1,143 research outputs found
Simulation of primordial object formation
We have included the chemical rate network responsible for the formation of
molecular Hydrogen in the N-body hydrodynamic code, Hydra, in order to study
the formation of the first cosmological at redshifts between 10 and 50. We have
tested our implementation of the chemical and cooling processes by comparing
N-body top hat simulations with theoretical predictions from a semi-analytic
model and found them to be in good agreement. We find that post-virialization
properties are insensitive to the initial abundance of molecular hydrogen. Our
main objective was to determine the minimum mass () of perturbations
that could become self gravitating (a prerequisite for star formation), and the
redshift at which this occurred. We have developed a robust indicator for
detecting the presence of a self-gravitating cloud in our simulations and find
that we can do so with a baryonic particle mass-resolution of 40 solar masses.
We have performed cosmological simulations of primordial objects and find that
the object's mass and redshift at which they become self gravitating agree well
with the results from the top hat simulations. Once a critical
molecular hydrogen fractional abundance of about 0.0005 has formed in an
object, the cooling time drops below the dynamical time at the centre of the
cloud and the gas free falls in the dark matter potential wells, becoming self
gravitating a dynamical time later.Comment: 45 pages, 17 figures, submitted to Ap
Recent Insights into Cell Surface Heparan Sulphate Proteoglycans and Cancer
A small group of cell surface receptors are proteoglycans, possessing a core protein with one or more covalently attached glycosaminoglycan chains. They are virtually ubiquitous and their chains are major sites at which protein ligands of many types interact. These proteoglycans can signal and regulate important cell processes, such as adhesion, migration, proliferation, and differentiation. Since many protein ligands, such as growth factors, morphogens, and cytokines, are also implicated in tumour progression, it is increasingly apparent that cell surface proteoglycans impact tumour cell behaviour. Here, we review some recent advances, emphasising that many tumour-related functions of proteoglycans are revealed only after their modification in processes subsequent to synthesis and export to the cell surface. These include enzymes that modify heparan sulphate structure, recycling of whole or fragmented proteoglycans into exosomes that can be paracrine effectors or biomarkers, and lateral interactions between some proteoglycans and calcium channels that impact the actin cytoskeleton
Gapped and gapless short range ordered magnetic states with wavevectors in the pyrochlore magnet TbTiO
Recent low temperature heat capacity (C) measurements on polycrystalline
samples of the pyrochlore antiferromagnet TbTiO
have shown a strong sensitivity to the precise Tb concentration , with a
large anomaly exhibited for at K and no such
anomaly and corresponding phase transition for . We have grown single
crystal samples of TbTiO, with approximate
composition , and , where the single
crystal exhibits a large C anomaly at =0.45 K, but neither the
nor the single crystals display any such anomaly. We
present new time-of-flight neutron scattering measurements on the
and the samples which show strong
quasi-Bragg peaks at low
temperatures characteristic of short range antiferromagnetic spin ice (AFSI)
order at zero magnetic field but only under field-cooled conditions, as was
previously observed in our single crystal. These results show that
the strong quasi-Bragg peaks
and gapped AFSI state at low temperatures under field cooled conditions are
robust features of TbTiO, and are not correlated with the presence
or absence of the C anomaly and phase transition at low temperatures.
Further, these results show that the ordered state giving rise to the C
anomaly is confined to for
TbTiO, and is not obviously connected with
conventional order of magnetic dipole degrees of freedom.Comment: 7 pages, 3 figure
A New Algorithm for Computing Statistics of Weak Lensing by Large-Scale Structure
We describe an efficient algorithm for calculating the statistics of weak
lensing by large-scale structure based on a tiled set of independent
particle-mesh N-body simulations which telescope in resolution along the line
of sight. This efficiency allows us to predict not only the mean properties of
lensing observables such as the power spectrum, skewness and kurtosis of the
convergence, but also their sampling errors for finite fields of view, which
are themselves crucial for assessing the cosmological significance of
observations. We find that the nongaussianity of the distribution substantially
increases the sampling errors for the skewness and kurtosis in the several to
tens of arcminutes regime, whereas those for the power spectrum are only
fractionally increased even out to wavenumbers where shot noise from the
intrinsic ellipticities of the galaxies will likely dominate the errors.Comment: 12 pages, 13 figures; minor changes reflect accepted versio
Radio Signatures of HI at High Redshift: Mapping the End of the ``Dark Ages''
The emission of 21-cm radiation from a neutral intergalactic medium (IGM) at
high redshift is discussed in connection with the thermal and ionization
history of the universe. The physical mechanisms that make such radiation
detectable against the cosmic microwave background include Ly_alpha coupling of
the hydrogen spin temperature to the kinetic temperature of the gas and
preheating of the IGM by the first generation of stars and quasars. Three
different signatures are investigated in detail: (a) the fluctuations in the
redshifted 21-cm emission induced by the gas density inhomogeneities that
develop at early times in cold dark matter (CDM) dominated cosmologies; (b) the
sharp absorption feature in the radio sky due to the rapid rise of the Ly_alpha
continuum background that marks the birth of the first UV sources in the
universe; and (c) the 21-cm emission and absorption shells that are generated
on several Mpc scales around the first bright quasars. Future radio
observations with projected facilities like the Giant Metrewave Radio Telescope
and the Square Kilometer Array may shed light on the power spectrum of density
fluctuations at z>5, and map the end of the "dark ages", i.e. the transition
from the post-recombination universe to one populated with radiation sources.Comment: LateX, 19 pages, 5 figures, significantly revised version to be
published in the Ap
Peculiar Velocities of Galaxy Clusters
We investigate the peculiar velocities predicted for galaxy clusters by
theories in the cold dark matter family. A widely used hypothesis identifies
rich clusters with high peaks of a suitably smoothed version of the linear
density fluctuation field. Their peculiar velocities are then obtained by
extrapolating the similarly smoothed linear peculiar velocities at the
positions of these peaks. We test these ideas using large high resolution
N-body simulations carried out within the Virgo supercomputing consortium. We
find that at early times the barycentre of the material which ends up in a rich
cluster is generally very close to a high peak of the initial density field.
Furthermore the mean peculiar velocity of this material agrees well with the
linear value at the peak. The late-time growth of peculiar velocities is,
however, systematically underestimated by linear theory. At the time clusters
are identified we find their rms peculiar velocity to be about 40% larger than
predicted. Nonlinear effects are particularly important in superclusters. These
systematics must be borne in mind when using cluster peculiar velocities to
estimate the parameter combination .Comment: 8 pages, 4 figures; submitted to MNRA
Reionization of the Universe and the Early Production of Metals
We simulate a plausible cosmological model in considerable physical and
numerical detail through the successive phases of reheating (at 10<z<20),
formation of Pop III stars at z=15 (due to molecular hydrogen cooling), with
subsequent reionization at z=7. We assume an efficiency of high mass star
formation appropriate to leave the universe, after it becomes transparent, with
an ionizing background J_21=0.4 (at z=4), near (and perhaps slightly below) the
observed value. Since the same stars produce the ionizing radiation and the
first generation of heavy elements, a mean metallicity of Z/Z_\sun=1/200 is
produced in this early phase, but there is a large variation about this mean,
with the high density regions having Z/Z_\sun=1/30 and low density regions
essentially no metals.
Reionization, when it occurs, is very rapid, which will leave a signature
which may be detectable by very large area meter-wavelength radio instruments.
Also, the background UV radiation field will show a sharp drop from 1Ryd to
4Ryd due to absorption edges.
The simulated volume is too small to form L_* galaxies, but the smaller
objects which are found in the simulation obey the Faber-Jackson relation.
In order to explore theoretically this domain of "the end of the dark ages"
quantitatively, numerical simulations must have a mass resolution of the order
of 10^{4.5} M_\sun} in baryons, high spatial resolution (1 kpc) to resolve
strong clumping, and allow for detailed and accurate treatment of both the
radiation field and atomic/molecular physics.Comment: submitted to Ap
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