578 research outputs found
Faint blue counts from formation of dwarf galaxies at z approximately equals 1
The nature of faint blue objects (FBO's) has been a source of much speculation since their detection in deep CCD images of the sky. Their high surface density argues against them being progenitors of present-day bright galaxies and since they are only weakly clustered on small scales, they cannot be entities that merged together to form present-day galaxies. Babul & Rees (1992) have suggested that the observed faint blue counts may be due to dwarf elliptical galaxies undergoing their initial starburst at z is approximately equal to 1. In generic hierarchical clustering scenarios, however, dwarf galaxy halos (M is approximately 10(exp 9) solar mass) are expected to form at an earlier epoch; for example, typical 10(exp 9) solar mass halos will virialize at z is approximately equal to 2.3 if the power-spectrum for the density fluctuations is that of the standard b = 2 cold dark matter (CDM) model. Under 'ordinary conditions' the gas would rapidly cool, collect in the cores, and undergo star-formation. Conditions at high redshifts are far from 'ordinary'. The intense UV background will prevent the gas in the dwarf halos from cooling, the halos being released from their suspended state only when the UV flux has diminished sufficiently
The Observable Thermal and Kinetic Sunyaev-Zel'dovich Effect in Merging Galaxy Clusters
The advent of high-resolution imaging of galaxy clusters using the
Sunyaev-Zel'dovich Effect (SZE) provides a unique probe of the astrophysics of
the intracluster medium (ICM) out to high redshifts. To investigate the effects
of cluster mergers on resolved SZE images, we present a high-resolution
cosmological simulation of a 1.5E15 M_sun adiabatic cluster using the TreeSPH
code ChaNGa. This massive cluster undergoes a 10:3:1 ratio triple merger
accompanied by a dramatic rise in its integrated Compton-Y, peaking at z =
0.05. By modeling the thermal SZE (tSZ) and kinetic SZE (kSZ) spectral
distortions of the Cosmic Microwave Background (CMB) at this redshift with
relativistic corrections, we produce various mock images of the cluster at
frequencies and resolutions achievable with current high-resolution SZE
instruments. The two gravitationally-bound merging subclusters account for 10%
and 1% of the main cluster's integrated Compton-Y, and have extended merger
shock features in the background ICM visible in our mock images. We show that
along certain projections and at specific frequencies, the kSZ CMB intensity
distortion can dominate over the tSZ due to the large line of sight velocities
of the subcluster gas and the unique frequency-dependence of these effects. We
estimate that a one-velocity assumption in estimation of line of sight
velocities of the merging subclusters from the kSZ induces a bias of ~10%. This
velocity bias is small relative to other sources of uncertainty in
observations, partially due to helpful bulk motions in the background ICM
induced by the merger. Our results show that high-resolution SZE observations,
which have recently detected strong kSZ signals in subclusters of merging
systems, can robustly probe the dynamical as well as the thermal state of the
ICM.Comment: MNRAS, accepted. 13 pages, 9 figure
The time-evolution of bias
We study the evolution of the bias factor b and the mass-galaxy correlation
coefficient r in a simple analytic model for galaxy formation and the
gravitational growth of clustering. The model shows that b and r can be
strongly time-dependent, but tend to approach unity even if galaxy formation
never ends as the gravitational growth of clustering debiases the older
galaxies. The presence of random fluctuations in the sites of galaxy formation
relative to the mass distribution can cause large and rapidly falling bias
values at high redshift.Comment: 4 pages, with 2 figures included. Typos corrected to match published
ApJL version. Color figure and links at http://www.sns.ias.edu/~max/bias.html
or from [email protected]
Joint Analysis of Cluster Observations: II. Chandra/XMM-Newton X-ray and Weak Lensing Scaling Relations for a Sample of 50 Rich Clusters of Galaxies
We present a study of multiwavelength X-ray and weak lensing scaling
relations for a sample of 50 clusters of galaxies. Our analysis combines
Chandra and XMM-Newton data using an energy-dependent cross-calibration. After
considering a number of scaling relations, we find that gas mass is the most
robust estimator of weak lensing mass, yielding 15 +/- 6% intrinsic scatter at
r500 (the pseudo-pressure YX has a consistent scatter of 22%+/-5%). The scatter
does not change when measured within a fixed physical radius of 1 Mpc. Clusters
with small BCG to X-ray peak offsets constitute a very regular population whose
members have the same gas mass fractions and whose even smaller <10% deviations
from regularity can be ascribed to line of sight geometrical effects alone.
Cool-core clusters, while a somewhat different population, also show the same
(<10%) scatter in the gas mass-lensing mass relation. There is a good
correlation and a hint of bimodality in the plane defined by BCG offset and
central entropy (or central cooling time). The pseudo-pressure YX does not
discriminate between the more relaxed and less relaxed populations, making it
perhaps the more even-handed mass proxy for surveys. Overall, hydrostatic
masses underestimate weak lensing masses by 10% on the average at r500; but
cool-core clusters are consistent with no bias, while non-cool-core clusters
have a large and constant 15-20% bias between r2500 and r500, in agreement with
N-body simulations incorporating unthermalized gas. For non-cool-core clusters,
the bias correlates well with BCG ellipticity. We also examine centroid shift
variance and and power ratios to quantify substructure; these quantities do not
correlate with residuals in the scaling relations. Individual clusters have for
the most part forgotten the source of their departures from self-similarity.Comment: Corrects an error in the X-ray luminosities (erratum
submitted)---none of the other results are affected. Go to
http://sfstar.sfsu.edu/jaco for an electronic fitter and updated quick data
download link
A Low Upper Limit to the Lyman Continuum Emission of two galaxies at z 3
Long exposure, long-slit spectra have been obtained in the UV/optical bands
for two galaxies at z=2.96 and z=3.32 to investigate the fraction of ionizing
UV photons escaping from high redshifts galaxies. The two targets are among the
brightest galaxies discovered by Steidel and collaborators and they have
different properties in terms of Lyman-alpha emission and dust reddening. No
significant Lyman continuum emission has been detected. The noise level in the
spectra implies an upper limit of f_{rel,esc}\equiv 3 f(900)/f(1500)< 16% for
the relative escape fraction of ionizing photons, after correction for
absorption by the intervening intergalactic medium. This upper limit is 4 times
lower than the previous detection derived from a composite spectrum of 29 Lyman
break galaxies at z 3.4. If this value is typical of the escape fraction of the
z 3 galaxies, and is added to the expected contribution of the QSO population,
the derived UV background is in good agreement with the one derived by the
proximity effect.Comment: 16 pages, 2 figures, ApJ Letters in pres
Cosmic Voids and Galaxy Bias in the Halo Occupation Framework
(Abridged) We investigate the power of void statistics to constrain galaxy
bias and the amplitude of dark matter fluctuations. We use the halo occupation
distribution (HOD) framework to describe the relation between galaxies and dark
matter. After choosing HOD parameters that reproduce the mean space density
n_gal and projected correlation function w_p measured for galaxy samples with
M_r<-19 and M_r<-21 from the Sloan Digital Sky Survey (SDSS), we predict the
void probability function (VPF) and underdensity probability function (UPF) of
these samples by populating the halos of a large, high-resolution N-body
simulation. If we make the conventional assumption that the HOD is independent
of large scale environment at fixed halo mass, then models constrained to match
n_gal and w_p predict nearly identical void statistics, independent of the
scatter between halo mass and central galaxy luminosity or uncertainties in HOD
parameters. Models with sigma_8=0.7 and sigma_8=0.9 also predict very similar
void statistics. However, the VPF and UPF are sensitive to environmental
variations of the HOD in a regime where these variations have little impact on
w_p. For example, doubling the minimum host halo mass in regions with large
scale (5 Mpc/h) density contrast delta<-0.65 has a readily detectable impact on
void probabilities of M_r<-19 galaxies, and a similar change for delta<-0.2
alters the void probabilities of M_r<-21 galaxies at a detectable level. The
VPF and UPF provide complementary information about the onset and magnitude of
density- dependence in the HOD. By detecting or ruling out HOD changes in low
density regions, void statistics can reduce systematic uncertainties in the
cosmological constraints derived from HOD modeling, and, more importantly,
reveal connections between halo formation history and galaxy properties.Comment: emulateapj, 16 pages, 13 figure
Semi-analytical dark matter halos and the Jeans equation
Although N-body studies of dark matter halos show that the density profiles,
rho(r), are not simple power-laws, the quantity rho/sigma^3, where sigma(r) is
the velocity dispersion, is in fact a featureless power-law over ~3 decades in
radius. In the first part of the paper we demonstrate, using the semi-analytic
Extended Secondary Infall Model (ESIM), that the nearly scale-free nature of
rho/sigma^3 is a robust feature of virialized halos in equilibrium. By
examining the processes in common between numerical N-body and semi-analytic
approaches, we argue that the scale-free nature of rho/sigma^3 cannot be the
result of hierarchical merging, rather it must be an outcome of violent
relaxation. The empirical results of the first part of the paper motivate the
analytical work of the second part of the paper, where we use rho/sigma^3
proportional to r^{-alpha} as an additional constraint in the isotropic Jeans
equation of hydrostatic equilibrium. Our analysis shows that the constrained
Jeans equation has different types of solutions, and in particular, it admits a
unique ``periodic'' solution with alpha=1.9444. We derive the analytic
expression for this density profile, which asymptotes to inner and outer
profiles of rho ~ r^{-0.78}, and rho ~ r^{-3.44}, respectively.Comment: 37 pg, 14 fig. Accepted to ApJ: added two figures and extended
discussion. Note that an earlier related paper (conference proceedings)
astro-ph/0412442 has a mistake in eq.(2.2); the correct version is eq.(5) of
the present submissio
Stochasticity of Bias and Nonlocality of Galaxy Formation: Linear Scales
If one wants to represent the galaxy number density at some point in terms of
only the mass density at the same point, there appears the stochasticity in
such a relation, which is referred to as ``stochastic bias''. The stochasticity
is there because the galaxy number density is not merely a local function of a
mass density field, but it is a nonlocal functional, instead. Thus, the
phenomenological stochasticity of the bias should be accounted for by nonlocal
features of galaxy formation processes. Based on mathematical arguments, we
show that there are simple relations between biasing and nonlocality on linear
scales of density fluctuations, and that the stochasticity in Fourier space
does not exist on linear scales under a certain condition, even if the galaxy
formation itself is a complex nonlinear and nonlocal precess. The stochasticity
in real space, however, arise from the scale-dependence of bias parameter, .
As examples, we derive the stochastic bias parameters of simple nonlocal models
of galaxy formation, i.e., the local Lagrangian bias models, the cooperative
model, and the peak model. We show that the stochasticity in real space is also
weak, except on the scales of nonlocality of the galaxy formation. Therefore,
we do not have to worry too much about the stochasticity on linear scales,
especially in Fourier space, even if we do not know the details of galaxy
formation process.Comment: 24 pages, latex, including 2 figures, ApJ, in pres
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