176 research outputs found
The Great Observatories Origins Deep Survey: Constraints on the Lyman Continuum Escape Fraction Distribution of Lyman--Break Galaxies at 3.4<z<4.5
We use ultra-deep ultraviolet VLT/VIMOS intermediate-band and VLT/FORS1
narrow-band imaging in the GOODS Southern field to derive limits on the
distribution of the escape fraction (f_esc) of ionizing radiation for L >~
L*(z=3) Lyman Break Galaxies (LBGs) at redshift 3.4--4.5. Only one LBG, at
redshift z=3.795, is detected in its Lyman continuum (LyC; S/N~5.5), the
highest redshift galaxy currently known with a direct detection. Its
ultraviolet morphology is quite compact (R_eff=0.8, kpc physical). Three out of
seven AGN are also detected in their LyC, including one at redshift z=3.951 and
z850 = 26.1. From stacked data (LBGs) we set an upper limit to the average
f_esc in the range 5%--20%, depending on the how the data are selected (e.g.,
by magnitude and/or redshift). We undertake extensive Monte Carlo simulations
that take into account intergalactic attenuation, stellar population synthesis
models, dust extinction and photometric noise in order to explore the moments
of the distribution of the escaping radiation. Various distributions
(exponential, log-normal and Gaussian) are explored. We find that the median
f_esc is lower than ~6% with an 84% percentile limit not larger than 20%. If
this result remains valid for fainter LBGs down to current observational
limits, then the LBG population might be not sufficient to account for the
entire photoionization budget at the redshifts considered here, with the exact
details dependent upon the assumed ionizing background and QSO contribution
thereto. It is possible that f_esc depends on the UV luminosity of the
galaxies, with fainter galaxies having higher f_esc, and estimates of f_esc
from a sample of faint LBG from the HUDF (i775<28.5) are in broad quantitative
agreement with such a scenario.Comment: 58 pages, 23 figures; submitted to ApJ, revised version in response
to referee's comment
On Pulsar Distance Measurements and their Uncertainties
Accurate distances to pulsars can be used for a variety of studies of the
Galaxy and its electron content. However, most distance measures to pulsars
have been derived from the absorption (or lack thereof) of pulsar emission by
Galactic HI gas, which typically implies that only upper or lower limits on the
pulsar distance are available. We present a critical analysis of all measured
HI distance limits to pulsars and other neutron stars, and translate these
limits into actual distance estimates through a likelihood analysis that
simultaneously corrects for statistical biases. We also apply this analysis to
parallax measurements of pulsars in order to obtain accurate distance estimates
and find that the parallax and HI distance measurements are biased in different
ways, because of differences in the sampled populations. Parallax measurements
typically underestimate a pulsar's distance because of the limited distance to
which this technique works and the consequential strong effect of the Galactic
pulsar distribution (i.e. the original Lutz-Kelker bias), in HI distance
limits, however, the luminosity bias dominates the Lutz-Kelker effect, leading
to overestimated distances because the bright pulsars on which this technique
is applicable are more likely to be nearby given their brightness.Comment: 32 pages, 1 figure, 2 tables; Accepted for publication in the
Astrophysical Journa
Central Powering of the Largest Lyman-alpha Nebula is Revealed by Polarized Radiation
High-redshift Lyman-alpha blobs are extended, luminous, but rare structures
that appear to be associated with the highest peaks in the matter density of
the Universe. Their energy output and morphology are similar to powerful radio
galaxies, but the source of the luminosity is unclear. Some blobs are
associated with ultraviolet or infrared bright galaxies, suggesting an extreme
starburst event or accretion onto a central black hole. Another possibility is
gas that is shock excited by supernovae. However some blobs are not associated
with galaxies, and may instead be heated by gas falling into a dark matter
halo. The polarization of the Ly-alpha emission can in principle distinguish
between these options, but a previous attempt to detect this signature returned
a null detection. Here we report on the detection of polarized Ly-alpha from
the blob LAB1. Although the central region shows no measurable polarization,
the polarized fraction (P) increases to ~20 per cent at a radius of 45 kpc,
forming an almost complete polarized ring. The detection of polarized radiation
is inconsistent with the in situ production of Ly-alpha photons, and we
conclude that they must have been produced in the galaxies hosted within the
nebula, and re-scattered by neutral hydrogen.Comment: Published in the August 18 issue of Nature. 1750 words, 3 figures,
and full Supplementary Information. Version has not undergone proofing.
Reduced and processed data products are available here:
http://obswww.unige.ch/people/matthew.hayes/LymanAlpha/LabPol
The Late Reionization of Filaments
We study the topology of reionization using accurate three-dimensional
radiative transfer calculations post-processed on outputs from cosmological
hydrodynamic simulations. In our simulations, reionization begins in overdense
regions and then "leaks" directly into voids, with filaments reionizing last
owing to their combination of high recombination rate and low emissivity. This
result depends on the uniquely-biased emissivity field predicted by our
prescriptions for star formation and feedback, which have previously been shown
to account for a wide array of measurements of the post-reionization Universe.
It is qualitatively robust to our choice of simulation volume, ionizing escape
fraction, and spatial resolution (in fact it grows stronger at higher spatial
resolution) even though the exact overlap redshift is sensitive to each of
these. However, it weakens slightly as the escape fraction is increased owing
to the reduced density contrast at higher redshift. We also explore whether our
results are sensitive to commonly-employed approximations such as using
optically-thin Eddington tensors or substantially altering the speed of light.
Such approximations do not qualitatively change the topology of reionization.
However, they can systematically shift the overlap redshift by up to , indicating that accurate radiative transfer is essential for
computing reionization. Our model cannot simultaneously reproduce the observed
optical depth to Thomson scattering and ionization rate per hydrogen atom at
, which could owe to numerical effects and/or missing early sources of
ionization.Comment: 16 pages, 9 figures, accepted to MNRA
Simultaneous Determination of Thermal Conductivity and Specific Heat for Refractory Materials
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66352/1/j.1151-2916.1990.tb07594.x.pd
An Analytic Model for the Evolution of the Stellar, Gas, and Metal Content of Galaxies
We present an analytic formalism that describes the evolution of the stellar,
gas, and metal content of galaxies. It is based on the idea, inspired by
hydrodynamic simulations, that galaxies live in a slowly-evolving equilibrium
between inflow, outflow, and star formation. We argue that this formalism
broadly captures the behavior of galaxy properties evolving in simulations. The
resulting equilibrium equations for the star formation rate, gas fraction, and
metallicity depend on three key free parameters that represent ejective
feedback, preventive feedback, and re-accretion of ejected material. We
schematically describe how these parameters are constrained by models and
observations. Galaxies perturbed off the equilibrium relations owing to inflow
stochasticity tend to be driven back towards equilibrium, such that deviations
in star formation rate at a given mass are correlated with gas fraction and
anti-correlated with metallicity. After an early gas accumulation epoch,
quiescently star-forming galaxies are expected to be in equilibrium over most
of cosmic time. The equilibrium model provides a simple intuitive framework for
understanding the cosmic evolution of galaxy properties, and centrally features
the cycle of baryons between galaxies and surrounding gas as the driver of
galaxy growth.Comment: 11 pages, MNRAS, accepte
The Circumgalactic Medium in Massive Halos
This chapter presents a review of the current state of knowledge on the cool
(T ~ 1e4 K) halo gas content around massive galaxies at z ~ 0.2-2. Over the
last decade, significant progress has been made in characterizing the cool
circumgalactic gas in massive halos of Mh ~ 1e12-1e14 Msun at intermediate
redshifts using absorption spectroscopy. Systematic studies of halo gas around
massive galaxies beyond the nearby universe are made possible by large
spectroscopic samples of galaxies and quasars in public archives. In addition
to accurate and precise constraints for the incidence of cool gas in massive
halos, detailed characterizations of gas kinematics and chemical compositions
around massive quiescent galaxies at z ~ 0.5 have also been obtained. Combining
all available measurements shows that infalling clouds from external sources
are likely the primary source of cool gas detected at d >~ 100 kpc from massive
quiescent galaxies. The origin of the gas closer in is currently less certain,
but SNe Ia driven winds appear to contribute significantly to cool gas found at
d < 100 kpc. In contrast, cool gas observed at d <~ 200 kpc from luminous
quasars appears to be intimately connected to quasar activities on parsec
scales. The observed strong correlation between cool gas covering fraction in
quasar host halos and quasar bolometric luminosity remains a puzzle. Combining
absorption-line studies with spatially-resolved emission measurements of both
gas and galaxies is the necessary next step to address remaining questions.Comment: 29 pages, 7 figures, invited review to appear in "Gas Accretion onto
Galaxies", Astrophysics and Space Science Library, eds. A. Fox & R. Dave, to
be published by Springe
School Absenteeism As an Adjunct Surveillance Indicator: Experience during the Second Wave of the 2009 H1N1 Pandemic in Quebec, Canada
A school absenteeism surveillance system was implemented in the province of Quebec, Canada during the second wave of the 2009 H1N1 pandemic. This paper compares this surveillance approach with other available indicators.All (3432) elementary and high schools from Quebec were included. Each school was required to report through a web-based system any day where the proportion of students absent for influenza-like illness (ILI) exceeded 10% of current school enrolment.Between October 18 and December 12 2009, 35.6% of all schools met the 10% absenteeism threshold. This proportion was greater in elementary compared to high schools (40% vs 19%) and in smaller compared to larger schools (44% vs 22%). The maximum absenteeism rate was reached the first day of reporting or within the next two days in 55% and 31% of schools respectively. The first reports and subsequent peak in school absenteeism provincially preceded the peak in paediatric hospitalization by two and one weeks, respectively. Trends in school surveillance otherwise mirrored other indicators.During a pandemic, school outbreak surveillance based on a 10% threshold appears insufficient to trigger timely intervention within a given affected school. However, school surveillance appears well-correlated and slightly anticipatory compared to other population indicators. As such, school absenteeism warrants further evaluation as an adjunct surveillance indicator whose overall utility will depend upon specified objectives, and other existing capacity for monitoring and response
A consistent determination of the temperature of the intergalactic medium at redshift z=2.4
We present new measurements of the thermal state of the intergalactic medium (IGM) at z∼2.4 derived from absorption line profiles in the Lyα forest. We use a large set of high-resolution hydrodynamical simulations to calibrate the relationship between the temperature-density (T--Δ) relation in the IGM and the distribution of HI column densities, NHI, and velocity widths, bHI, of discrete Lyα forest absorbers. This calibration is then applied to the measurement of the lower cut-off of the bHI--NHI distribution recently presented by Rudie et al. (2012). We infer a power-law T--Δ relation, T=T0Δγ−1, with a temperature at mean density, T0=[1.00+0.32−0.21]×104K and slope (γ−1)=0.54±0.11. The slope is fully consistent with that advocated by the analysis of Rudie et al (2012); however, the temperature at mean density is lower by almost a factor of two, primarily due to an adjustment in the relationship between column density and physical density assumed by these authors. These new results are in excellent agreement with the recent temperature measurements of Becker et al. (2011), based on the curvature of the transmitted flux in the Lyα forest. This suggests that the thermal state of the IGM at this redshift is reasonably well characterised over the range of densities probed by these methods
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