178 research outputs found
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Gender-based differences in letters of recommendation written for ophthalmology residency applicants.
BACKGROUND:To determine whether gender-based differences may be present in letters of recommendation written for ophthalmology residency applicants. METHODS:All applications submitted through SF Match to the UCLA Stein Eye Institute Residency Training Program from the 2017-2018 application cycle were analyzed using validated text analysis software (Linguistic Inquiry and Word Count (Austin, TX)). The main outcome measures were differences in language use in letters of recommendation by gender of applicant. RESULTS:Of 440 applicants, 254 (58%) were male and 186 (42%) were female. The two gender groups had similar United States Medical Licensing Exam (USMLE) Step 1 scores, undergraduate grade point averages (uGPA's), proportions of underrepresented minority (URM) applicants and Gold Humanism Honor Society members, numbers of academic and service activities listed, and gender distributions of their letter writers (all P values > 0.05). However, letters written for male applicants were determined to use more "authentic" words than those written for female applicants (mean difference, 0.800; 95% CI, 0.001-1.590; P = 0.047). Letters written for male applicants also contained more "leisure" words (mean difference, 0.056; 95% CI, 0.008-0.104; P = 0.023) and fewer "feel" words (mean difference, 0.033; 95% CI, 0.001-0.065; P = 0.041) and "biological processes" words (mean difference, 0.157; 95% CI, 0.017-0.297; P = 0.028). CONCLUSIONS:There were gender differences detected in recommendation letters in ophthalmology consistent with prior studies from other fields. Awareness of these differences may improve residency selection processes
A smoother end to the dark ages
Independent lines of evidence suggest that the first stars, which ended the
cosmic dark ages, came in pairs, rather than singly. This could change the
prevailing view that the early Universe had a Swiss-cheese-like appearance.Comment: Nature News and Views, April 7, 201
Chandra Observations of Galaxy Zoo Mergers: Frequency of Binary Active Nuclei in Massive Mergers
We present the results from a Chandra pilot study of 12 massive galaxy
mergers selected from Galaxy Zoo. The sample includes major mergers down to a
host galaxy mass of 10 that already have optical AGN
signatures in at least one of the progenitors. We find that the coincidences of
optically selected active nuclei with mildly obscured ( cm) X-ray nuclei are relatively common (8/12), but the
detections are too faint ( counts per nucleus; erg s cm) to reliably separate starburst and
nuclear activity as the origin of the X-ray emission. Only one merger is found
to have confirmed binary X-ray nuclei, though the X-ray emission from its
southern nucleus could be due solely to star formation. Thus, the occurrences
of binary AGN in these mergers are rare (0-8%), unless most merger-induced
active nuclei are very heavily obscured or Compton thick.Comment: 8 pages, including 5 figures and 1 table. Accepted by Ap
The First Stars: Mass Growth Under Protostellar Feedback
We perform three-dimensional cosmological simulations to examine the growth
of metal-free, Population III (Pop III) stars under radiative feedback. We
begin our simulation at z=100 and trace the evolution of gas and dark matter
until the formation of the first minihalo. We then follow the collapse of the
gas within the minihalo up to densities of n = 10^12 cm^-3, at which point we
replace the high-density particles with a sink particle to represent the
growing protostar. We model the effect of Lyman-Werner (LW) radiation emitted
by the protostar, and employ a ray-tracing scheme to follow the growth of the
surrounding H II region over the next 5000 yr. We find that a disk assembles
around the first protostar, and that radiative feedback will not prevent
further fragmentation of the disk to form multiple Pop III stars. Ionization of
neutral hydrogen and photodissociation of H_2 by LW radiation leads to heating
of the dense gas to several thousand Kelvin, and this warm region expands
outward at the gas sound speed. Once the extent of this warm region becomes
equivalent to the size of the disk, the disk mass declines while the accretion
rate onto the protostars is reduced by an order of magnitude. This occurs when
the largest sink has grown to ~ 20 M_sol while the second sink has grown to 7
M_sol, and we estimate the main sink will approach an asymptotic value of ~ 30
M_sol by the time it reaches the main sequence. Our simulation thus indicates
that the most likely outcome is a massive Pop III binary. However, we simulate
only one minihalo, and the statistical variation between minihaloes may be
substantial. If Pop III stars were typically unable to grow to more than a few
tens of solar masses, this would have important consequences for the occurence
of pair-instability supernovae in the early Universe as well as the Pop III
chemical signature in the oldest stars observable today.Comment: 21 pages, 11 figures, to appear in MNRA
The impact of the supersonic baryon-dark matter velocity difference on the z~20 21cm background
Recently, Tseliakhovich and Hirata (2010) showed that during the cosmic Dark
Ages the baryons were typically moving supersonically with respect to the dark
matter with a spatially variable Mach number. Such supersonic motion may source
shocks that heat the Universe. This motion may also suppress star formation in
the first halos. Even a small amount of coupling of the 21cm signal to this
motion has the potential to vastly enhance the 21cm brightness temperature
fluctuations at 15<z<40 as well as to imprint acoustic oscillations in this
signal. We present estimates for the size of this coupling, which we calibrate
with a suite of cosmological simulations. Our simulations, discussed in detail
in a companion paper, are initialized to self-consistently account for gas
pressure and the dark matter-baryon relative velocity, v_bc (in contrast to
prior simulations). We find that the supersonic velocity difference
dramatically suppresses structure formation at 10-100 comoving kpc scales, it
sources shocks throughout the Universe, and it impacts the accretion of gas
onto the first star-forming minihalos (even for halo masses as large as ~10^7
Msun). However, we find that the v_bc-sourced temperature fluctuations can
contribute only as much as ~10% of the fluctuations in the 21cm signal. We do
find that v_bc could source an O(1) component in the power spectrum of the 21cm
signal via the X-ray (but not ultraviolet) backgrounds produced once the first
stars formed. In a scenario in which ~10^6 Msun minihalos reheated the Universe
via their X-ray backgrounds, we find that the pre-reionization 21cm signal
would be larger than previously anticipated and exhibit significant acoustic
features. We show that structure formation shocks are unable to heat the
Universe sufficiently to erase a strong 21cm absorption trough at z ~ 20 that
is found in most models of the sky-averaged 21cm intensity.Comment: 17 pages, 11 figures, accepted to ApJ; for movies see
http://astro.berkeley.edu/~mmcquinn/firstligh
Suppression of HD-cooling in protogalactic gas clouds by Lyman-Werner radiation
It has been shown that HD molecules can form efficiently in metal-free gas
collapsing into massive protogalactic halos at high redshift. The resulting
radiative cooling by HD can lower the gas temperature to that of the cosmic
microwave background, T_CMB=2.7(1+z)K, significantly below the temperature of a
few 100 K achievable via H_2-cooling alone, and thus reduce the masses of the
first generation of stars. Here we consider the suppression of HD-cooling by UV
irradiation in the Lyman-Werner (LW) bands. We include photo-dissociation of
both H_2 and HD, and explicitly compute the self-shielding and shielding of
both molecules by neutral hydrogen as well as the shielding of HD by H_2. We
use a simplified dynamical collapse model, and follow the chemical and thermal
evolution of the gas, in the presence of a UV background. We find that a LW
flux of J_crit = 1e-22 erg/cm^2/sr/s/Hz is able to suppress HD cooling and thus
prevent collapsing primordial gas from reaching temperatures below 100 K. The
main reason for the lack of HD cooling for J>J_crit is the partial
photo-dissociation of H_2, which prevents the gas from reaching sufficiently
low temperatures (T<150K) for HD to become the dominant coolant; direct HD
photo-dissociation is unimportant except for a narrow range of fluxes and
column densities. Since the prevention of HD-cooling requires only partial H_2
photo-dissociation, the critical flux J_crit is modest, and is below the UV
background required to reionize the universe at redshift z=10-20. We conclude
that HD-cooling can reduce the masses of typical stars only in rare halos
forming well before the epoch of reionization.Comment: 14 pages with 9 figures, submitted to MNRA
The Radio Signatures of the First Supernovae
Primordial stars are key to primeval structure formation as the first stellar
components of primeval galaxies, the sources of cosmic chemical enrichment and
likely cosmic reionization, and they possibly gave rise to the supermassive
black holes residing at the centres of galaxies today. While the direct
detection of individual Pop III stars will likely remain beyond reach for
decades to come, we show their supernova remnants may soon be detectable in the
radio. We calculate radio synchrotron signatures between 0.5 - 35 GHz from
hydrodynamical computations of the supernova remnants of Pop III stars in
minihaloes. We find that hypernovae yield the brightest systems, with observed
radio fluxes as high as 1 - 10 muJy. Less energetic Type II supernovae yield
remnants about a factor of 30 dimmer and pair-instability supernova remnants
are dimmer by a factor of more than 10,000. Because of the high gas densities
of the progenitor environments, synchrotron losses severely limit the maximum
emission frequencies, producing a distinctive peaked radio spectrum
distinguishable from normal galactic supernova remnant spectra. Hypernovae
radio remnants should be detectable by existing radio facilities like eVLA and
eMERLIN while Type II supernova remnants will require the Square Kilometre
Array. The number counts of hypernova remnants at z > 20 with fluxes above 1
muJy are expected to be one per hundred square degree field, increasing to a
few per square degree if they form down to z = 10. The detection of a z > 20
Type II supernova remnant brighter than 1 nJy would require a 100 - 200 square
degree field, although only a 1 - 2 square degree field for those forming down
to z = 10. Hypernova and Type II supernova remnants are easily separated from
one another by their light curves, which will enable future surveys to use them
to constrain the initial mass function of Pop III stars.Comment: 12 pages, 9 figures; major revision; to appear in MNRA
X-ray emission from high-redshift miniquasars: self-regulating the population of massive black holes through global warming
Observations of high-redshift quasars at z>6 imply that supermassive black
holes (SMBHs) with masses over a billion solar masses were in place less than 1
Gyr after the Big Bang. If these SMBHs assembled from "seed" BHs left behind by
the first stars, then they must have accreted gas at close to the Eddington
limit during a large fraction (>50%) of the time. A generic problem with this
scenario, however, is that the mass density in million-solar-mass SMBHs at z=6
already exceeds the locally observed SMBH mass density by several orders of
magnitude; in order to avoid this overproduction, BH seed formation and growth
must become significantly less efficient in less massive protogalaxies, while
proceeding uninterrupted in the most massive galaxies that formed first. Using
Monte-Carlo realizations of the merger and growth history of BHs, we show that
X-rays from the earliest accreting BHs can provide such a feedback mechanism.
Our calculations paint a self-consistent picture of black-hole-made climate
change, in which the first miniquasars---among them the ancestors of the z>6
quasar SMBHs---globally warm the IGM and suppress the formation and growth of
subsequent generations of BHs. We present two specific models with global
miniquasar feedback that provide excellent agreement with recent estimates of
the z=6 SMBH mass function. For each of these models, we estimate the rate of
BH mergers at z>6 that could be detected by the proposed gravitational-wave
observatory eLISA/NGO.Comment: 15 pages, 6 figures, accepted to MNRAS; v2 includes minor changes,
mostly to references, to match version to be publishe
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