5,349 research outputs found
Star Formation, Supernovae Feedback and the Angular Momentum Problem in Numerical CDM Cosmogony: Half Way There?
We present a smoothed particle hydrodynamic (SPH) simulation that reproduces
a galaxy that is a moderate facsimile of those observed. The primary failing
point of previous simulations of disk formation, namely excessive transport of
angular momentum from gas to dark matter, is ameliorated by the inclusion of a
supernova feedback algorithm that allows energy to persist in the model ISM for
a period corresponding to the lifetime of stellar associations. The inclusion
of feedback leads to a disk at a redshift , with a specific angular
momentum content within 10% of the value required to fit observations. An
exponential fit to the disk baryon surface density gives a scale length within
17% of the theoretical value. Runs without feedback, with or without star
formation, exhibit the drastic angular momentum transport observed elsewhere.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
Mg II Absorption Systems in SDSS QSO Spectra
We present the results of a MgII absorption-line survey using QSO spectra
from the SDSS EDR. Over 1,300 doublets with rest equivalent widths greater than
0.3\AA and redshifts were identified and measured. We
find that the rest equivalent width ()
distribution is described very well by an exponential function , with
and \AA. Previously reported power law
fits drastically over-predict the number of strong lines. Extrapolating our
exponential fit under-predicts the number of \AA systems,
indicating a transition in near \AA. A combination of
two exponentials reproduces the observed distribution well, suggesting that
MgII absorbers are the superposition of at least two physically distinct
populations of absorbing clouds. We also derive a new redshift parameterization
for the number density of \AA lines:
and \AA. We find that the distribution steepens with decreasing redshift,
with decreasing from \AA at to \AA at
. The incidence of moderately strong MgII lines does not
show evidence for evolution with redshift. However, lines stronger than
\AA show a decrease relative to the no-evolution prediction with
decreasing redshift for . The evolution is stronger for
increasingly stronger lines. Since in saturated absorption lines is an
indicator of the velocity spread of the absorbing clouds, we interpret this as
an evolution in the kinematic properties of galaxies from moderate to low z.Comment: 50 pages, 26 figures, accepted for publication in Ap
Study of the local field distribution on a single-molecule magnet-by a single paramagnetic crystal; a DPPH crystal on the surface of an Mn12-acetate crystal
The local magnetic field distribution on the subsurface of a single-molecule
magnet crystal, SMM, above blocking temperature (T >> Tb) detected for a very
short time interval (~ 10-10 s), has been investigated. Electron Paramagnetic
Resonance (EPR) spectroscopy using a local paramagnetic probe was employed as a
simple alternative detection method. An SMM crystal of
[Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-acetate) and a crystal of 2,2-
diphenyl-1-picrylhydrazyl (DPPH) as the paramagnetic probe were chosen for this
study. The EPR spectra of DPPH deposited on Mn12-acetate show additional
broadening and shifting in the magnetic field in comparison to the spectra of
the DPPH in the absence of the SMM crystal. The additional broadening of the
DPPH linewidth was considered in terms of the two dominant electron spin
interactions (dipolar and exchange) and the local magnetic field distribution
on the crystal surface. The temperature dependence of the linewidth of the
Gaussian distribution of local fields at the SMM surface was extrapolated for
the low temperature interval (70-5 K)
Electrically detected magnetic resonance of carbon dangling bonds at the Si-face 4H-SiC/SiO interface
SiC based metal-oxide-semiconductor field-effect transistors (MOSFETs) have
gained a significant importance in power electronics applications. However,
electrically active defects at the SiC/SiO interface degrade the ideal
behavior of the devices. The relevant microscopic defects can be identified by
electron paramagnetic resonance (EPR) or electrically detected magnetic
resonance (EDMR). This helps to decide which changes to the fabrication process
will likely lead to further increases of device performance and reliability.
EDMR measurements have shown very similar dominant hyperfine (HF) spectra in
differently processed MOSFETs although some discrepancies were observed in the
measured -factors. Here, the HF spectra measured of different SiC MOSFETs
are compared and it is argued that the same dominant defect is present in all
devices. A comparison of the data with simulated spectra of the C dangling bond
(P) center and the silicon vacancy (V) demonstrates
that the P center is a more suitable candidate to explain the
observed HF spectra.Comment: Accepted for publication in the Journal of Applied Physic
Second-Generation Objects in the Universe: Radiative Cooling and Collapse of Halos with Virial Temperatures Above 10^4 Kelvin
The first generation of protogalaxies likely formed out of primordial gas via
H2-cooling in cosmological minihalos with virial temperatures of a few 1000K.
However, their abundance is likely to have been severely limited by feedback
processes which suppressed H2 formation. The formation of the protogalaxies
responsible for reionization and metal-enrichment of the intergalactic medium,
then had to await the collapse of larger halos. Here we investigate the
radiative cooling and collapse of gas in halos with virial temperatures Tvir >
10^4K. In these halos, efficient atomic line radiation allows rapid cooling of
the gas to 8000 K; subsequently the gas can contract nearly isothermally at
this temperature. Without an additional coolant, the gas would likely settle
into a locally gravitationally stable disk; only disks with unusually low spin
would be unstable. However, we find that the initial atomic line cooling leaves
a large, out-of-equilibrium residual free electron fraction. This allows the
molecular fraction to build up to a universal value of about x(H2) = 10^-3,
almost independently of initial density and temperature. We show that this is a
non--equilibrium freezeout value that can be understood in terms of timescale
arguments. Furthermore, unlike in less massive halos, H2 formation is largely
impervious to feedback from external UV fields, due to the high initial
densities achieved by atomic cooling. The H2 molecules cool the gas further to
about 100K, and allow the gas to fragment on scales of a few 100 Msun. We
investigate the importance of various feedback effects such as
H2-photodissociation from internal UV fields and radiation pressure due to
Ly-alpha photon trapping, which are likely to regulate the efficiency of star
formation.Comment: Revised version accepted by ApJ; some reorganization for clarit
From algebra to logic: there and back again -- the story of a hierarchy
This is an extended survey of the results concerning a hierarchy of languages
that is tightly connected with the quantifier alternation hierarchy within the
two-variable fragment of first order logic of the linear order.Comment: Developments in Language Theory 2014, Ekaterinburg : Russian
Federation (2014
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