42,878 research outputs found
The Mass Function and Average Mass Loss Rate of Dark Matter Subhaloes
We present a simple, semi-analytical model to compute the mass functions of
dark matter subhaloes. The masses of subhaloes at their time of accretion are
obtained from a standard merger tree. During the subsequent evolution, the
subhaloes experience mass loss due to the combined effect of dynamical
friction, tidal stripping, and tidal heating. Rather than integrating these
effects along individual subhalo orbits, we consider the average mass loss
rate, where the average is taken over all possible orbital configurations. This
allows us to write the average mass loss rate as a simple function that depends
only on redshift and on the instantaneous mass ratio of subhalo and parent
halo. After calibrating the model by matching the subhalo mass function (SHMF)
of cluster-sized dark matter haloes obtained from numerical simulations, we
investigate the predicted mass and redshift dependence of the SHMF.We find
that, contrary to previous claims, the subhalo mass function is not universal.
Instead, both the slope and the normalization depend on the ratio of the parent
halo mass, M, and the characteristic non-linear mass M*. This simply reflects a
halo formation time dependence; more massive parent haloes form later, thus
allowing less time for mass loss to operate. We analyze the halo-to-halo
scatter, and show that the subhalo mass fraction of individual haloes depends
most strongly on their accretion history in the last Gyr. Finally we provide a
simple fitting function for the average SHMF of a parent halo of any mass at
any redshift and for any cosmology, and briefly discuss several implications of
our findings.Comment: Replaced to match version accepted for publication in MNRAS. Small
section added that discusses higher-order moments of subhalo occupation
distribution (including a new figure). Otherwise, few small change
Inverse Bremsstrahlung in Shocked Astrophysical Plasmas
There has recently been interest in the role of inverse bremsstrahlung, the
emission of photons by fast suprathermal ions in collisions with ambient
electrons possessing relatively low velocities, in tenuous plasmas in various
astrophysical contexts. This follows a long hiatus in the application of
suprathermal ion bremsstrahlung to astrophysical models since the early 1970s.
The potential importance of inverse bremsstrahlung relative to normal
bremsstrahlung, i.e. where ions are at rest, hinges upon the underlying
velocity distributions of the interacting species. In this paper, we identify
the conditions under which the inverse bremsstrahlung emissivity is significant
relative to that for normal bremsstrahlung in shocked astrophysical plasmas. We
determine that, since both observational and theoretical evidence favors
electron temperatures almost comparable to, and certainly not very deficient
relative to proton temperatures in shocked plasmas, these environments
generally render inverse bremsstrahlung at best a minor contributor to the
overall emission. Hence inverse bremsstrahlung can be safely neglected in most
models invoking shock acceleration in discrete sources such as supernova
remnants. However, on scales > 100pc distant from these sources, Coulomb
collisional losses can deplete the cosmic ray electrons, rendering inverse
bremsstrahlung, and perhaps bremsstrahlung from knock-on electrons, possibly
detectable.Comment: 13 pages, including 2 figures, using apjgalley format; to appear in
the January 10, 2000 issue, of the Astrophysical Journa
Thermal effects on cephalopod energy metabolism - A case study for Sepia officinalis
Cephalopods are the largest, most active invertebrates and there is considerable evidence for their convergent evolution with fishes. However, most active cephalopods display standard and active metabolic rates that are several-fold higher than comparably sized fishes. Shifting habitat temperatures due to climate change will therefore affect a cephalopods energy metabolism much more than that of a fish. Prediction of the probable outcome of cephalopod-fish competition thus requires quantitative information concerning whole animal energetics and corresponding efficiencies. Migrating cephalopods such as squid and cuttlefish grow rapidly to maturity, carry few food reserves and have little overlap of generations. This "live fast, die young" life history strategy means that they require niches capable of sustaining high power requirements and rapid growth. This presentation aims to draw a bottom-up picture of the cellular basis of energy metabolism of the cuttlefish Sepia officinalis, from its molecular basis to whole animal energetics based on laboratory experiments and field data. We assessed the proportionality of standard vs active metabolic rate and the daily energetic requirements using field tracking data in combination with lab based respirometry and video analysis. Effects of environmental temperature on mitochondrial energy coupling were investigated in whole animals using in vivo 31P-NMR spectroscopy. As efficient energy turnover needs sufficient oxygen supply, also thermal effects on the blood oxygen-binding capacities of the respiratory pigment haemocyanin and the differential expression of its isoforms were investigated.Supported by NERC grant NERC/A/S/2002/00812
Integration of tools for the Design and Assessment of High-Performance, Highly Reliable Computing Systems (DAHPHRS), phase 1
Systems for Space Defense Initiative (SDI) space applications typically require both high performance and very high reliability. These requirements present the systems engineer evaluating such systems with the extremely difficult problem of conducting performance and reliability trade-offs over large design spaces. A controlled development process supported by appropriate automated tools must be used to assure that the system will meet design objectives. This report describes an investigation of methods, tools, and techniques necessary to support performance and reliability modeling for SDI systems development. Models of the JPL Hypercubes, the Encore Multimax, and the C.S. Draper Lab Fault-Tolerant Parallel Processor (FTPP) parallel-computing architectures using candidate SDI weapons-to-target assignment algorithms as workloads were built and analyzed as a means of identifying the necessary system models, how the models interact, and what experiments and analyses should be performed. As a result of this effort, weaknesses in the existing methods and tools were revealed and capabilities that will be required for both individual tools and an integrated toolset were identified
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