3,241 research outputs found
Interplay of shear and bulk viscosity in generating flow in heavy-ion collisions
We perform viscous hydrodynamic calculations in 2+1 dimensions to investigate
the influence of bulk viscosity on the viscous suppression of elliptic flow in
non-central heavy-ion collisions at RHIC energies. Bulk and shear viscous
effects on the evolution of radial and elliptic flow are studied with different
model assumptions for the transport coefficients. We find that the temperature
dependence of the relaxation time for the bulk viscous pressure, especially its
critical slowing down near the quark-hadron phase transition at T_c, partially
offsets effects from the strong growth of the bulk viscosity itself near T_c,
and that even small values of the specific shear viscosity eta/s of the
fireball matter can be extracted without large uncertainties from poorly
controlled bulk viscous effects.Comment: 13 pages, 7 figures, 1 table. Submitted to Physical Review C. v2:
corrected typos in several entries in Table
Models of Disk Evolution: Confrontation with Observations
We present simple models for disk evolution based on two different
approaches: a forward approach based on predictions generic to hierarchical
models for structure formation (e.g., Mo, Mao, & White 1998) and a backwards
approach based on detailed modeling of the Milky Way galaxy (e.g., Bouwens,
Cayon, & Silk 1997). We normalize these models to local observations and
predict high-redshift luminosities, sizes, circular velocities, and surface
brightnesses. Both approaches yield somewhat similar predictions for size,
surface brightness, and luminosity evolution though they clearly differ in the
amount of number evolution. These predictions seem to be broadly consistent
with the high-redshift observations of Simard et al. (1999), suggesting that
the B-band surface brightness of disks has indeed evolved by ~1.5 mag from z~0
to z~1 similar to the models and is not an artifact of selection effects as
previously claimed. We also find a lack of low surface brightness galaxies in
several high redshift samples relative to model predictions based on local
samples (de Jong & van der Kruit 1994; Mathewson, Ford, & Buchhorn 1992).Comment: 34 pages, 9 figures, accepted to Ap
A Look At Three Different Scenarios for Bulge Formation
In this paper, we present three qualitatively different scenarios for bulge
formation: a secular evolution model in which bulges form after disks and
undergo several central starbursts, a primordial collapse model in which bulges
and disks form simultaneously, and an early bulge formation model in which
bulges form prior to disks. We normalize our models to the local z=0
observations of de Jong & van der Kruit (1994) and Peletier & Balcells (1996)
and make comparisons with high redshift observations. We consider model
predictions relating directly to bulge-to-disk properties. As expected, smaller
bulge-to-disk ratios and bluer bulge colors are predicted by the secular
evolution model at all redshifts, although uncertainties in the data are
currently too large to differentiate strongly between the models.Comment: 19 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Observational Constraints on the Self Interacting Dark Matter Scenario and the Growth of Supermassive Black Holes
We consider the consequences of SIDM for a velocity dependent cross section
per unit mass. Accretion of SIDM onto seed black holes can produce supermassive
black holes that are too large for certain combinations of parameters,which is
used to obtain a new constraint on the dark matter interaction. Constraints due
to other considerations are presented and previous ones are generalized. The
black hole constraint is extremely sensitive to the slope \alpha, of the inner
density profile of dark halos. For the most probable value of \alpha=1.3, there
exists a narrow range in parameter space, such that all constraints are
satisfied. However, the adiabatic compression of the dark halo by baryons as
they cool and contract in normal galaxies yields a steeper cusp, \alpha=1.7.
This gives a tighter constraint, which would exclude SIDM as a possible
solution to the purported problems with CDM in the absence of other dynamical
processes. Nevertheless, SIDM with parameters consistent with this stronger
constraint, can explain the ubiquity of supermassive black holes in the centers
of galaxies. A ``best fit'' model is presented which reproduces the
supermassive black hole masses and their observed correlations with the
velocity dispersion of the host bulges. Specifically, the fourth power
dependence of black hole mass on velocity dispersion is a direct consequence of
the power spectrum having an index of n=-2. Although the dark matter collision
rates for this model are too small to directly remedy problems with CDM,
mergers between dark halos harboring supermassive black holes at high redshift
could ameliorate the cuspy halo problem. This scenario also explains the lack
of comparable supermassive black holes in bulgeless galaxies like M33.Comment: 30 pages, 6 figures, significant improvements: added new constraint,
revised old constraints, changed figure
Sharp A₂ inequality for haar shift operators
"Vegeu el resum a l'inici del document del fitxer adjunt"
The cluster abundance in cosmic string models for structure formation
We use the present observed number density of large X-ray clusters to
constrain the amplitude of matter density perturbations induced by cosmic
strings on the scale of Mpc (), in both open cosmologies
and flat models with a non-zero cosmological constant. We find a slightly lower
value of than that obtained in the context of primordial Gaussian
fluctuations generated during inflation. This lower normalization of
results from the mild non-Gaussianity on cluster scales, where the one point
probability distribution function is well approximated by a
distribution. We use our estimate of to constrain the string linear
energy density and show that it is consistent with the COBE
normalization.Comment: 5 pages, 2 figure
Semi-Analytical Models for Lensing by Dark Halos: I. Splitting Angles
We use the semi-analytical approach to analyze gravitational lensing of
quasars by dark halos in various cold dark matter (CDM) cosmologies, in order
to determine the sensitivity of the prediction probabilities of images
separations to the input assumptions regarding halos and cosmologies. The mass
function of dark halos is assumed to be given by the Press-Schechter function.
The mass density profile of dark halos is alternatively taken to be the
singular isothermal sphere (SIS), the Navarro-Frenk-White (NFW) profile, or the
generalized NFW profile. The cosmologies include: the Einstein-de Sitter model
(SCDM), the open model (OCDM), and the flat \Lambda-model (LCDM). As expected,
we find that the lensing probability is extremely sensitive to the mass density
profile of dark halos, and somewhat less so to the mean mass density in the
universe, and the amplitude of primordial fluctuations. NFW halos are very much
less effective in producing multiple images than SIS halos. However, none of
these models can completely explain the current observations: the SIS models
predict too many large splitting lenses, while the NFW models predict too few
small splitting lenses. This indicates that there must be at least two
populations of halos in the universe. A combination of SIS and NFW halos can
reasonably reproduce the current observations if we choose the mass for the
transition from SIS to NFW to be ~ 10^{13} solar masses. Additionally, there is
a tendency for CDM models to have too much power on small scales, i.e. too much
mass concentration; and it appears that the cures proposed for other apparent
difficulties of CDM would help here as well, an example being the warm dark
matter (WDM) variant which is shown to produce large splitting lenses fewer
than the corresponding CDM model by one order of magnitude.Comment: 46 pages, including 13 figures. Revised version with significant
improvemen
Cluster Alignments and Ellipticities in LCDM Cosmology
The ellipticities and alignments of clusters of galaxies, and their evolution
with redshift, are examined in the context of a Lambda-dominated cold dark
matter cosmology. We use a large-scale, high-resolution N-body simulation to
model the matter distribution in a light cone containing ~10^6 clusters out to
redshifts of z=3. Cluster ellipticities are determined as a function of mass,
radius, and redshift, both in 3D and in projection. We find strong cluster
ellipticities: the mean ellipticity increases with redshift from 0.3 at z=0 to
0.5 at z=3, for both 3D and 2D ellipticities; the evolution is well-fit by
e=0.33+0.05z. The ellipticities increase with cluster mass and with cluster
radius; the main cluster body is more elliptical than the cluster cores, but
the increase of ellipticities with redshift is preserved. Using the fitted
cluster ellipsoids, we determine the alignment of clusters as a function of
their separation. We find strong alignment of clusters for separations <100
Mpc/h; the alignment increases with decreasing separation and with increasing
redshift. The evolution of clusters from highly aligned and elongated systems
at early times to lower alignment and elongation at present reflects the
hierarchical and filamentary nature of structure formation. These measures of
cluster ellipticity and alignment will provide a new test of the current
cosmological model when compared with upcoming cluster surveys.Comment: 29 pages including 13 figures, to appear in ApJ Jan. 2005 (corrected
typos, added reference
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