640 research outputs found
The scale-dependence of relative galaxy bias: encouragement for the halo model description
We investigate the relationship between the colors, luminosities, and
environments of galaxies in the Sloan Digital Sky Survey spectroscopic sample,
using environmental measurements on scales ranging from 0.2 to 6 Mpc/h. We
find: (1) that the relationship between color and environment persists even to
the lowest luminosities we probe (absolute magnitude in the r band of about -14
for h=1); (2) at luminosities and colors for which the galaxy correlation
function has a large amplitude, it also has a steep slope; and (3) in regions
of a given overdensity on small scales (1 Mpc/h), the overdensity on large
scales (6 Mpc/h) does not appear to relate to the recent star formation history
of the galaxies. Of these results, the last has the most immediate application
to galaxy formation theory. In particular, it lends support to the notion that
a galaxy's properties are related only to the mass of its host dark matter
halo, and not to the larger scale environment.Comment: submitted to ApJ; full resolution figures and slide material
available at http://cosmo.nyu.edu/blanton/scale_density.htm
Cosmological Parameters from Velocities, CMB and Supernovae
We compare and combine likelihood functions of the cosmological parameters
Omega_m, h and sigma_8, from peculiar velocities, CMB and type Ia supernovae.
These three data sets directly probe the mass in the Universe, without the need
to relate the galaxy distribution to the underlying mass via a "biasing"
relation. We include the recent results from the CMB experiments BOOMERANG and
MAXIMA-1. Our analysis assumes a flat Lambda CDM cosmology with a
scale-invariant adiabatic initial power spectrum and baryonic fraction as
inferred from big-bang nucleosynthesis. We find that all three data sets agree
well, overlapping significantly at the 2 sigma level. This therefore justifies
a joint analysis, in which we find a joint best fit point and 95 per cent
confidence limits of Omega_m=0.28 (0.17,0.39), h=0.74 (0.64,0.86), and
sigma_8=1.17 (0.98,1.37). In terms of the natural parameter combinations for
these data sigma_8 Omega_m^0.6 = 0.54 (0.40,0.73), Omega_m h = 0.21
(0.16,0.27). Also for the best fit point, Q_rms-ps = 19.7 muK and the age of
the universe is 13.2 Gyr.Comment: 8 pages, 5 figures. Submitted to MNRA
Constraints on the relationship between stellar mass and halo mass at low and high redshift
We use a statistical approach to determine the relationship between the
stellar masses of galaxies and the masses of the dark matter halos in which
they reside. We obtain a parameterized stellar-to-halo mass (SHM) relation by
populating halos and subhalos in an N-body simulation with galaxies and
requiring that the observed stellar mass function be reproduced. We find good
agreement with constraints from galaxy-galaxy lensing and predictions of
semi-analytic models. Using this mapping, and the positions of the halos and
subhalos obtained from the simulation, we find that our model predictions for
the galaxy two-point correlation function (CF) as a function of stellar mass
are in excellent agreement with the observed clustering properties in the SDSS
at z=0. We show that the clustering data do not provide additional strong
constraints on the SHM function and conclude that our model can therefore
predict clustering as a function of stellar mass. We compute the conditional
mass function, which yields the average number of galaxies with stellar masses
in the range [m, m+dm] that reside in a halo of mass M. We study the redshift
dependence of the SHM relation and show that, for low mass halos, the SHM ratio
is lower at higher redshift. The derived SHM relation is used to predict the
stellar mass dependent galaxy CF and bias at high redshift. Our model predicts
that not only are massive galaxies more biased than low mass ones at all
redshifts, but the bias increases more rapidly with increasing redshift for
massive galaxies than for low mass ones. We present convenient fitting
functions for the SHM relation as a function of redshift, the conditional mass
function, and the bias as a function of stellar mass and redshift.Comment: 21 pages, 17 figures, discussion enlarged, one more figure, updated
references, accepted for publication in Ap
The impact of assembly bias on the halo occupation in hydrodynamical simulations
We investigate the variations in galaxy occupancy of the dark matter haloes with the large-scale environment and halo formation time, using two state-of-the-art hydrodynamical cosmological simulations, EAGLE and Illustris. For both simulations, we use three galaxy samples with a fixed number density ranked by stellar mass. For these samples, we find that low-mass haloes in the most dense environments are more likely to host a central galaxy than those in the least dense environments. When splitting the halo population by formation time, these relations are stronger. Hence, at a fixed low halo mass, early-formed haloes are more likely to host a central galaxy than late-formed haloes since they have had more time to assemble. The satellite occupation shows a reverse trend where early-formed haloes host fewer satellites due to having more time to merge with the central galaxy. We also analyse the stellar mass–halo mass relation for central galaxies in terms of the large-scale environment and formation time of the haloes. We find that low-mass haloes in the most dense environment host relatively more massive central galaxies. This trend is also found when splitting the halo population by age, with early-formed haloes hosting more massive galaxies. Our results are in agreement with previous findings from semi-analytical models, providing robust predictions for the occupancy variation signature in the halo occupation distribution of galaxy formation models
A theoretical framework for combining techniques that probe the link between galaxies and dark matter
We develop a theoretical framework that combines measurements of
galaxy-galaxy lensing, galaxy clustering, and the galaxy stellar mass function
in a self-consistent manner. While considerable effort has been invested in
exploring each of these probes individually, attempts to combine them are still
in their infancy despite the potential of such combinations to elucidate the
galaxy-dark matter connection, to constrain cosmological parameters, and to
test the nature of gravity. In this paper, we focus on a theoretical model that
describes the galaxy-dark matter connection based on standard halo occupation
distribution techniques. Several key modifications enable us to extract
additional parameters that determine the stellar-to-halo mass relation and to
simultaneously fit data from multiple probes while allowing for independent
binning schemes for each probe. In a companion paper, we demonstrate that the
model presented here provides an excellent fit to galaxy-galaxy lensing, galaxy
clustering, and stellar mass functions measured in the COSMOS survey from z=0.2
to z=1.0. We construct mock catalogs from numerical simulations to investigate
the effects of sample variance and covariance on each of the three probes.
Finally, we analyze and discuss how trends in each of the three observables
impact the derived parameters of the model. In particular, we investigate the
various features of the observed galaxy stellar mass function (low-mass slope,
plateau, knee, and high-mass cut-off) and show how each feature is related to
the underlying relationship between stellar and halo mass. We demonstrate that
the observed plateau feature in the stellar mass function at Mstellar~2x10^10
Msun is due to the transition that occurs in the stellar-to-halo mass relation
at Mhalo ~ 10^12 Msun from a low-mass power-law regime to a sub-exponential
function at higher stellar mass.Comment: 21 pages. Accepted to Ap
A Local Hubble Bubble from SNe Ia?
We analyze the monopole in the peculiar velocities of 44 Type Ia supernovae
(SNe Ia) to test for a local void. The sample extends from 20 to 300 Mpc/h,
with distances, deduced from light-curve shapes, accurate to ~6%. Assuming
Omega_m=1 and Omega_lambda=0, the most significant deviation we find from the
Hubble law is an outwards flow of (6.6+/-2.2)% inside a sphere of radius 70
Mpc/h as would be produced by a void of ~20% underdensity surrounded by a dense
shell. This shell roughly coincides with the local Great Walls. Monte Carlo
analyses, using Gaussian errors or bootstrap resampling, show the probability
for chance occurrence of this result out of a pure Hubble flow to be ~2%. The
monopole could be contaminated by higher moments of the velocity field,
especially a quadrupole, which are not properly probed by the current limited
sky coverage. The void would be less significant if Omega_m is low and
Omega_lambda is high. It would be more significant if one outlier is removed
from the sample, or if the size of the void is constrained a-priori. This
putative void is not in significant conflict with any of the standard
cosmological scenarios. It suggests that the Hubble constant as determined
within 70 Mpc/h could be overestimated by ~6% and the local value of Omega may
be underestimated by ~20%. While the present evidence for a local void is
marginal in this data set, the analysis shows that the accumulation of SNe Ia
distances will soon provide useful constraints on elusive and important aspects
of regional cosmic dynamics.Comment: 21 pages, 3 figures. Slightly revised version. To appear in ApJ, 503,
Aug. 20, 199
Multi-User OFDM Based on Braided Convolutional Codes
Braided convolutional codes (BCCs) form a class of iteratively decodable convolutional codes that are constructed from component convolutional codes. In braided code division multiple access (BCDMA), these very efficient error correcting codes are combined with a multiple access method and inherent interleaving for channel diversity exploitation into one single scheme. In this paper, we describe the BCDMA principle and present simulation results for a frequency selective Rayleigh fading channel. Results for bit interleaved coded modulation (BICM) based on turbo and LDPC codes are also given for comparison
Gravitational Collapse with a Cosmological Constant
We consider the effect of a positive cosmological constant on spherical
gravitational collapse to a black hole for a few simple, analytic cases. We
construct the complete Oppenheimer-Snyder-deSitter (OSdS) spacetime, the
generalization of the Oppenheimer-Snyder solution for collapse from rest of a
homogeneous dust ball in an exterior vacuum. In OSdS collapse, the cosmological
constant may affect the onset of collapse and decelerate the implosion
initially, but it plays a diminishing role as the collapse proceeds. We also
construct spacetimes in which a collapsing dust ball can bounce, or hover in
unstable equilibrium, due to the repulsive force of the cosmological constant.
We explore the causal structure of the different spacetimes and identify any
cosmological and black hole event horizons which may be present.Comment: 7 pages, 10 figures; To appear in Phys. Rev.
Single parameter galaxy classification: The Principal Curve through the multi-dimensional space of galaxy properties
We propose to describe the variety of galaxies from SDSS by using only one
affine parameter. To this aim, we build the Principal Curve (P-curve) passing
through the spine of the data point cloud, considering the eigenspace derived
from Principal Component Analysis of morphological, physical and photometric
galaxy properties. Thus, galaxies can be labeled, ranked and classified by a
single arc length value of the curve, measured at the unique closest projection
of the data points on the P-curve. We find that the P-curve has a "W" letter
shape with 3 turning points, defining 4 branches that represent distinct galaxy
populations. This behavior is controlled mainly by 2 properties, namely u-r and
SFR. We further present the variations of several galaxy properties as a
function of arc length. Luminosity functions variate from steep Schechter fits
at low arc length, to double power law and ending in Log-normal fits at high
arc length. Galaxy clustering shows increasing autocorrelation power at large
scales as arc length increases. PCA analysis allowed to find peculiar galaxy
populations located apart from the main cloud of data points, such as small red
galaxies dominated by a disk, of relatively high stellar mass-to-light ratio
and surface mass density. The P-curve allows not only dimensionality reduction,
but also provides supporting evidence for relevant physical models and
scenarios in extragalactic astronomy: 1) Evidence for the hierarchical merging
scenario in the formation of a selected group of red massive galaxies. These
galaxies present a log-normal r-band luminosity function, which might arise
from multiplicative processes involved in this scenario. 2) Connection between
the onset of AGN activity and star formation quenching, which appears in green
galaxies when transitioning from blue to red populations. (Full abstract in
downloadable version)Comment: Full abstract in downloadable versio
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