81 research outputs found
Metal Lines in Cosmological Models of Lyman-Alpha Absorbers
The metal absorption lines found in association with \lya absorbers of
moderate to high HI column density contain valuable information about the
metallicity and ionization conditions within the absorbers and offer a stronger
test of models of the intergalactic medium at \sim 3 than HI absorption lines
alone.
We have developed a method to predict the strengths of metal absorption lines
within the framework of cosmological models for the \lya forest. The method
consists of evaluating a quantity, the Line Observability Index, for a database
of hundreds of candidate metal lines, allowing a comprehensive identification
of the lines the model predicts to be detectable associated with a \lya
absorber of a given HI column density and metallicity.
Applying this technique to a particular class of models at \sim 2-4, we
predict that the OVI(1032 \AA, 1038 \AA) doublet is the only practical probe of
the metallicity of low column density absorbers (HI \simlt 10^{14.5} cm^{-2}\lambda_r >
1216, and that the strongest metal lines should be
CIII(977 \AA) and SiIII(1206.5 \AA), which peak at HI \sim 10^{17} cm^{-2}$.Comment: To appear in the proceedings from the 13th IAP Workshop (1-5 July
1997) "Evolution of the Intergalactic Medium From QSO Absorption Line
Systems
The Observability of Metal Lines Associated with the Lyman-alpha Forest
We develop a prescription for characterizing the strengths of metal lines
associated with Lyman-alpha forest absorbers (LYFAs) of a given neutral
hydrogen column density N_HI and metallicity [Fe/H]. This Line Observability
Index (LOX) is line-specific and translates, for weak lines, into a measure of
the equivalent width. It can be evaluated quickly for thousands of transitions
within the framework of a given model of the Lyman-alpha forest, providing a
ranking of the lines in terms of their strengths and enabling model builders to
select the lines that should be detectable in observed spectra of a given
resolution and signal-to-noise ratio. We compute the LOX for a large number of
elements and transitions in two cosmological models of the Lyman-alpha forest
at z=3 derived from a hydrodynamic simulation of structure formation, and we
discuss how the LOX depends on redshift and on model parameters such as the
mean baryonic density and radiation field. We find that the OVI (1032,1038)
doublet is the best probe of the metallicity in low column density LYFAs N_{HI}
\approx 10^{14.5} cm^{-2}). Metallicities down to [O/H] \sim -3 ([Fe/H] \sim
-3.5 with the assumed [O/Fe] ratio) yield OVI absorption features that should
be detectable in current high-quality spectra, provided that the expected
position of the OVI feature is not contaminated by HI absorption. The strongest
transitions in lower ionisation states of oxygen are OV(630), OIV(788), and
OIII(833), and are likely to be detected with next generation UV instruments.
Of the lines with rest wavelengths \lambda_r > 1216, which can potentially be
observed redwards of the \lya forest, the CIV(1548,1551) doublet is expected to
dominate in all LYFAs, regardless of the value of N_HI.Comment: Substantially revised version: larger line database, additional
cosmological model analyzed. Accepted for Ap
Constraining the Metallicity of the Low Density Lyman-alpha Forest Using OVI Absorption
We search for OVI absorption in a Keck HIRES spectrum of the z=3.62 quasar
Q1422+231. Comparison of CIV measurements to cosmological simulations shows
that \lya forest absorbers with N_HI > 10^{14.5} have [C/H]~=-2.5, for the UV
background spectrum of Haardt & Madau (HM). Lower column density absorption
arises in lower density gas, where OVI is the most sensitive metal tracer.
Since OVI lines lie at wavelengths contaminated by Lyman series absorption, we
interpret our Q1422 results by comparing to artificial spectra drawn from an
SPH simulation of a Lambda-dominated CDM model. A search for deep, narrow
features in Q1422 yields only a few candidate OVI lines, statistically
consistent with the number in artificial spectra with no metals; spectra
generated with the HM background and [O/H] >= -2.5 predict too many narrow
lines. However, applying the optical depth ratio technique of Songaila (1998),
we DO find significant OVI associated with CIV systems; matching Q1422 requires
[O/C]~=+0.5, implying [O/H]~=-2.0. Taken together these results imply that (a)
the metallicity in the low density IGM is at least a factor of three below that
in the overdense regions where CIV absorption is detectable, and (b) oxygen is
overabundant in these regions, consistent with the enrichment pattern of old
halo stars. If the UV background is heavily truncated above 4 Ry, an
implausibly high oxygen overabundance ([O/C]>+2) is required by the data; thus
a majority of the volume of the universe must have undergone helium
reionization by z=3.(Abridged)Comment: Submitted to ApJ, 48 pp including 14 ps figures, uses aaspp4.st
Accelerated gene evolution and subfunctionalization in the pseudotetraploid frog Xenopus laevis
Ancient whole genome duplications have been implicated in the vertebrate and teleost radiations, and in the emergence of diverse angiosperm lineages, but the evolutionary response to such a perturbation is still poorly understood. The African clawed frog Xenopus laevis experienced a relatively recent tetraploidization {approx} 40 million years ago. Analysis of the considerable amount of EST sequence available for this species together with the genome sequence of the related diploid Xenopus tropicalis provides a unique opportunity to study the genomic response to whole genome duplication
The X-Ray Forest: A New Prediction of Hierarchical Structure Formation Models
We use numerical simulations of structure formation in a Cold Dark Matter
model to predict the absorption lines in the soft X-rays produced by heavy
elements in the shock-heated intergalactic medium at low redshift. The
simulation incorporates a model for metal production in galaxies and the
subsequent dispersion of the metals to the intergalactic medium. We analyze in
particular absorption lines produced by oxygen, and calculate the ionization
stage taking into account the observed X-ray background at the present time. We
find that oxygen is fully ionized by the X-ray background in low-density voids,
and is mostly in the form of OVII and OVIII in the sheets and filamentary
regions. Strong absorption lines of OVII and OVIII with equivalent widths W ~
100 km/s are produced in filamentary regions of overdensities ~ 100 and
temperatures ~ 10^6 K, located in the outskirts of groups and clusters of
galaxies. The OVII line at E = 574 eV is generally the strongest one in these
systems. Our model predicts that any X-ray source (such as a quasar) should
typically show about one OVII absorption line with W > 100 km/s in the interval
from z=0 to z=0.3. These lines could be detected with the upcoming generation
of X-ray telescopes, and their origin in intervening systems could be confirmed
by the association with groups of galaxies and X-ray emitting halos near the
line-of-sight at the same redshift. The hot intergalactic medium may be one of
the main reservoirs of baryons in the present universe, and the heavy element
X-ray absorption lines offer a promising possibility of detecting this new
component in the near future.Comment: Accepted by ApJ. The full resolution color GIF version of Fig.2 is
include
The Structure of Isothermal, Self-gravitating Gas Spheres for Softened Gravity
A theory for the structure of isothermal, self-gravitating gas spheres in
pressure equilibrium in a softened gravitational field is developed. The one
parameter spline softening proposed by Hernquist & Katz (1989) is used. We show
that the addition of this extra scale parameter implies that the set of
equilibrium solutions constitute a one-parameter family, rather than the one
and only one isothermal sphere solution for Newtonian gravity. We demonstrate
the perhaps somewhat surprising result that for any finite choice of softening
length and temperature, it is possible to deposit an arbitrarily large mass of
gas in pressure equilibrium and with a non-singular density distribution inside
of r_0 for any r_0 > 0. The theoretical predictions of our models are compared
with the properties of the small, massive, quasi-isothermal gas clumps which
typically form in numerical Tree-SPH simulations of 'passive' galaxy formation
of Milky Way sized galaxies. We find reasonable agreement despite the neglect
of rotational support in the models. We comment on whether the hydrodynamical
resolution in our numerical simulation of galaxy formation is sufficient, and
finally we conclude that one should be cautious, when comparing results of
numerical simulations involving gravitational softening and hydrodynamical
smoothing, with reality.Comment: 22 pages Latex + 12 figure
Genome resequencing reveals multiscale geographic structure and extensive linkage disequilibrium in the forest tree Populus trichocarpa
This is the publisher’s final pdf. The article is copyrighted by the New Phytologist Trust and published by John Wiley & Sons, Inc. It can be found at: http://onlinelibrary.wiley.com/journal/10.1111/%28ISSN%291469-8137. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work.•Plant population genomics informs evolutionary biology, breeding, conservation and bioenergy feedstock development. For example, the detection of reliable phenotype–genotype associations and molecular signatures of selection requires a detailed knowledge about genome-wide patterns of allele frequency variation, linkage disequilibrium and recombination.\ud
•We resequenced 16 genomes of the model tree Populus trichocarpa and genotyped 120 trees from 10 subpopulations using 29 213 single-nucleotide polymorphisms.\ud
•Significant geographic differentiation was present at multiple spatial scales, and range-wide latitudinal allele frequency gradients were strikingly common across the genome. The decay of linkage disequilibrium with physical distance was slower than expected from previous studies in Populus, with r² dropping below 0.2 within 3–6 kb. Consistent with this, estimates of recent effective population size from linkage disequilibrium (N[subscript e] ≈ 4000–6000) were remarkably low relative to the large census sizes of P. trichocarpa stands. Fine-scale rates of recombination varied widely across the genome, but were largely predictable on the basis of DNA sequence and methylation features.\ud
•Our results suggest that genetic drift has played a significant role in the recent evolutionary history of P. trichocarpa. Most importantly, the extensive linkage disequilibrium detected suggests that genome-wide association studies and genomic selection in undomesticated populations may be more feasible in Populus than previously assumed
The amphioxus genome and the evolution of the chordate karyotype
Lancelets ('amphioxus') are the modern survivors of an ancient chordate lineage, with a fossil record dating back to the Cambrian period. Here we describe the structure and gene content of the highly polymorphic approx520-megabase genome of the Florida lancelet Branchiostoma floridae, and analyse it in the context of chordate evolution. Whole-genome comparisons illuminate the murky relationships among the three chordate groups (tunicates, lancelets and vertebrates), and allow not only reconstruction of the gene complement of the last common chordate ancestor but also partial reconstruction of its genomic organization, as well as a description of two genome-wide duplications and subsequent reorganizations in the vertebrate lineage. These genome-scale events shaped the vertebrate genome and provided additional genetic variation for exploitation during vertebrate evolution
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