203 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
On the importance of local sources of radiation for quasar absorption line systems
A generic assumption of ionization models of quasar absorption systems is
that radiation from local sources is negligible compared with the cosmological
background. We test this assumption and find that it is unlikely to hold for
absorbers as rare as H I Lyman limit systems. Assuming that the absorption
systems are gas clouds centered on sources of radiation, we derive analytic
estimates for the cross-section weighted moments of the flux seen by the
absorbers, of the impact parameter, and of the luminosity of the central
source. In addition, we compute the corresponding medians numerically. For the
one class of absorbers for which the flux has been measured: damped Ly-alpha
systems at z~3, our prediction is in excellent agreement with the observations
if we assume that the absorption arises in clouds centered on Lyman-break
galaxies. Finally, we show that if Lyman-break galaxies dominate the UV
background at redshift 3, then consistency between observations of the UV
background, the UV luminosity density from galaxies, and the number density of
Lyman limit systems requires escape fractions of order 10 percent.Comment: Accepted for publication in the Astrophysical Journal, 11 pages, 1
figure. Version 2: Added alternative method. Decreased fiducial escape
fraction to guarantee consistency between observed luminosity density, mean
free path, and UV background. This increased the column density above which
local radiation is importan
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
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
The missing metals problem. III How many metals are expelled from galaxies?
[Abridged] We revisit the metal budget at z~2. In the first two papers of
this series, we already showed that ~30% (to <60% if extrapolating the LF) of
the metals are observed in all z~2.5 galaxies detected in current surveys.
Here, we extend our analysis to the metals outside galaxies, i.e. in
intergalactic medium (IGM), using observational data and analytical
calculations. Our results for the two are strikingly similar: (1)
Observationally, we find that, besides the small (5%) contribution of DLAs, the
forest and sub-DLAs contribute subtantially to make <30--45% of the metal
budget, but neither of these appear to be sufficient to close the metal budget.
The forest accounts for 15--30% depending on the UV background, and sub-DLAs
for >2% to <17% depending on the ionization fraction. Together, the `missing
metals' problem is substantially eased. (2) We perform analytical calculations
based on the effective yield--mass relation. At z=2, we find that the method
predicts that 2$--50% of the metals have been ejected from galaxies into the
IGM, consistent with the observations. The metal ejection is predominantly by
L<1/3L_B^*(z=2) galaxies, which are responsible for 90% the metal enrichment,
while the 50 percentile is at L~1/10L^*_B(z=2). As a consequence, if indeed 50%
of the metals have been ejected from galaxies, 3--5 bursts of star formation
are required per galaxy prior to z=2. The ratio between the mass of metals
outside galaxies to those in stars has changed from z=2 to z=0: it was 2:1 or
1:1 and is now 1:8 or 1:9. This evolution implies that a significant fraction
of the IGM metals will cool and fall back into galaxies.Comment: 18pages, MNRAS, in press; small changes to match proofs; extended
version with summary tabl
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
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