1,215 research outputs found
Commentary : Disentangling the contributions of childhood and adult weight to cardiovascular disease risk
Non peer reviewe
Direct evidence for an early reionization of the Universe?
We examine the possible reionization of the intergalactic medium (IGM) by the
source UDF033238.7-274839.8 (hereafter HUDF-JD2), which was discovered in deep
{\it HST}/VLT/{\it Spitzer} images obtained as part of the Great Observatory
Origins Deep Survey and {\it Hubble} Ultra-Deep Field projects. Mobasher et al
(2005) have identified HUDF-JD2 as a massive ()
post-starburst galaxy at redshift z. We find that HUDF-JD2 may be
capable of reionizing its surrounding region of the Universe, starting the
process at a redshift as high as z.Comment: 6 pages, 2 figures. Accepted for publication in ApJ Letter
Epigenetic Chromatin Silencing: Bistability and Front Propagation
The role of post-translational modification of histones in eukaryotic gene
regulation is well recognized. Epigenetic silencing of genes via heritable
chromatin modifications plays a major role in cell fate specification in higher
organisms. We formulate a coarse-grained model of chromatin silencing in yeast
and study the conditions under which the system becomes bistable, allowing for
different epigenetic states. We also study the dynamics of the boundary between
the two locally stable states of chromatin: silenced and unsilenced. The model
could be of use in guiding the discussion on chromatin silencing in general. In
the context of silencing in budding yeast, it helps us understand the phenotype
of various mutants, some of which may be non-trivial to see without the help of
a mathematical model. One such example is a mutation that reduces the rate of
background acetylation of particular histone side-chains that competes with the
deacetylation by Sir2p. The resulting negative feedback due to a Sir protein
depletion effect gives rise to interesting counter-intuitive consequences. Our
mathematical analysis brings forth the different dynamical behaviors possible
within the same molecular model and guides the formulation of more refined
hypotheses that could be addressed experimentally.Comment: 19 pages, 5 figure
GMRT detection of HI 21 cm associated absorption towards the z=1.2 red quasar 3C 190
We report the GMRT detection of associated HI 21 cm-line absorption in the
z=1.1946 red quasar 3C 190. Most of the absorption is blue-shifted with respect
to the systemic redshift. The absorption, at 647.7 MHz, is broad and
complex, spanning a velocity width of 600 \kms. Since the core is
self-absorbed at this frequency, the absorption is most likely towards the
hotspots. Comparison of the radio and deep optical images reveal linear
filaments in the optical which overlap with the brighter radio jet towards the
south-west. We therefore suggest that most of the HI 21 cm-line absorption
could be occurring in the atomic gas shocked by the south-west jet.Comment: 8 pages, 1 fugure. To appear in Journal of Astrophysics and Astronom
The Rest-Frame UV Luminosity Density of Star-Forming Galaxies at Redshifts z>3.5
We have measured the rest--frame lambda~1500 Ang comoving specific luminosity
density of star--forming galaxies at redshift 3.5<z<6.5 from deep images taken
with the Hubble Space Telescope (HST)and the Advanced Camera for Surveys (ACS),
obtained as part of the Great Observatories Origins Deep Survey (GOODS). We
used color selection criteria to construct samples of star--forming galaxies at
redshifts z~4, 5 and 6, identified by the signature of the 912 Ang Lyman
continuum discontinuity and Lyman-alpha forest blanketing in their rest--frame
UV colors (Lyman--break galaxies). The ACS samples cover ~0.09 square degree,
and are also relatively deep, reaching between 0.2 and 0.5 L_3^*, depending on
the redshift, where is the characteristic UV luminosity of Lyman--break
galaxies at z~3. The specific luminosity density of Lyman--break galaxies
appears to be nearly constant with redshift from z~3 to z~6, although the
measure at z~6 remains relatively uncertain, because it depends on the accurate
estimate of the faint counts of the z~6 sample. If Lyman--break galaxies are
fair tracers of the cosmic star formation activity, our results suggest that at
z~6 the universe was already producing stars as vigorously as it did near its
maximum several Gyr later, at 1<~z<~3. Thus, the onset of large--scale star
formation in the universe is to be sought at around z~6 or higher, namely at
less than ~7% of the current cosmic age.Comment: AAS LaTeX macros 4.0, 11 pages, 1 postscript figure. Accepted for
publication in The Astrophysical Journal, Letter. Minor changes to the figure
caption. The data and the GOODS-group papers can be found at
http://www.stsci.edu/science/goods
Resolving the Formation of Protogalaxies. I. Virialization
(Abridged) Galaxies form in hierarchically assembling dark matter halos. With
cosmological three dimensional adaptive mesh refinement simulations, we explore
in detail the virialization of baryons in the concordance cosmology, including
optically thin primordial gas cooling. We focus on early protogalaxies with
virial temperatures of 10^4 K and their progenitors. Without cooling, virial
heating occurs in shocks close to the virial radius for material falling in
from voids. Material in dense filaments penetrates deeper to about half that
radius. With cooling the virial shock position shrinks and also the filaments
reach scales as small as a third the virial radius. The temperatures in
protogalaxies found in adiabatic simulations decrease by a factor of two from
the center and show flat entropy cores. In cooling halos the gas reaches virial
equilibrium with the dark matter potential through its turbulent velocities. We
observe turbulent Mach numbers ranging from one to three in the cooling cases.
This turbulence is driven by the large scale merging and interestingly remains
supersonic in the centers of these early galaxies even in the absence of any
feedback processes. The virial theorem is shown to approximately hold over 3
orders of magnitude in length scale with the turbulent pressure prevailing over
the thermal energy. The turbulent velocity distributions are Maxwellian and by
far dominate the small rotation velocities associated with the total angular
momentum of the galaxies. Decomposing the velocity field using the
Cauchy-Stokes theorem, we show that ample amounts of vorticity are present
around shocks even at the very centers of these objects.Comment: 13 pages, 6 figures. Submitted to ApJ on 8 March 2007. Revised
manuscript. Comments welcom
Association between maternal folate concentrations during pregnancy and insulin resistance in Indian children
AIMS/HYPOTHESIS: In an Indian birth cohort, higher maternal homocysteine concentration in pregnancy was associated with lower birthweight of the offspring. Lower maternal vitamin B12 and higher folate concentrations were associated with higher offspring insulin resistance. Disordered one-carbon metabolism during early development may increase later metabolic risk. We explored these associations in another birth cohort in India at three age points.METHODS: We measured plasma vitamin B12, folate and homocysteine concentrations at 30 ± 2 weeks' gestation in 654 women who delivered at one hospital. Neonatal anthropometry was recorded, and the children's glucose and insulin concentrations were measured at 5, 9.5 and 13.5 years of age. Insulin resistance was estimated using HOMA of insulin resistance (HOMA-IR).RESULTS: Maternal homocysteine concentrations were inversely associated with all neonatal anthropometric measurements (p < 0.05), and positively associated with glucose concentrations in the children at 5 (30 min; p = 0.007) and 9.5 years of age (120 min; p = 0.02). Higher maternal folate concentrations were associated with higher HOMA-IR in the children at 9.5 (p = 0.03) and 13.5 years of age (p = 0.03). Maternal vitamin B12 concentrations were unrelated to offspring outcomes.CONCLUSIONS/INTERPRETATION: Maternal vitamin B12 status did not predict insulin resistance in our cohort. However, associations of maternal homocysteine and folate concentrations with birth size, and with childhood insulin resistance and glycaemia in the offspring, suggest a role for nutritionally driven disturbances in one-carbon metabolism in fetal programming of diabetes
Cosmological Evolution of Supergiant Star-Forming Clouds
In an exploration of the birthplaces of globular clusters, we present a
careful examination of the formation of self-gravitating gas clouds within
assembling dark matter haloes in a hierarchical cosmological model. Our
high-resolution smoothed particle hydrodynamical simulations are designed to
determine whether or not hypothesized supergiant molecular clouds (SGMCs) form
and, if they do, to determine their physical properties and mass spectra. It
was suggested in earlier work that clouds with a median mass of several 10^8
M_sun are expected to assemble during the formation of a galaxy, and that
globular clusters form within these SGMCs. Our simulations show that clouds
with the predicted properties are indeed produced as smaller clouds collide and
agglomerate within the merging dark matter haloes of our cosmological model. We
find that the mass spectrum of these clouds obeys the same power-law form
observed for globular clusters, molecular clouds, and their internal clumps in
galaxies, and predicted for the supergiant clouds in which globular clusters
may form. We follow the evolution and physical properties of gas clouds within
small dark matter haloes up to z = 1, after which prolific star formation is
expected to occur. Finally, we discuss how our results may lead to more
physically motivated "rules" for star formation in cosmological simulations of
galaxy formation.Comment: Accepted to The Astrophysical Journal; 17 pages, 8 figure
On Star Formation and the Non-Existence of Dark Galaxies
We investigate whether a baryonic dark galaxy or `galaxy without stars' could
persist indefinitely in the local universe, while remaining stable against star
formation. To this end, a simple model has been constructed to determine the
equilibrium distribution and composition of a gaseous protogalactic disk.
Specifically, we determine the amount of gas that will transit to a Toomre
unstable cold phase via the H2 cooling channel in the presence of a UV--X-ray
cosmic background radiation field.
All but one of the models are predicted to become unstable to star formation.
Moreover, we find that all our model objects would be detectable via HI line
emission, even in the case that star formation is potentially avoided. These
results are consistent with the non-detection of isolated extragalactic HI
clouds with no optical counterpart (galaxies without stars) by HIPASS.
Additionally, where star formation is predicted to occur, we determine the
minimum interstellar radiation field required to restore gravothermal
stability, which we then relate to a minimum global star formation rate. This
leads to the prediction of a previously undocumented relation between HI mass
and star formation rate that is observed for a wide variety of dwarf galaxies
in the HI mass range 10^8--10^10 M_sun. The existence of such a relation
strongly supports the notion that the well observed population of dwarf
galaxies represent the minimum rates of self-regulating star formation in the
universe. (Barely abridged)Comment: 19 pages, 8 figures, TeX using emulateapj.cls, v2 accepted for
publication in ApJ (16/8/5) with one figure deleted and a number of minor
clarifying revision
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