2,591 research outputs found
Was the Universe Reionized by Massive Population-III Stars?
The WMAP satellite has measured a large optical depth to electron scattering
after cosmological recombination of 0.17+-0.04, implying significant
reionization of the primordial gas only ~200 million years after the big bang.
However, the most recent overlap of intergalactic HII regions must have occured
at z<9 based on the Lyman-alpha forest constraint on the thermal history of the
intergalactic medium. Here we argue that a first generation of metal-free stars
with a heavy (rather than Salpeter) mass function is therefore required to
account for much of the inferred optical depth. This conclusion holds if
feedback regulates star formation in early dwarf galaxies as observed in
present-day dwarfs.Comment: 4 pages, 1 figure, replaced to match version accepted by ApJ Letter
Large scale distribution of total mass versus luminous matter from Baryon Acoustic Oscillations: First search in the SDSS-III BOSS Data Release 10
Baryon Acoustic Oscillations (BAOs) in the early Universe are predicted to
leave an as yet undetected signature on the relative clustering of total mass
versus luminous matter. A detection of this effect would provide an important
confirmation of the standard cosmological paradigm and constrain alternatives
to dark matter as well as non-standard fluctuations such as Compensated
Isocurvature Perturbations (CIPs). We conduct the first observational search
for this effect, by comparing the number-weighted and luminosity-weighted
correlation functions, using the SDSS-III BOSS Data Release 10 CMASS sample.
When including CIPs in our model, we formally obtain evidence at of
the relative clustering signature and a limit that matches the existing upper
limits on the amplitude of CIPs. However, various tests suggest that these
results are not yet robust, perhaps due to systematic biases in the data. The
method developed in this Letter, used with more accurate future data such as
that from DESI, is likely to confirm or disprove our preliminary evidence.Comment: 6 pages, 2 figures, accepted for publication in PR
Measuring the Size of Quasar Broad-Line Clouds Through Time Delay Light-Curve Anomalies of Gravitational Lenses
Intensive monitoring campaigns have recently attempted to measure the time
delays between multiple images of gravitational lenses. Some of the resulting
light-curves show puzzling low-level, rapid variability which is unique to
individual images, superimposed on top of (and concurrent with) longer
time-scale intrinsic quasar variations which repeat in all images. We
demonstrate that both the amplitude and variability time-scale of the rapid
light-curve anomalies, as well as the correlation observed between intrinsic
and microlensed variability, are naturally explained by stellar microlensing of
a smooth accretion disk which is occulted by optically-thick broad-line clouds.
The rapid time-scale is caused by the high velocities of the clouds (~5x10^3
km/s), and the low amplitude results from the large number of clouds covering
the magnified or demagnified parts of the disk. The observed amplitudes of
variations in specific lenses implies that the number of broad-line clouds that
cover ~10% of the quasar sky is ~10^5 per 4 pi steradian. This is comparable to
the expected number of broad line clouds in models where the clouds originate
from bloated stars.Comment: 19 pages, 9 figures. Submitted to Ap
Unintentional miRNA Ablation Is a Risk Factor in Gene Knockout Studies: A Short Report
One of the most powerful techniques for studying the function of a gene is to disrupt the expression of that gene using genetic engineering strategies such as targeted recombination or viral integration of gene trap cassettes. The tremendous utility of these tools was recognized this year with the awarding of the Nobel Prize in Physiology or Medicine to Capecchi, Evans, and Smithies for their pioneering work in targeted recombination mutagenesis in mammals. Another noteworthy discovery made nearly a decade ago was the identification of a novel class of non-coding genes called microRNAs. MicroRNAs are among the largest known classes of regulatory elements with more than 1000 predicted to exist in the mouse genome. Over 50% of known microRNAs are located within introns of coding genes. Given that currently about half of the genes in mouse have been knocked out, we investigated the possibility that intronic microRNAs may have been coincidentally deleted or disrupted in some of these mouse models. We searched published murine knockout studies and gene trap embryonic stem cell line databases for cases where a microRNA was located within or near the manipulated genomic loci, finding almost 200 cases where microRNA expression may have been disrupted along with another gene. Our results draw attention to the need for careful planning in future knockout studies to minimize the unintentional disruption of microRNAs. These data also raise the possibility that many knockout studies may need to be reexamined to determine if loss of a microRNA contributes to the phenotypic consequences attributed to loss of a protein-encoding gene
The Photo-Evaporation of Dwarf Galaxies During Reionization
During the period of reionization the Universe was filled with a cosmological
background of ionizing radiation. By that time a significant fraction of the
cosmic gas had already been incorporated into collapsed galactic halos with
virial temperatures below about 10000 K that were unable to cool efficiently.
We show that photoionization of this gas by the fresh cosmic UV background
boiled the gas out of the gravitational potential wells of its host halos. We
calculate the photoionization heating of gas inside spherically symmetric dark
matter halos, and assume that gas which is heated above its virial temperature
is expelled. In popular Cold Dark Matter models, the Press-Schechter halo
abundance implies that about 50-90% of the collapsed gas was evaporated at
reionization. The gas originated from halos below a threshold circular velocity
of 10-15 km/s. The resulting outflows from the dwarf galaxy population at
redshifts 5-10 affected the metallicity, thermal and hydrodynamic state of the
surrounding intergalactic medium. Our results suggest that stellar systems with
a velocity dispersion below about 10 km/s, such as globular clusters or the
dwarf spheroidal galaxies of the Local Group, did not form directly through
cosmological collapse at high redshifts.Comment: 29 pages, 7 PostScript figures, accepted for ApJ. Final version,
revised due to referee comments. Figures 6 & 7 have been corrected for a
small numerical erro
Is a Classical Language Adequate in Assessing the Detectability of the Redshifted 21cm Signal from the Early Universe?
The classical radiometer equation is commonly used to calculate the
detectability of the 21cm emission by diffuse cosmic hydrogen at high
redshifts. However, the classical description is only valid in the regime where
the occupation number of the photons in phase space is much larger than unity
and they collectively behave as a classical electromagnetic field. At redshifts
z<20, the spin temperature of the intergalactic gas is dictated by the
radiation from galaxies and the brightness temperature of the emitting gas is
in the range of mK, independently from the existence of the cosmic microwave
background. In regions where the observed brightness temperature of the 21cm
signal is smaller than the observed photon energy, of 68/(1+z) mK, the
occupation number of the signal photons is smaller than unity. Neverethless,
the radiometer equation can still be used in this regime because the weak
signal is accompanied by a flood of foreground photons with a high occupation
number (involving the synchrotron Galactic emission and the cosmic microwave
background). As the signal photons are not individually distinguishable, the
combined signal+foreground population of photons has a high occupation number,
thus justifying the use of the radiometer equation.Comment: 4 pages, Accepted for publication in JCA
Measuring the Small-Scale Power Spectrum of Cosmic Density Fluctuations Through 21 cm Tomography Prior to the Epoch of Structure Formation
The thermal evolution of the cosmic gas decoupled from that of the cosmic
microwave background (CMB) at a redshift z~200. Afterwards and before the first
stars had formed, the cosmic neutral hydrogen absorbed the CMB flux at its
resonant 21cm spin-flip transition. We calculate the evolution of the spin
temperature for this transition and the resulting anisotropies that are
imprinted on the CMB sky due to linear density fluctuations during this epoch.
These anisotropies at an observed wavelength of 10.56[(1+z)/50] meters, contain
an amount of information that is orders of magnitude larger than any other
cosmological probe. Their detection, although challenging, could tightly
constrain any possible running of the spectral index from inflation (as
suggested by WMAP), small deviations from Gaussianity, or any significant
contribution from neutrinos or warm dark matter to the cosmic mass budget.Comment: 4 pages, 3 figures, accepted for publication in Physical Review
Letter
Probing Red Giant Atmospheres with Gravitational Microlensing
Gravitational microlensing provides a new technique for studying the surfaces
of distant stars. Microlensing events are detected in real time and can be
followed up with precision photometry and spectroscopy. This method is
particularly adequate for studying red giants in the Galactic bulge. Recently
we developed an efficient method capable of computing the lensing effect for
thousands of frequencies in a high-resolution stellar spectrum. Here we
demonstrate the effects of microlensing on synthesized optical spectra of red
giant model atmospheres. We show that different properties of the stellar
surface can be recovered from time-dependent photometry and spectroscopy of a
point-mass microlensing event with a small impact parameter. In this study we
concentrate on center-to-limb variation of spectral features. Measuring such
variations can reveal the depth structure of the atmosphere of the source star.Comment: 23 pages with 11 Postscript figures, submitted to ApJ; Section 2
expanded, references added, text revise
Dwarf Galaxy Formation Was Suppressed By Cosmic Reionization
A large number of faint galaxies, born less than a billion years after the
big bang, have recently been discovered. The fluctuations in the distribution
of these galaxies contributed to a scatter in the ionization fraction of cosmic
hydrogen on scales of tens of Mpc, as observed along the lines of sight to the
earliest known quasars. Theoretical simulations predict that the formation of
dwarf galaxies should have been suppressed after cosmic hydrogen was reionized,
leading to a drop in the cosmic star formation rate. Here we present evidence
for this suppression. We show that the post-reionization galaxies which
produced most of the ionizing radiation at a redshift z~5.5, must have had a
mass in excess of ~10^{10.6+/-0.4} solar masses or else the aforementioned
scatter would have been smaller than observed. This limiting mass is two orders
of magnitude larger than the galaxy mass that is thought to have dominated the
reionization of cosmic hydrogen (~10^8 solar masses). We predict that future
surveys with space-based infrared telescopes will detect a population of
smaller galaxies that reionized the Universe at an earlier time, prior to the
epoch of dwarf galaxy suppression.Comment: 19 pages, 3 figures. Accepted for publication in Nature; press
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