11,575 research outputs found
The Keck+Magellan Survey for Lyman Limit Absorption II: A Case Study on Metallicity Variations
We present an absorption line analysis of the Lyman limit system (LLS) at
z=3.55 in our Magellan/MIKE spectrum of PKS2000-330. Our analysis of the Lyman
limit and full HI Lyman series constrains the total HI column density of the
LLS (N_HI = 10^[18.0 +/- 0.25] cm^{-2} for b_HI >= 20 km/s) and also the N_HI
values of the velocity subsystems comprising the absorber. We measure ionic
column densities for metal-line transitions associated with the subsystems and
use these values to constrain the ionization state (>90% ionized) and relative
abundances of the gas. We find an order of magnitude dispersion in the
metallicities of the subsystems, marking the first detailed analysis of
metallicity variations in an optically thick absorber. The results indicate
that metals are not well mixed within the gas surrounding high galaxies.
Assuming a single-phase photoionization model, we also derive an N_H-weighted
metallicity, = -1.66 +/- 0.25, which matches the mean metallicity in
the neutral ISM in high z damped Lya systems (DLAs). Because the line density
of LLSs is ~10 times higher than the DLAs, we propose that the former dominate
the metal mass-density at z~3 and that these metals reside in the galaxy/IGM
interface. Considerations of a multi-phase model do not qualitatively change
these conclusions. Finally, we comment on an anomalously large O^0/Si^+ ratio
in the LLS that suggests an ionizing radiation field dominated by soft UV
sources (e.g. a starburst galaxy). Additional abundance analysis is performed
on the super-LLS systems at z=3.19.Comment: 20 pages, 7 figures (most in color). Accepted to Ap
The geometry effects of an expanding Universe on the detection of cool neutral gas at high redshift
Recent high redshift surveys for 21-cm absorption in damped Lyman-alpha
absorption systems (DLAs) take the number of published searches at z > 2 to 25,
the same number as at z < 2, although the detection rate at high redshift
remains significantly lower (20% cf. 60%). Using the known properties of the
DLAs to estimate the unknown profile widths of the 21-cm non-detections and
including the limits via a survival analysis, we show that the mean spin
temperature/covering factor degeneracy at high redshift is, on average, double
that of the low redshift sample. This value is significantly lower than the
previous factor of eight for the spin temperatures and is about the same factor
as in the angular diameter distance ratios between the low and high redshift
samples. That is, without the need for the several pivotal assumptions, which
lead to an evolution in the spin temperature, we show that the observed
distribution of 21-cm detections in DLAs can be accounted for by the geometry
effects of an expanding Universe. That is, as yet there is no evidence of the
spin temperature of gas rich galaxies evolving with redshift.Comment: Accepted by ApJ Letter
Understanding Physical Conditions in High Redshift Galaxies through C I Fine Structure Lines: Data and Methodology
We probe the physical conditions in high redshift galaxies, specifically, the
Damped Lyman-alpha Systems (DLAs) using neutral carbon (CI) fine structure
lines and molecular hydrogen (H2). We report five new detections of CI and
analyze the CI in an additional 2 DLAs with previously published data. We also
present one new detection of H2 in a DLA. We present a new method of analysis
that simultaneously constrains \emph{both} the volume density and the
temperature of the gas, as opposed to previous studies that a priori assumed a
gas temperature. We use only the column density of CI measured in the fine
structure states and the assumption of ionization equilibrium in order to
constrain the physical conditions in the gas. We present a sample of 11 CI
velocity components in 6 DLAs and compare their properties to those derived by
the global CII* technique. The resulting median values for this sample are:
= 69 cm^{-3}, = 50 K, and = 3.86 cm^{-3} K, with
standard deviations, sigma_{n(HI)} = 134 cm^{-3}, sigma_T = 52 K, and
sigma_{log(P/k)} = 3.68 cm^{-3} K. This can be compared with the integrated
median values for the same DLAs : = 2.8 cm^{-3}, = 139 K, and
= 2.57 cm^{-3} K, with standard deviations sigma_{n(HI)} = 3.0
cm^{-3}, sigma_T = 43 K, and sigma_{log(P/k)} = 0.22 cm^{-3} K. Interestingly,
the pressures measured in these high redshift CI clouds are similar to those
found in the Milky Way. We conclude that the CI gas is tracing a
higher-density, higher-pressure region, possibly indicative of post-shock gas
or a photodissociation region on the edge of a molecular cloud. We speculate
that these clouds may be direct probes of the precursor sites of star formation
in normal galaxies at high redshift.Comment: Accepted for publication in Ap
Effects of Ultraviolet Background and Local Stellar Radiation on the H_I Column Density Distribution
We study the impact of ultraviolet background (UVB) radiation field and the
local stellar radiation on the H_I column density distribution f(N_HI) of
damped Ly-alpha systems (DLAs) and sub-DLAs at z=3 using cosmological smoothed
particle hydrodynamics simulations. We find that, in the previous simulations
with an optically thin approximation, the UVB was sinking into the H_I cloud
too deeply, and therefore we underestimated the f(N_HI) at 19 < log(N_HI) <
21.2 compared to the observations. If the UVB is shut off in the high-density
regions with n_gas > 6 x 10^{-3} cm^{-3}, then we reproduce the observed
f(N_HI) at z=3 very well. We also investigate the effect of local stellar
radiation by post-processing our simulation with a radiative transfer code, and
find that the local stellar radiation does not change the f(N_HI) very much.
Our results show that the shape of f(N_HI) is determined primarily by the UVB
with a much weaker effect by the local stellar radiation and that the optically
thin approximation often used in cosmological simulation is inadequate to
properly treat the ionization structure of neutral gas in and out of DLAs. Our
result also indicates that the DLA gas is closely related to the transition
region from optically-thick neutral gas to optically-thin ionized gas within
dark matter halos.Comment: 5 pages, 3 figures, uses emulateapj. ApJL in press. Corrected a
plotting error in Fig. 3, which reduced the effect of local stellar radiation
on f(N_HI
Metallicities and dust content of proximate damped Lyman alpha systems in the Sloan Digital Sky Survey
Composite spectra of 85 proximate absorbers (log N(HI)>20 and velocity
difference between the absorption and emission redshift, dv<10,000 km/s) in the
SDSS are used to investigate the trends of metal line strengths with velocity
separation from the QSO. We construct composites in 3 velocity bins: dv<3000
km/s, 30006000 km/s, with further sub-samples to
investigate the metal line dependence on N(HI) and QSO luminosity. Low (e.g.
SiII and FeII) and high ionization (e.g. SiIV and CIV) species alike have
equivalent widths (EWs) that are larger by factors of 1.5 -- 3 in the dv<3000
km/s composite, compared to the dv>6000 km/s spectrum. The EWs show an even
stronger dependence on dv if only the highest neutral hydrogen column density
(log N(HI)>20.7) absorbers are considered. We conclude that PDLAs generally
have higher metallicities than intervening absorbers, with the enhancement
being a function of both dv and N(HI). It is also found that absorbers near
QSOs with lower rest-frame UV luminosities have significantly stronger metal
lines. We speculate that absorbers near to high luminosity QSOs may have had
their star formation prematurely quenched. Finally, we search for the signature
of dust reddening by the PDLAs, based on an analysis of the QSO continuum
slopes relative to a control sample and determine a limit of E(B-V)<0.014 for
an SMC extinction curve. This work provides an empirical motivation for
distinguishing between proximate and intervening DLAs, and establishes a
connection between the QSO environment and galaxy properties at high redshifts.Comment: Accepted for publication in MNRA
The color excess of quasars with intervening DLA systems- Analysis of the SDSS data release five
We analyzed the spectroscopic and photometric database of the 5th data
release of the Sloan Digital Sky Survey (SDSS) to search for evidence of the
quasar reddening produced by dust embedded in intervening damped Ly alpha (DLA)
systems. From a list of 5164 quasars in the interval of emission redshift 2.25
/= 4, we built
up an "absorption sample" of 248 QSOs with a single DLA system in the interval
of absorption redshift 2.2 < z_a </= 3.5 and a "pool" of 1959 control QSOs
without DLA systems or strong metal systems. For each QSO of the absorption
sample we extracted from the pool a subset of control QSOs that are closest in
redshift and magnitude. The mean color of this subset was used as a zero point
to measure the "deviation from the mean color" of individual DLA-QSOs, Delta_i.
The colors were measured using "BEST" ugriz SDSS imaging data. The mean color
excess of the absorption sample, , was estimated by averaging the individual
color deviations Delta_i. We find = 27 +/- 9 x 10**(-3) mag and
= 54 +/- 12 x 10**(-3) mag. These values are representative of the
reddening of DLA systems at z_a ~ 2.7 in SDSS QSOs with limiting magnitude r
=/~ 20.2. The detection of the mean reddening is confirmed by several
statistical tests. Analysis of the results suggests an origin of the reddening
in dust embedded in the DLA systems, with an SMC-type extinction curve. By
converting the reddening into rest-frame extinction, we derive a mean
dust-to-gas ratio ~ 2 to 4 x 10**(-23) mag cm^2. This value is ~
-1.25 dex lower than the mean dust-to-gas ratio of the Milky Way, in line with
the lower level of metallicity in the present DLA sample.Comment: Accepted for publication on Astronomy & Astrophysics, 17 pages, 10
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