1,074 research outputs found
Glaciological and volcanological studies in the Wrangell Mountains, Alaska
There are no author-identified significant results in this report
The rationality of Sol manifolds
Let be the fundamental group of a manifold modeled on three
dimensional Sol geometry. We prove that has a finite index subgroup
which has a rational growth series with respect to a natural generating
set. We do this by enumerating by a regular language. However, in contrast
to most earlier proofs of this sort our regular language is not a language of
words in the generating set, but rather reflects a different geometric
structure in .Comment: 30 pages; author's name changed to agree with published version; to
appear in Journal of Algebr
The nonlinear evolution of baryonic overdensities in the early universe: Initial conditions of numerical simulations
We run very large cosmological N-body hydrodynamical simulations in order to
study statistically the baryon fractions in early dark matter halos. We
critically examine how differences in the initial conditions affect the gas
fraction in the redshift range z = 11--21. We test three different linear power
spectra for the initial conditions: (1) A complete heating model, which is our
fiducial model; this model follows the evolution of overdensities correctly,
according to Naoz & Barkana (2005), in particular including the spatial
variation of the speed of sound of the gas due to Compton heating from the CMB.
(2) An equal-{\delta} model, which assumes that the initial baryon fluctuations
are equal to those of the dark matter, while conserving sigma8 of the total
matter. (3) A mean cs model, which assumes a uniform speed of sound of the gas.
The latter two models are often used in the literature. We calculate the baryon
fractions for a large sample of halos in our simulations. Our fiducial model
implies that before reionization and significant stellar heating took place,
the minimum mass needed for a minihalo to keep most of its baryons throughout
its formation was ~ 3 * 10^4 Msun. However, the alternative models yield a
wrong (higher by about 50%) minimum mass, since the system retains a memory of
the initial conditions. We also demonstrate this using the "filtering mass"
from linear theory, which accurately describes the evolution of the baryon
fraction throughout the simulated redshift range.Comment: 6 figures 1 table, accepted to MNRA
Two temperature viscous accretion flows around rotating black holes: Description of under-fed systems to ultra-luminous X-ray sources
We discuss two temperature accretion disk flows around rotating black holes.
As we know that to explain observed hard X-rays the choice of Keplerian angular
momentum profile is not unique, we consider the sub-Keplerian regime of the
disk. Without any strict knowledge of the magnetic field structure, we assume
the cooling mechanism is dominated by bremsstrahlung process. We show that in a
range of Shakura-Sunyaev viscosity parameter 0.2\gsim\alpha\gsim0.0005, flow
behavior varies widely, particularly by means of the size of disk, efficiency
of cooling and corresponding temperatures of ions and electrons. We also show
that the disk around a rotating black hole is hotter compared to that around a
Schwarzschild black hole, rendering a larger difference between ion and
electron temperatures in the former case. With all the theoretical solutions in
hand, finally we reproduce the observed luminosities () of two extreme cases
-- the under-fed AGNs and quasars (e.g. Sgr ) with L\gsim 10^{33}
erg/sec to ultra-luminous X-ray sources with erg/sec, at
different combinations of mass accretion rate, ratio of specific heats,
Shakura-Sunyaev viscosity parameter and Kerr parameter, and conclude that Sgr
may be an intermediate spinning black hole.Comment: 21 pages including 5 figures; few typos corrected; to appear in New
Astronom
Cosmological Reionization Around the First Stars: Monte Carlo Radiative Transfer
We study the evolution of ionization fronts around the first proto-galaxies
by using high resolution numerical cosmological (Lambda+CDM model) simulations
and Monte Carlo radiative transfer methods. We present the numerical scheme in
detail and show the results of test runs from which we conclude that the scheme
is both fast and accurate. As an example of interesting cosmological
application, we study the reionization produced by a stellar source of total
mass M=2 10^8 M_\odot turning on at z=12, located at a node of the cosmic web.
The study includes a Spectral Energy Distribution of a zero-metallicity stellar
population, and two Initial Mass Functions (Salpeter/Larson). The expansion of
the I-front is followed as it breaks out from the galaxy and it is channeled by
the filaments into the voids, assuming, in a 2D representation, a
characteristic butterfly shape. The ionization evolution is very well tracked
by our scheme, as realized by the correct treatment of the channeling and
shadowing effects due to overdensities. We confirm previous claims that both
the shape of the IMF and the ionizing power metallicity dependence are
important to correctly determine the reionization of the universe.Comment: 8 pages, 8 figures. Revised version, accepted for publication by
MNRA
Disk heating agents across the Hubble sequence
We measure the shape of the velocity ellipsoid in two late-type spiral
galaxies (Hubble types Sc and Scd) and combine these results with our previous
analyses of six early-type spirals (Sa to Sbc) to probe the relation between
galaxy morphology and the ratio of the vertical and radial dispersions. We
confirm at much higher significance (99.9 percent) our prior detection of a
tight correlation between these quantities. We explore the trends of the
magnitude and shape of the velocity ellipsoid axes with galaxy properties
(colour, gas surface mass density, and spiral arm structure). The observed
relationships allow for an observational identification of the radial and
vertical disk heating agents in external disk galaxies.Comment: 11 pages, 8 figures, 4 tables. Accepted for publication in MNRA
The Ursinus Weekly, November 23, 1977
Ursinus News in Brief: U.S.G.A. threatens boycott petition; Cafeteria dinner affirmed; Jessup announces computer courses; Williamson nominates two; Four elected to task force; Ursinus appliance team • Vandalism to exceed \u2776 totals: President\u27s dining room vandalized • U.C. unaware of breakout: How safe is Ursinus? • Enrollments decrease • Five call for U.S.G.A. action • Comment • Letters to the editor • Movie attack: MacArthur • A view of Wismer • Burns debuts as deity • Sex and drugs • Medical credentials • Opinion: S.F.A.R.C. • Grizzly Gridiron closes at 2-7 • Women\u27s volleyball: Wrap-up • Soccer finale • 3 & 4 end season 9-1 • Varsity hockey • X-Country dominateshttps://digitalcommons.ursinus.edu/weekly/1077/thumbnail.jp
The Ursinus Weekly, November 23, 1977
Ursinus News in Brief: U.S.G.A. threatens boycott petition; Cafeteria dinner affirmed; Jessup announces computer courses; Williamson nominates two; Four elected to task force; Ursinus appliance team • Vandalism to exceed \u2776 totals: President\u27s dining room vandalized • U.C. unaware of breakout: How safe is Ursinus? • Enrollments decrease • Five call for U.S.G.A. action • Comment • Letters to the editor • Movie attack: MacArthur • A view of Wismer • Burns debuts as deity • Sex and drugs • Medical credentials • Opinion: S.F.A.R.C. • Grizzly Gridiron closes at 2-7 • Women\u27s volleyball: Wrap-up • Soccer finale • 3 & 4 end season 9-1 • Varsity hockey • X-Country dominateshttps://digitalcommons.ursinus.edu/weekly/1077/thumbnail.jp
Galaxy Formation Theory
We review the current theory of how galaxies form within the cosmological
framework provided by the cold dark matter paradigm for structure formation.
Beginning with the pre-galactic evolution of baryonic material we describe the
analytical and numerical understanding of how baryons condense into galaxies,
what determines the structure of those galaxies and how internal and external
processes (including star formation, merging, active galactic nuclei etc.)
determine their gross properties and evolution. Throughout, we highlight
successes and failings of current galaxy formation theory. We include a review
of computational implementations of galaxy formation theory and assess their
ability to provide reliable modelling of this complex phenomenon. We finish
with a discussion of several "hot topics" in contemporary galaxy formation
theory and assess future directions for this field.Comment: 58 pages, to appear in Physics Reports. This version includes minor
corrections and a handful of additional reference
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