174,112 research outputs found
The effect of supernova heating on cluster properties and constraints on galaxy formation models
Models of galaxy formation should be able to predict the properties of
clusters of galaxies, in particular their gas fractions, metallicities, X-ray
luminosity-temperature relation, temperature function and mass-deposition-rate
function. Fitting these properties places important constaints on galaxy
formation on all scales. By following gas processes in detail, our
semi-analytic model (based on that of Nulsen & Fabian 1997) is the only such
model able to predict all of the above cluster properties. We use realistic gas
fractions and gas density profiles, and as required by observations we break
the self-similarity of cluster structure by including supernova heating of
intracluster gas, the amount of which is indicated by the observed
metallicities. We also highlight the importance of the mass-deposition-rate
function as an independent and very sensitive probe of cluster structure.Comment: 5 pages, 4 figures, accepted for publication in MNRAS as a lette
Charge pumping in monolayer graphene driven by a series of time-periodic potentials
We applied the Floquet scattering-matrix formalism to studying the electronic
transport properties in a mesoscopic Dirac system. Using the method, we
investigate theoretically quantum pumping driven by a series of time-periodic
potentials in graphene monolayer both in the adiabatic and non-adiabatic
regimes. Our numerical results demonstrate that adding harmonic modulated
potentials can break the time reversal symmetry when no voltage bias is applied
to the graphene monolayer. Thus, when the system is pumped with proper dynamic
parameters, these scatterers can produce a nonzero dc pumped current. We also
find that the transmission is anisotropic as the incident angle is changed.Comment: 8 pages, 6 figure
The soft X-ray background: evidence for widespread disruption of the gas halos of galaxy groups
Almost all of the extragalactic X-ray background (XRB) at 0.25 keV can be
accounted for by radio-quiet quasars, allowing us to derive an upper limit of 4
\bgunit\ for the remaining background at 0.25 keV. However, the XRB from the
gas halos of groups of galaxies, with gas removal due to cooling accounted for,
exceeds this upper limit by an order of magnitude if non-gravitational heating
is not included. We calculate this using simulations of halo merger trees and
realistic gas density profiles, which we require to reproduce the observed gas
fractions and abundances of X-ray clusters. In addition, we find that the
entire mass range of groups, from to \Ms,
contributes to the 0.25 keV background in this case. In a further study, we
reduce the luminosities of groups by maximally heating their gas halos while
maintaining the same gas fractions. This only reduces the XRB by a factor of 2
or less. We thus argue that most of the gas associated with groups must be
outside their virial radii. This conclusion is supported by X-ray studies of
individual groups. The properties of both groups and X-ray clusters can be
naturally explained by a model in which the gas is given excess specific
energies of keV/particle by non-gravitational heating. With this
excess energy, the gas is gravitationally unbound from groups, but recollapses
with the formation of a cluster of temperature \ga 1 keV. This is similar to
a model proposed by Pen, but is contrary to the evolution of baryons described
by Cen \& Ostriker. (abridged)Comment: 14 pages, 14 figures, submitted to MNRA
Analysis of a single-fold deployable truss beam preloaded by extension of selected face diagonal members
A technique for preloading a deployable box truss beam by extension of one face diagonal per bay was studied to determine if it would result in uniform loading of truss joints without causing excessive truss deformations. Results indicate that it is possible to accomplish uniform loading in the beam region way from beam boundaries, whereas in the regions near boundaries the member loading becomes non-uniform with magnitudes greater than those in the uniform load region. Also, the type of deformation which results in the beam depends on the pattern of preloaded members
- …