2,342 research outputs found
Fractional bidromy in the vibrational spectrum of HOCl
We introduce the notion of fractional bidromy which is the combination of
fractional monodromy and bidromy, two recent generalizations of Hamiltonian
monodromy. We consider the vibrational spectrum of the HOCl molecule which is
used as an illustrative example to show the presence of nontrivial fractional
bidromy. To our knowledge, this is the first example of a molecular system
where such a generalized monodromy is exhibited.Comment: 9 pages, 2 figue
Formation of early-type galaxies from cosmological initial conditions
We describe high resolution Smoothed Particle Hydrodynamics (SPH) simulations
of three approximately field galaxies starting from \LCDM initial
conditions. The simulations are made intentionally simple, and include
photoionization, cooling of the intergalactic medium, and star formation but
not feedback from AGN or supernovae. All of the galaxies undergo an initial
burst of star formation at , accompanied by the formation of a
bubble of heated gas. Two out of three galaxies show early-type properties at
present whereas only one of them experienced a major merger. Heating from
shocks and -PdV work dominates over cooling so that for most of the gas the
temperature is an increasing function of time. By a significant
fraction of the final stellar mass is in place and the spectral energy
distribution resembles those of observed massive red galaxies. The galaxies
have grown from on average by 25% in mass and in size by gas poor
(dry) stellar mergers. By the present day, the simulated galaxies are old
(), kinematically hot stellar systems surrounded by hot
gaseous haloes. Stars dominate the mass of the galaxies up to
effective radii ( kpc). Kinematic and most photometric properties
are in good agreement with those of observed elliptical galaxies. The galaxy
with a major merger develops a counter-rotating core. Our simulations show that
realistic intermediate mass giant elliptical galaxies with plausible formation
histories can be formed from \LCDM initial conditions even without requiring
recent major mergers or feedback from supernovae or AGN.Comment: accepted for publication in Ap
An Isocurvature CDM Cosmogony. I. A Worked Example of Evolution Through Inflation
I present a specific worked example of evolution through inflation to the
initial conditions for an isocurvature CDM model for structure formation. The
model invokes three scalar fields, one that drives power law inflation, one
that survives to become the present-day CDM, and one that gives the CDM field a
mass that slowly decreases during inflation and so ``tilts'' the primeval mass
fluctuation spectrum of the CDM. The functional forms for the potentials and
the parameter values that lead to an observationally acceptable model for
structure formation do not seem to be out of line with current ideas about the
physics of the very early universe. I argue in an accompanying paper that the
model offers an acceptable fit to main observational constraints.Comment: 11 pages, 3 postscript figures, uses aas2pp4.st
AGN Obscuring Tori Supported by Infrared Radiation Pressure
Explicit 2-d axisymmetric solutions are found to the hydrostatic equilibrium,
energy balance, and photon diffusion equations within obscuring tori around
active galactic nuclei. These solutions demonstrate that infrared radiation
pressure can support geometrically thick structures in AGN environments subject
to certain constraints: the bolometric luminosity must be roughly 0.03--1 times
the Eddington luminosity; and the Compton optical depth of matter in the
equatorial plane should be order unity, with a tolerance of about an order of
magnitude up or down. Both of these constraints are at least roughly consistent
with observations. In addition, angular momentum must be redistributed so that
the fractional rotational support against gravity rises from the inner edge of
the torus to the outer in a manner specific to the detailed shape of the
gravitational potential. This model also predicts that the column densities
observed in obscured AGN should range from about 10^{22} to about 10^{24}
cm^{-2}.Comment: ApJ, in pres
Galaxy Luminosity Functions from Deep Spectroscopic Samples of Rich Clusters
Using a new spectroscopic sample and methods accounting for spectroscopic
sampling fractions that vary in magnitude and surface brightness, we present
R-band galaxy luminosity functions (GLFs) for six nearby galaxy clusters with
redshifts 4000 < cz < 20000 km/s and velocity dispersions 700 < sigma < 1250
km/s. In the case of the nearest cluster, Abell 1060, our sample extends to
M_R=-14 (7 magnitudes below M*), making this the deepest spectroscopic
determination of the cluster GLF to date. Our methods also yield composite GLFs
for cluster and field galaxies to M_R=-17 (M*+4), including the GLFs of
subsamples of star forming and quiescent galaxies. The composite GLFs are
consistent with Schechter functions (M*_R=-21.14^{+0.17}_{-0.17},
alpha=-1.21^{+0.08}_{-0.07} for the clusters, M*_R=-21.15^{+0.16}_{-0.16},
alpha=-1.28^{+0.12}_{-0.11} for the field). All six cluster samples are
individually consistent with the composite GLF down to their respective
absolute magnitude limits, but the GLF of the quiescent population in clusters
is not universal. There are also significant variations in the GLF of quiescent
galaxies between the field and clusters that can be described as a steepening
of the faint end slope. The overall GLF in clusters is consistent with that of
field galaxies, except for the most luminous tip, which is enhanced in clusters
versus the field. The star formation properties of giant galaxies are more
strongly correlated with the environment than those of fainter galaxies.Comment: 53 pages, 8 figures, 1 ASCII table; accepted for publication in Ap
Fundamental Discreteness Limitations of Cosmological N-Body Clustering Simulations
We explore some of the effects that discreteness and two-body scattering may
have on N-body simulations with ``realistic'' cosmological initial conditions.
We use an identical subset of particles from the initial conditions for a
Particle-Mesh (PM) calculation as the initial conditions for a variety
PM and Tree code runs. We investigate the effect of mass resolution (the
mean interparticle separation) since most ``high resolution'' codes only have
high resolution in gravitational force. The phase-insensitive two--point
statistics, such as the power spectrum (autocorrelation) are somewhat affected
by these variations, but phase-sensitive statistics show greater differences.
Results converge at the mean interparticle separation scale of the lowest
mass-resolution code. As more particles are added, but the force resolution is
held constant, the PM and the Tree runs agree more and more strongly with
each other and with the PM run which had the same initial conditions. This
shows high particle density is necessary for correct time evolution, since many
different results cannot all be correct. However, they do not so converge to a
PM run which continued the fluctuations to small scales. Our results show that
ignoring them is a major source of error on comoving scales of the missing
wavelengths. This can be resolved by putting in a high particle density. Since
the codes never agree well on scales below the mean comoving interparticle
separation, we find little justification for quantitative predictions on this
scale. Some measures vary by 50%, but others can be off by a factor of three or
more. Our results suggest possible problems with the density of galaxy halos,
formation of early generation objects such as QSO absorber clouds, etc.Comment: Revised version to be published in Astrophysical Journal. One figure
changed; expanded discussion, more information on code parameters. Latex, 44
pages, including 19 figures. Higher resolution versions of Figures 10-15
available at: ftp://kusmos.phsx.ukans.edu/preprints/nbod
The Arecibo Dual-Beam Survey: The HI Mass Function of Galaxies
We use the HI-selected galaxy sample from the Arecibo Dual-Beam Survey
(Rosenberg & Schneider 2000) to determine the shape of the HI mass function of
galaxies in the local universe using both the step-wise maximum likelihood and
the 1/V_tot methods. Our survey region spanned all 24 hours of right ascension
at selected declinations between 8 and 29 degrees covering ~430 deg^2 of sky in
the main beam. The survey is not as deep as some previous Arecibo surveys, but
it has a larger total search volume and samples a much larger area of the sky.
We conducted extensive tests on all aspects of the galaxy detection process,
allowing us to empirically correct for our sensitivity limits, unlike the
previous surveys. The mass function for the entire sample is quite steep, with
a power-law slope of \alpha ~ -1.5. We find indications that the slope of the
HI mass function is flatter near the Virgo cluster, suggesting that
evolutionary effects in high density environments may alter the shape of the HI
mass function. These evolutionary effects may help to explain differences in
the HI mass function derived by different groups. We are sensitive to the most
massive sources (log M > 5x10^10 M\solar) over most of the declination range,
\~1 sr, and do not detect any massive low surface brightness galaxies. These
statistics restrict the population of Malin 1-like galaxies to <5.5x10^-6
Mpc^-3.Comment: ApJ accepted, 12 page
Inflation, dark matter and dark energy in the string landscape
We consider the conditions needed to unify the description of dark matter,
dark energy and inflation in the context of the string landscape. We find that
incomplete decay of the inflaton field gives the possibility that a single
field is responsible for all three phenomena. By contrast, unifying dark matter
and dark energy into a single field, separate from the inflaton, appears rather
difficult.Comment: 4 pages RevTex4. Updated to include a toy model of reheating. Matches
version accepted by Phys Rev Let
The Omega Dependence of the Evolution of xi(r)
The evolution of the two-point correlation function, xi(r,z), and the
pairwise velocity dispersion, sigma(r,z), for both the matter and halo
population, in three different cosmological models:
(Omega_M,Omega_Lambda)=(1,0), (0.2,0) and (0.2,0.8) are described. If the
evolution of xi is parameterized by xi(r,z)=(1+z)^{-(3+eps)}xi(r,0), where
xi(r,0)=(r/r_0)^{-gamma}, then eps(mass) ranges from 1.04 +/- 0.09 for (1,0) to
0.18 +/- 0.12 for (0.2,0), as measured by the evolution of at 1 Mpc (from z ~ 5
to the present epoch). For halos, eps depends on their mean overdensity. Halos
with a mean overdensity of about 2000 were used to compute the halo two-point
correlation function tested with two different group finding algorithms: the
friends of friends and the spherical overdensity algorithm. It is certainly
believed that the rate of growth of this xihh will give a good estimate of the
evolution of the galaxy two-point correlation function, at least from z ~ 1 to
the present epoch. The values we get for eps(halos) range from 1.54 for (1,0)
to -0.36 for (0.2,0), as measured by the evolution of xi(halos) from z ~ 1.0 to
the present epoch. These values could be used to constrain the cosmological
scenario. The evolution of the pairwise velocity dispersion for the mass and
halo distribution is measured and compared with the evolution predicted by the
Cosmic Virial Theorem (CVT). According to the CVT, sigma(r,z)^2 ~ G Q rho(z)
r^2 xi(r,z) or sigma proportional to (1+z)^{-eps/2}. The values of eps measured
from our simulated velocities differ from those given by the evolution of xi
and the CVT, keeping gamma and Q constant: eps(CVT) = 1.78 +/- 0.13 for (1,0)
or 1.40 +/- 0.28 for (0.2,0).Comment: Accepted for publication in the ApJ. Also available at
http://manaslu.astro.utoronto.ca/~carlberg/cnoc/xiev/xi_evo.ps.g
Impact of Reionization on the Stellar Populations of Nearby Dwarf Galaxies
Cold dark matter models for galaxy formation predict that low-mass systems
will be the first sites of star formation. As these objects have shallow
gravitational potential wells, the subsequent growth of their stellar
populations may be halted by heating and gas loss due to reionization. This
effect has been suggested to have profoundly influenced properties of
present-day dwarf galaxies, including their stellar populations and even
survival as visible galaxies. In this Letter we draw on results from
quantitative studies of Local Group dwarf galaxy star formation histories,
especially for Milky Way satellites, to show that no clear signature exists for
a widespread evolutionary impact from reionization. All nearby dwarf galaxies
studied in sufficient detail contain ancient populations indistinguishable in
age from the oldest Galactic globular clusters. Ancient star formation activity
proceeded over several Gyr, and some dwarf spheroidal galaxies even experienced
fairly continuous star formation until just a few Gyr ago. Despite their
uniformly low masses, their star formation histories differ considerably. The
evolutionary histories of nearby dwarf galaxies appear to reflect influences
from a variety of local processes rather than a dominant effect from
reionization.Comment: Accepted by The Astrophysical Journal Letters. 5 pages, one figur
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