-rich galaxies allow to
estimate the neutral gas mass over cosmic time, which is a possible indicator
of gas consumption as star formation proceeds. The DLAs and sub-DLAs are
believed to contain a large fraction of neutral gas mass in the Universe. In
Paper I of the series, we present the results of a search for DLAs and sub-DLAs
in the ESO-UVES Advanced Data Products dataset of 250 quasars. Here we use an
unbiased sub-sample of sub-DLAs from this dataset. We build a subset of 122
quasars ranging from 1.5 <z_em < 5.0, suitable for statistical analysis. The
statistical sample is analyzed in conjunction with other sub-DLA samples from
the literature. This makes up a combined sample of 89 sub-DLAs over a redshift
path of . Redshift evolution of the number density and the line
density are derived for sub-DLAs and compared with the LLSs and DLAs
measurements from the literature. The results indicate that these three classes
of absorbers are evolving in the redshift interval 1 < z < 5. The column
density distribution, f(N,z), down to the sub-DLA limit is determined. The
flattening of f_(N,z) in the sub-DLA regime is present in the observations. The
redshift evolution of f_(N,z) down to sub-DLA regime is also presented,
indicating the presence of more sub-DLAs at high-redshift as compared to
low-redshift. f_(N,z) is further used to determine the neutral gas mass
density, Omega_g, at 1.5 < z < 5.0. The complete sample shows that sub-DLAs
contribute 8-20% to the total Omega_g from 1.5 < z < 5.0. In agreement with
previous studies, no evolution of Omega_g is seen from low-redshift to
high-redshift, suggesting that star formation solely cannot explain this
non-evolution and replenishment of gas and/or recombination of ionized gas is
needed. (Abridged)Comment: 20 pages, 10 figures, 7 table
Element Abundances in a Gas-rich Galaxy at z = 5: Clues to the Early Chemical Enrichment of Galaxies
Element abundances in high-redshift quasar absorbers offer excellent probes
of the chemical enrichment of distant galaxies, and can constrain models for
population III and early population II stars. Recent observations indicate that
the sub-damped Lyman-alpha (sub-DLA) absorbers are more metal-rich than DLA
absorbers at redshifts 03. It has also been suggested that the DLA
metallicity drops suddenly at 4.7. However, only 3 DLAs at 4.5 and
none at 3.5 have "dust-free" metallicity measurements of undepleted
elements. We report the first quasar sub-DLA metallicity measurement at
3.5, from detections of undepleted elements in high-resolution data for a
sub-DLA at =5.0. We obtain fairly robust abundances of C, O, Si, and Fe,
using lines outside the Lyman-alpha forest. This absorber is metal-poor, with
O/H]=-2.000.12, which is 4 below the level expected from
extrapolation of the trend for 3.5 sub-DLAs. The C/O ratio is
1.8 times lower than in the Sun. More strikingly, Si/O is
3.2 times lower than in the Sun, while Si/Fe is nearly
(1.2 times) solar. This absorber does not display a clear
alpha/Fe enhancement. Dust depletion may have removed more Si from the gas
phase than is common in the Milky Way interstellar medium, which may be
expected if high-redshift supernovae form more silicate-rich dust. C/O and Si/O
vary substantially between different velocity components, indicating spatial
variations in dust depletion and/or early stellar nucleosynethesis (e.g.,
population III star initial mass function). The higher velocity gas may trace
an outflow enriched by early stars.Comment: 42 pages including 9 figures, accepted for publication in Ap
Keck and VLT Observations of Super-damped Lyman-alpha Absorbers at z=2=2.5: Constraints on Chemical Compositions and Physical Conditions
We report Keck/ESI and VLT/UVES observations of three super-damped
Lyman-alpha quasar absorbers with H I column densities log N(HI) >= 21.7 at
redshifts z=2-2.5. All three absorbers show similar metallicities (-1.3 to -1.5
dex), and dust depletion of Fe, Ni, and Mn. Two of the absorbers show
supersolar [S/Zn] and [Si/Zn]. We combine our results with those for other DLAs
to examine trends between N(HI), metallicity, dust depletion. A larger fraction
of the super-DLAs lie close to or above the line [X/H]=20.59-log N(HI) in the
metallicity vs. N(HI) plot, compared to the less gas-rich DLAs, suggesting that
super-DLAs are more likely to be rich in molecules. Unfortunately, our data for
Q0230-0334 and Q0743+1421 do not cover H2 absorption lines. For Q1418+0718,
some H2 lines are covered, but not detected. CO is not detected in any of our
absorbers. For DLAs with log N(HI) < 21.7, we confirm strong correlation
between metallicity and Fe depletion, and find a correlation between
metallicity and Si depletion. For super-DLAs, these correlations are weaker or
absent. The absorbers toward Q0230-0334 and Q1418+0718 show potential
detections of weak Ly-alpha emission, implying star formation rates of about
1.6 and 0.7 solar masses per year, respectively (ignoring dust extinction).
Upper limits on the electron densities from C II*/C II or Si II*/Si II are low,
but are higher than the median values in less gas-rich DLAs. Finally, systems
with log N(HI) > 21.7 may have somewhat narrower velocity dispersions delta
v_90 than the less gas-rich DLAs, and may arise in cooler and/or less turbulent
gas.Comment: 57 pages, 15 figures. Accepted for publication in Ap
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