4,359 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
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
Abundances in Damped Ly-alpha Galaxies
Damped Ly_alpha galaxies provide a sample of young galaxies where chemical
abundances can be derived throughout the whole universe with an accuracy
comparable to that for the local universe. Despite a large spread in redshift,
HI column density and metallicity, DLA galaxies show a remarkable uniformity in
the elemental ratios rather suggestive of similar chemical evolution if not of
an unique population. These galaxies are characterized by a moderate, if any,
enhancement of alpha-elements over Fe-peak elemental abundance with [S/Zn]
about 0 and [O/Zn] about 0.2, rather similarly to the dwarfs galaxies in the
Local Group. Nitrogen shows a peculiar behaviour with a bimodal distribution
and possibly two plateaux. In particular, the plateau at low N abundances
([N/H] < -3), is not observed in other atrophysical sites and might be evidence
for primary N production by massive stars.Comment: To appear in the Proceedings of the ESO/Arcetri Workshop on "Chemical
Abundances and Mixing in Stars in the Milky Way and its Satellites", eds., L.
Pasquini and S. Randich (Springer-Verlag Series, "ESO Astrophysics Symposia"
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
figure
Examination Of Larval Transport, Distribution And Hybridization Patterns Of Two Blue Mussel Species Mytilus Edulis And M. Trossulus In Machias Bay, Maine
Using an interdisciplinary approach, this study examined the population connectivity of the blue mussel Mytilus trossulus within Machias Bay, Maine, near its inshore southern boundary range. The Eastern Maine Coast Current (EMCC) flows southwestward along the northern Gulf of Maine coastline, just outside of Machias Bay and is a potential pathway for M. trossulus larvae. This study compared results to two historical studies that assayed M. trossulus and M. edulis species distributions to evaluate any temporal changes. Both similarities and differences were found in species composition in Machias Bay. Historical data suggested that the lower Machias Bay was supplied by larvae via the EMCC, and that conclusion was consistent with this study due to similar species composition in the lower bay. However, in upper Machias Bay, this study found a complete shift in species composition. Since earlier sampling, a M. trossulus population has become established in the upper bay and appears to be maintained by self-seeding via locally produced larvae. Additionally, hybrids between M. trossulus and M. edulis are now abundant in upper Machias Bay, likely due to a breakdown in conspecific sperm precedence at low densities of M. trossulus. The persistence of hybrids will probably be dependent upon the size of future M. trossulus populations
A High Signal-to-Noise Ratio Composite Spectrum of Gamma-ray Burst Afterglows
We present a composite spectrum of 60 long duration gamma-ray burst (GRB)
afterglows with redshifts in the range 0.35<z<6.7 observed with low resolution
optical spectra. The composite spectrum covers the wavelength range 700-6600 A
in the rest frame and has a mean signal-to-noise ratio of 150 per 1 A pixel and
reaches a maximum of ~300 in the range 2500-3500 A. Equivalent widths are
measured from metal absorption lines from the Lya line to ~5200 A, and
associated metal and hydrogen lines are identified between the Lyman break and
Lya line. The average transmission within the Lyman forest is consistent with
that found along quasar lines of sight. We find a temporal variation in fine
structure lines when dividing the sample into bursts observed within 2 hours
from their trigger and those observed later. Other lines in the predominantly
neutral gas show variations too, but this is most likely a random effect caused
by weighting of individual strong absorption lines and which mimics a temporal
variation. Bursts characterized with high or low prompt GRB energy release
produce afterglows with similar absorption line strengths, and likewise for
bursts with bright or faint optical afterglows. Bursts defined as dark from
their optical to X-ray spectral index have stronger absorption lines relative
to the optically bright bursts. The composite spectrum has strong CaII and MgII
absorption lines as commonly found in dusty galaxies, however, we find no
evidence for dust or a significant molecular content based on the non-detection
of diffuse interstellar bands. Compared to starburst galaxy spectra, the GRB
composite has much stronger fine structure lines, while metal absorption lines
are weaker.Comment: Accepted for publication in ApJ, 24 page
The SDSS Damped Lya Survey: Data Release 1
We present the results from an automated search for damped Lya (DLA) systems
in the quasar spectra of Data Release 1 from the Sloan Digital Sky Survey
(SDSS-DR1). At z~2.5, this homogeneous dataset has greater statistical
significance than the previous two decades of research. We derive a statistical
sample of 71 damped Lya systems (>50 previously unpublished) at z>2.1 and
measure HI column densities directly from the SDSS spectra. The number of DLA
systems per unit redshift is consistent with previous measurements and we
expect our survey has >95% completeness. We examine the cosmological baryonic
mass density of neutral gas Omega_g inferred from the damped Lya systems from
the SDSS-DR1 survey and a combined sample drawn from the literature. Contrary
to previous results, the Omega_g values do not require a significant correction
from Lyman limit systems at any redshift. We also find that the Omega_g values
for the SDSS-DR1 sample do not decline at high redshift and the combined sample
shows a (statistically insignificant) decrease only at z>4. Future data
releases from SDSS will provide the definitive survey of DLA systems at z~2.5
and will significantly reduce the uncertainty in Omega_g at higher redshift.Comment: 12 pages, includes color figures. Accepted to PASP, April 20 200
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