634 research outputs found
Images of Bursting Sources of High-Energy Cosmic Rays. I: Effects of Magnetic Fields
It has recently been shown that the highest energy cosmic rays (CRs) may
originate in the same cosmological objects producing -ray bursts. This
model requires the presence of intergalactic magnetic fields (IGMF) to delay
the arrival times of eV CRs by 50 years or longer relative to
the -rays, of an amplitude that is consistent with other observational
constraints. Sources of CRs coming from individual bursts should be resolved
with the planned ``Auger'' experiment, with as many as hundreds of CRs for the
brightest sources. We analyze here the apparent angular and energy distribution
of CRs from bright sources below the pion production threshold (in the energy
range ) expected in this model.
This observable distribution depends on the structure of the IGMF: the apparent
spectral width is small, , if the
intergalactic field correlation length is much larger than , and large, , in the opposite limit . The apparent angular size is also larger for smaller . If the
sources of CRs we predict are found, they will corroborate the bursting model
and they will provide us with a technique to investigate the structure of the
IGMF.Comment: Submitted to the ApJL; 10 pages AASTeX, including 2 PostScript
figure
Gravitational Lensing in Clusters of Galaxies: New Clues Regarding the Dynamics of Intracluster Gas
Long arcs in clusters of galaxies, produced by gravitational lensing, can be
used to estimate the mass interior to the arcs and therefore, constrain the
cluster mass distribution. The radial density distribution of the intracluster
gas (ICM) can be extracted from the X-ray surface brightness observations. If
the gas temperature is also known, it is then possible to probe the dynamical
state of the gas and test whether the ICM is in hydro- static equilibrium
within the gravitational potential of the cluster as a result of thermal
pressure support. We analyze three clusters that exhibit large arcs, whose
X-ray surface brightness profiles have been observed, and whose gas
temperatures have been determined. In two of the clusters, A2218 and A1689, the
central mass implied by lensing is a factor of -- too large for the gas
at the observed temperature to be in hydrostatic equilibrium solely due to
thermal pressure support. In other words, if we accept the mass estimate
derived from the lensing analysis and demand that the X-ray surface brightness
profile be consistent with the observations, the required gas temperature is a
factor of -- higher than observed. The results for the third cluster,
A2163 (the most luminous and the hottest cluster known), are more ambiguous.
The discrepancy between the X-ray and the lensing mass estimates arise because
the presence of arcs imply a highly concentrated cluster mass distribution
whereas the observed X-ray profiles imply a more extended mass distribution.
The large X-ray core radii are not the result of the limited resolution of the
X-ray detectors. We consider various possibilities that could account for the
discrepancy.Comment: 20 pages, uuencoded compressed postscript, CITA/93/3
Impact of Cosmic Rays on Population III Star Formation
We explore the implications of a possible cosmic ray (CR) background
generated during the first supernova explosions that end the brief lives of
massive Population III stars. We show that such a CR background could have
significantly influenced the cooling and collapse of primordial gas clouds in
minihaloes around redshifts of z ~ 15 - 20, provided the CR flux was sufficient
to yield an ionization rate greater than about 10^-19 s^-1 near the center of
the minihalo. The presence of CRs with energies less than approximately 10^7 eV
would indirectly enhance the molecular cooling in these regions, and we
estimate that the resulting lower temperatures in these minihaloes would yield
a characteristic stellar mass as low as ~ 10 M_sun. CRs have a less pronounced
effect on the cooling and collapse of primordial gas clouds inside more massive
dark matter haloes with virial masses greater than approximately 10^8 M_sun at
the later stages of cosmological structure formation around z ~ 10 - 15. In
these clouds, even without CR flux the molecular abundance is already
sufficient to allow cooling to the floor set by the temperature of the cosmic
microwave background.Comment: MNRAS in press, 12 pages, 6 figure
How cells feel: stochastic model for a molecular mechanosensor
Understanding mechanosensitivity, i.e. how cells sense the stiffness of their
environment is very important, yet there is a fundamental difficulty in
understanding its mechanism: to measure an elastic modulus one requires two
points of application of force - a measuring and a reference point. The cell in
contact with substrate has only one (adhesion) point to work with, and thus a
new method of measurement needs to be invented. The aim of this theoretical
work is to develop a self-consistent physical model for mechanosensitivity, a
process by which a cell detects the mechanical stiffness of its environment
(e.g. a substrate it is attached to via adhesion points) and generates an
appropriate chemical signaling to remodel itself in response to this
environment. The model uses the molecular mechanosensing complex of latent
TGF- attached to the adhesion point as the biomarker. We show that the
underlying Brownian motion in the substrate is the reference element in the
measuring process. The model produces the closed expression for the rate of
release of active TGF-, which depends on the substrate stiffness and the
pulling force coming from the cell in a subtle and non-trivial way. It is
consistent with basic experimental data showing an increase in signal for
stiffer substrates and higher pulling forces. In addition, we find that for
each cell there is a range of stiffness where a homeostatic configuration of
the cell can be achieved, outside of which the cell either relaxes its
cytoskeletal forces and detaches from the very weak substrate, or generates an
increasingly strong pulling force through stress fibers with a positive
feedback loop on very stiff substrates. In this way, the theory offers the
underlying mechanism for the myofibroblast conversion in wound healing and
smooth muscle cell dysfunction in cardiac disease
Gravitational Collapse of Small-Scale Structure as the Origin of the Lyman Alpha Forest
If gravitational clustering is a hierarchical process, the present
large-scale structure of the galaxy distribution implies that structures on
smaller scales must have formed at high redshift. We simulate the formation of
small-scale structure (average cell mass: M)
and the evolution of photoionized gas, in the specific case of a CDM model with
a cosmological constant. The photoionized gas has a natural minimal scale of
collapse, the Jeans scale (M). We find that low
column density (\nhi \le 10^{14}\cm^{-2}) lines originate in regions
resembling Zel'dovich pancakes, where gas with overdensities in the range is enclosed by two shocks but is typically re-expanding at approximately
the Hubble velocity. However, higher column density (\nhi \ge
10^{15}\cm^{-2}) lines stem from more overdense regions where the shocked gas
is cooling. We show that this model can probably account for the observed
number of lines, their distribution in column density and b-parameters, as well
as the cloud physical sizes as observed in gravitationally lensed quasars. We
find a redshift evolution that isComment: 15p postscript file to appear in The Astrophysical Journal Letters
(1994
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