10,572 research outputs found
Energy and Charged Particle Flow in 10.8 A GeV/c Au+Au Collisions
Experimental results and a detailed analysis are presented of the transverse
energy and charged particle azimuthal distributions measured by the E877
collaboration for different centralities of Au+Au collisions at a beam momentum
of 10.8 A GeV/c. The anisotropy of these distributions is studied with respect
to the reaction plane reconstructed on an event-by-event basis using the
transverse energy distribution measured by calorimeters. Results are corrected
for the reaction plane resolution. For semicentral events we observe directed
flow signals of up to ten percent. We observe a stronger anisotropy for slow
charged particles. For both the charged particle and transverse energy
distributions we observe a small but non zero elliptic anisotropy with the
major axis pointing into the reaction plane. Combining the information on
transverse energy and charged particle flow we obtain information on the flow
of nucleons and pions. The data are compared to event generators and the need
to introduce a mean field or nucleon-nucleon potential is discussed.Comment: RevTex, 25 pages, 13 figures included as one Postscript file,
submitted to Phys. Rev.
A Map of the Universe
We have produced a new conformal map of the universe illustrating recent
discoveries, ranging from Kuiper belt objects in the Solar system, to the
galaxies and quasars from the Sloan Digital Sky Survey. This map projection,
based on the logarithm map of the complex plane, preserves shapes locally, and
yet is able to display the entire range of astronomical scales from the Earth's
neighborhood to the cosmic microwave background. The conformal nature of the
projection, preserving shapes locally, may be of particular use for analyzing
large scale structure. Prominent in the map is a Sloan Great Wall of galaxies
1.37 billion light years long, 80% longer than the Great Wall discovered by
Geller and Huchra and therefore the largest observed structure in the universe.Comment: Figure 8, and additional material accessible on the web at:
http://www.astro.princeton.edu/~mjuric/universe
The Halo Occupation Distribution: Towards an Empirical Determination of the Relation Between Galaxies and Mass
We investigate galaxy bias in the framework of the ``Halo Occupation
Distribution'' (HOD), which defines the bias of a population of galaxies by the
conditional probability P(N|M) that a dark matter halo of virial mass M
contains N galaxies, together with prescriptions that specify the relative
spatial and velocity distributions of galaxies and dark matter within halos. By
populating the halos of a cosmological N-body simulation using a variety of HOD
models, we examine the sensitivity of different galaxy clustering statistics to
properties of the HOD. The galaxy correlation function responds to different
aspects of P(N|M) on different scales. Obtaining the observed power-law form of
xi(r) requires rather specific combinations of HOD parameters, implying a
strong constraint on the physics of galaxy formation; the success of numerical
and semi-analytic models in reproducing this form is entirely non-trivial.
Other clustering statistics such as the galaxy-mass correlation function, the
bispectrum, the void probability function, the pairwise velocity dispersion,
and the group multiplicity function are sensitive to different combinations of
HOD parameters and thus provide complementary information about galaxy bias. We
outline a strategy for determining the HOD empirically from redshift survey
data. This method starts from an assumed cosmological model, but we argue that
cosmological and HOD parameters will have non-degenerate effects on galaxy
clustering, so that a substantially incorrect cosmological model will not
reproduce the observations for any choice of HOD. Empirical determinations of
the HOD as a function of galaxy type from the 2dF and SDSS redshift surveys
will provide a detailed target for theories of galaxy formation, insight into
the origin of galaxy properties, and sharper tests of cosmological models.Comment: 60 pages + 21 eps figures. Replaced with accepted ApJ version. Minor
changes + added reference
A Mid-Infrared Study of the Class 0 Cluster in LDN 1448
We present ground-based mid-infrared observations of Class 0 protostars in
LDN 1448. Of the five known protostars in this cloud, we detected two, L1448N:A
and L1448C, at 12.5, 17.9, 20.8, and 24.5 microns, and a third, L1448 IRS 2, at
24.5 microns. We present high-resolution images of the detected sources, and
photometry or upper limits for all five Class 0 sources in this cloud. With
these data, we are able to augment existing spectral energy distributions
(SEDs) for all five objects and place them on an evolutionary status diagram.Comment: Accepted by the Astronomical Journal; 26 pages, 9 figure
Locally Biased Galaxy Formation and Large Scale Structure
We examine the influence of the morphology-density(MD) relation and a wide
range of simple models for biased galaxy formation on statistical measures of
large scale structure. We contrast the behavior of local biasing models, in
which the efficiency of galaxy formation is determined by density, geometry, or
velocity dispersion of the local mass distribution, with that of non-local
biasing models, in which galaxy formation is modulated coherently over scales
larger than the galaxy correlation length. If morphological segregation of
galaxies is governed by a local MD relation, then the correlation function of
E/S0 galaxies should be steeper and stronger than that of spiral galaxies on
small scales, as observed, while on large scales the correlation functions of
E/S0 and spiral galaxies should have the same shape but different amplitudes.
Similarly, all of our local bias models produce scale-independent amplification
of the correlation function and power spectrum in the linear and mildly
non-linear regimes; only a non-local biasing mechanism can alter the shape of
the power spectrum on large scales. Moments of the biased galaxy distribution
retain the hierarchical pattern of the mass moments, but biasing alters the
values and scale-dependence of the hierarchical amplitudes S3 and S4.
Pair-weighted moments of the galaxy velocity distribution are sensitive to the
details of the biasing prescription. The non-linearity of the relation between
galaxy density and mass density depends on the biasing prescription and the
smoothing scale, and the scatter in this relation is a useful diagnostic of the
physical parameters that determine the bias. Although the sensitivity of galaxy
clustering statistics to the details of biasing is an obstacle to testing
cosmological models, it is an asset for testing galaxy formation theories.Comment: 47 pages including 17 Figures, submitted to Ap
Screening for Multiple Endocrine Neoplasia Type 2A with DNA-Polymorphism Analysis
Nine chromosome 10 DNA markers (FNRB, D10S34, D10Z1, MEN203, D10S94, RBP3, D10S15, MBP [48.11], D10S22) were typed in two large Canadian pedigrees with multiple endocrine neoplasia type 2A (MEN 2A). These markers and the gene for MEN 2A (MEN2A) are believed to be in one linkage group spanning approximately 15 cM (male). MEN203 and D10S94 were informative and tightly linked to MEN2A with no recombinants observed in 26 meiotic events. D10S15 (MCK2), widely used in DNA genotyping predictions, demonstrated two recombinants in these two families. The use of multiple flanking markers increases both the likelihood of informativeness and the accuracy of risk assessments for predictive testing. We were able to assign a risk estimate for all 10 at-risk individuals
A Multiphase Model for the Intracluster Medium
Constraints on the clustered mass density \Omega_m of the universe derived
from the mean intracluster gas fraction of X-ray clusters may be biased by a
single-phase assumption for the thermodynamic structure of the intracluster
medium (ICM). We propose a descriptive model for multiphase structure in which
a spherically symmetric ICM contains isobaric density perturbations with
radially dependent variance \sigma^2(r)=\sigma_c^2 (1+r^2/r_c^2)^{-\eps}. The
model extends the work of Gunn & Thomas (1996) which assumed radially
independent density fluctuations thoughout the ICM. Fixing the X-ray emission
profile and emission weighted temperature, we explore two independently
observable signatures of the model in the {\sigma_c,\eps} space. For
bremsstrahlung dominated emission, the central Sunyaev--Zeldovich (SZ)
decrement in the multiphase case is increased over the single-phase case and
multiphase X-ray spectra in the range 0.1-20 keV are flatter in the continuum
and exhibit stronger low energy emission lines than their single-phase
counterpart. We quantify these effects for a fiducial 10^8 K cluster and
demonstrate how the combination of SZ and X-ray spectroscopy can be used to
identify a preferred location in the {\sigma_c,\eps} plane. From these
parameters, the correct value of mean ICM gas fraction in the multiphase model
results, allowing an unbiased estimate of \Omega_m to be recovered. The
consistency of recent determinations of the Hubble constant from SZ and X-ray
observations with values determined by other methods suggests that biases in
ICM gas fractions are small, \ltsim 20%.Comment: Nine pages, submitted to Monthly Notices of the RAS. Seven postscript
figures incoporate
The DEEP2 Galaxy Redshift Survey: The Evolution of Void Statistics from z~1 to z~0
We present measurements of the void probability function (VPF) at z~1 using
data from the DEEP2 Redshift Survey and its evolution to z~0 using data from
the Sloan Digital Sky Survey (SDSS). We measure the VPF as a function of galaxy
color and luminosity in both surveys and find that it mimics trends displayed
in the two-point correlation function, ; namely that samples of brighter,
red galaxies have larger voids (i.e. are more strongly clustered) than fainter,
blue galaxies. We also clearly detect evolution in the VPF with cosmic time,
with voids being larger in comoving units at z~0. We find that the reduced VPF
matches the predictions of a `negative binomial' model for galaxies of all
colors, luminosities, and redshifts studied. This model lacks a physical
motivation, but produces a simple analytic prediction for sources of any number
density and integrated two-point correlation function, \bar{\xi}. This implies
that differences in the VPF across different galaxy populations are consistent
with being due entirely to differences in the population number density and
\bar{\xi}. The robust result that all galaxy populations follow the negative
binomial model appears to be due to primarily to the clustering of dark matter
halos. The reduced VPF is insensitive to changes in the parameters of the halo
occupation distribution, in the sense that halo models with the same \bar{\xi}
will produce the same VPF. For the wide range of galaxies studied, the VPF
therefore does not appear to provide useful constraints on galaxy evolution
models that cannot be gleaned from studies of \bar{\xi} alone. (abridged)Comment: 17 pages, 15 figures, ApJ accepte
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