2,460 research outputs found
The mass function
We present the mass functions for different mass estimators for a range of
cosmological models. We pay particular attention to how universal the mass
function is, and how it depends on the cosmology, halo identification and mass
estimator chosen. We investigate quantitatively how well we can relate observed
masses to theoretical mass functions.Comment: 14 pages, 12 figures, to appear in ApJ
Patchy He II reionization and the physical state of the IGM
We present a Monte-Carlo model of He II reionization by QSOs and its effect
on the thermal state of the clumpy intergalactic medium (IGM). The model
assumes that patchy reionization develops as a result of the discrete
distribution of QSOs. It includes various recipes for the propagation of the
ionizing photons, and treats photo-heating self-consistently. The model
provides the fraction of He III, the mean temperature in the IGM, and the He II
mean optical depth -- all as a function of redshift. It also predicts the
evolution of the local temperature versus density relation during reionization.
Our findings are as follows: The fraction of He III increases gradually until
it becomes close to unity at . The He II mean optical depth
decreases from at to at .
The mean temperature rises gradually between and and
declines slowly at lower redshifts. The model predicts a flattening of the
temperature-density relation with significant increase in the scatter during
reionization at . Towards the end of reionization the scatter is
reduced and a tight relation is re-established. This scatter should be
incorporated in the analysis of the Ly forest at . Comparison
with observational results of the optical depth and the mean temperature at
moderate redshifts constrains several key physical parameters.Comment: 18 pages, 9 figures; Changed content. Accepted for publication in
MNRA
Properties and use of CMB power spectrum likelihoods
Fast robust methods for calculating likelihoods from CMB observations on
small scales generally rely on approximations based on a set of power spectrum
estimators and their covariances. We investigate the optimality of these
approximation, how accurate the covariance needs to be, and how to estimate the
covariance from simulations. For a simple case with azimuthal symmetry we
compare optimality of hybrid pseudo-C_l CMB power spectrum estimators with the
exact result, indicating that the loss of information is not negligible, but
neither is it enough to have a large effect on standard parameter constraints.
We then discuss the number of samples required to estimate the covariance from
simulations, with and without a good analytic approximation, and assess the use
of shrinkage estimators. Finally we discuss how to combine an approximate
high-ell likelihood with a more exact low-ell harmonic-space likelihood as a
practical method for accurate likelihood calculation on all scales.Comment: 15 pages, 11 figures; updated to match version accepted by PR
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
557 GHz Observations of Water Vapor Outflows from VY CMa and W Hydrae
We report the first detection of thermal water vapor emission in the 557 GHz,
ground state transition of ortho-HO toward VY Canis
Majoris. In observations obtained with the Submillimeter Wave Astronomy
Satellite (SWAS), we measured a flux of Jy, in a spectrally resolved
line centered on a velocity km s with a full width half
maximum of km s, somewhat dependent on the assumed line shape.
We analyze the line shape in the context of three different radial outflow
models for which we provide analytical expressions. We also detected a weaker
557 GHz emission line from W Hydrae. We find that these and other HO
emission line strengths scale as suggested by Zubko and Elitzur (2000).Comment: Astrophysical Journal Letters, accepte
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
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
Starburst and AGN activity in ultraluminous infrared galaxies
(Abridged) We examine the power source of 41 local Ultraluminous Infrared
Galaxies using archival infrared and optical photometry. We fit the observed
Spectral Energy Distributions (SEDs) with starburst and AGN components; each
component being drawn from a family of templates. We find all of the sample
require a starburst, whereas only half require an AGN. In 90% of the sample the
starburst provides over half the IR emission, with a mean fractional luminosity
of 82%. When combined with other galaxy samples we find that starburst and AGN
luminosities correlate over 6 decades in IR luminosity suggesting that a common
factor governs both luminosities, plausibly the gas masses in the nuclear
regions. We find that the mid-IR 7.7 micron line-continuum ratio is no
indication of the starburst luminosity, or the fractional AGN luminosity, and
therefore that this ratio is not a reliable diagnostic of the power source in
ULIRGs. We propose that the scatter in the radio-IR correlation in ULIRGs is
due to a skewed starburst IMF and/or relic relativistic electrons from a
previous starburst, rather than contamination from an obscured AGN. We show
that most ULIRGs undergo multiple starbursts during their lifetime, and by
inference that mergers between more than two galaxies may be common amongst
ULIRGs. Our results support the evolutionary model for ULIRGs proposed by
Farrah et al 2001, where they can follow many different evolutionary paths of
starburst and AGN activity in transforming merging spiral galaxies into
elliptical galaxies, but that most do not go through an optical QSO phase. The
lower level of AGN activity in our local sample than in z~1 HLIRGs implies that
the two samples are distinct populations. We postulate that different galaxy
formation processes at high-z are responsible for this difference.Comment: 24 pages, 8 figures. Accepted for publication in MNRA
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Proton beam therapy and localised prostate cancer: current status and controversies
Proton therapy is a promising, but costly, treatment for prostate cancer. Theoretical physical advantages exist; yet to date, it has been shown only to be comparably safe and effective when compared with the alternatives and not necessarily superior. If clinically meaningful benefits do exist for patients, more rigorous study will be needed to detect them and society will require this to justify the investment of time and money. New technical advances in proton beam delivery coupled with shortened overall treatment times and declining device costs have the potential to make this a more cost-effective therapy in the years ahead
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