1,681 research outputs found
BOOMERanG Data Suggest a Purely Baryonic Universe
The amplitudes of peaks in the angular power spectrum of anisotropies in the
microwave background radiation depend on the mass content of the universe. The
second peak should be prominent when cold dark matter is dominant, but is
depressed when baryons dominate. Recent microwave background data are
consistent with a purely baryonic universe with Omega(matter) = Omega(baryon) ~
0.03 and Omega(Lambda) ~ 1.Comment: 10 pages AASTeX with 1 color postscript figure. Accepted for
publication in ApJ Letters. And yes, the prediction was in the literature
before the dat
Galaxy Cluster Mass Estimation from Stacked Spectroscopic Analysis
We use simulated galaxy surveys to study: i) how galaxy membership in
redMaPPer clusters maps to the underlying halo population, and ii) the accuracy
of a mean dynamical cluster mass, , derived from stacked
pairwise spectroscopy of clusters with richness . Using galaxy pairs patterned after the SDSS redMaPPer cluster sample study
of Rozo et al. (2015 RMIV), we show that the pairwise velocity PDF of
central--satellite pairs with in the simulation matches the form
seen in RMIV. Through joint membership matching, we deconstruct the main
Gaussian velocity component into its halo contributions, finding that the
top-ranked halo contributes of the stacked signal. The halo mass
scale inferred by applying the virial scaling of Evrard et al. (2008) to the
velocity normalization matches, to within a few percent, the log-mean halo mass
derived through galaxy membership matching. We apply this approach, along with
mis-centering and galaxy velocity bias corrections, to estimate the log-mean
matched halo mass at of SDSS redMaPPer clusters. Employing the velocity
bias constraints of Guo et al. (2015), we find with and .
Systematic uncertainty in the velocity bias of satellite galaxies
overwhelmingly dominates the error budget.Comment: 14 pages, 7 figure
The Ultimate Halo Mass in a LCDM Universe
In the far future of an accelerating LCDM cosmology, the cosmic web of
large-scale structure consists of a set of increasingly isolated halos in
dynamical equilibrium. We examine the approach of collisionless dark matter to
hydrostatic equilibrium using a large N-body simulation evolved to scale factor
a = 100, well beyond the vacuum--matter equality epoch, a_eq ~ 0.75, and 53/h
Gyr into the future for a concordance model universe (Omega_m ~ 0.3,
Omega_Lambda ~ 0.7). The radial phase-space structure of halos -- characterized
at a < a_eq by a pair of zero-velocity surfaces that bracket a dynamically
active accretion region -- simplifies at a > 10 a_eq when these surfaces merge
to create a single zero-velocity surface, clearly defining the halo outer
boundary, rhalo, and its enclosed mass, mhalo. This boundary approaches a fixed
physical size encompassing a mean interior density ~ 5 times the critical
density, similar to the turnaround value in a classical Einstein-deSitter
model. We relate mhalo to other scales currently used to define halo mass
(m200, mvir, m180b) and find that m200 is approximately half of the total
asymptotic cluster mass, while m180b follows the evolution of the inner zero
velocity surface for a < 2 but becomes much larger than the total bound mass
for a > 3. The radial density profile of all bound halo material is well fit by
a truncated Hernquist profile. An NFW profile provides a somewhat better fit
interior to r200 but is much too shallow in the range r200 < r < rhalo.Comment: 5 pages, 3 figures, submitted to MNRAS letter
Rhapsody-G simulations: galaxy clusters as baryonic closed boxes and the covariance between hot gas and galaxies
Within a sufficiently large cosmic volume, conservation of baryons implies a
simple `closed box' view in which the sum of the baryonic components must equal
a constant fraction of the total enclosed mass. We present evidence from
Rhapsody-G hydrodynamic simulations of massive galaxy clusters that the
closed-box expectation may hold to a surprising degree within the interior,
non-linear regions of haloes. At a fixed halo mass, we find a significant
anti-correlation between hot gas mass fraction and galaxy mass fraction (cold
gas + stars), with a rank correlation coefficient of -0.69 within .
Because of this anti-correlation, the total baryon mass serves as a low-scatter
proxy for total cluster mass. The fractional scatter of total baryon fraction
scales approximately as , while the scatter of
either gas mass or stellar mass is larger in magnitude and declines more slowly
with increasing radius. We discuss potential observational tests using cluster
samples selected by optical and hot gas properties; the simulations suggest
that joint selection on stellar and hot gas has potential to achieve 5% scatter
in total halo mass.Comment: 10 pages, 6 figures, 3 tables; replaced to match published versio
RHAPSODY-G simulations II - Baryonic growth and metal enrichment in massive galaxy clusters
We study the evolution of the stellar component and the metallicity of both
the intracluster medium and of stars in massive ( M) simulated galaxy clusters from the Rhapsody-G suite in
detail and compare them to observational results. The simulations were
performed with the AMR code RAMSES and include the effect of AGN feedback at
the sub-grid level. AGN feedback is required to produce realistic galaxy and
cluster properties and plays a role in mixing material in the central regions
and regulating star formation in the central galaxy. In both our low and high
resolution runs with fiducial stellar yields, we find that stellar and ICM
metallicities are a factor of two lower than in observations. We find that cool
core clusters exhibit steeper metallicity gradients than non-cool core
clusters, in qualitative agreement with observations. We verify that the ICM
metallicities measured in the simulation can be explained by a simple
"regulator" model in which the metallicity is set by a balance of stellar yield
and gas accretion. It is plausible that a combination of higher resolution and
higher metal yield in AMR simulation would allow the metallicity of simulated
clusters to match observed values; however this hypothesis needs to be tested
with future simulations. Comparison to recent literature highlights that
results concerning the metallicity of clusters and cluster galaxies might
depend sensitively on the scheme chosen to solve the hydrodynamics.Comment: 22 pages, 11 figures, 2 tables. Accepted for publication on MNRA
Is the Lambda CDM Model Consistent with Observations of Large-Scale Structure?
The claim that large-scale structure data independently prefers the Lambda
Cold Dark Matter model is a myth. However, an updated compilation of
large-scale structure observations cannot rule out Lambda CDM at 95%
confidence. We explore the possibility of improving the model by adding Hot
Dark Matter but the fit becomes worse; this allows us to set limits on the
neutrino mass.Comment: To appear in Proceedings of "Sources and Detection of Dark
Matter/Energy in the Universe", ed. D. B. Cline. 6 pages, including 2 color
figure
Off-Center Mergers of Clusters of Galaxies and Nonequipartition of Electrons and Ions in Intracluster Medium
We investigate the dynamical evolution of clusters of galaxies and their
observational consequences during off-center mergers, explicitly considering
the relaxation process between ions and electrons in intracluster medium by
N-body and hydrodynamical simulations. In the contracting phase a bow shock is
formed between the two subclusters. The observed temperature between two peaks
in this phase depends on the viewing angle even if the geometry of the system
seems to be very simple like head-on collisions. Around the most contracting
epoch, when we observe merging clusters nearly along the collision axis, they
look like spherical relaxed clusters with large temperature gradients. In the
expanding phase, spiral bow shocks occur. As in head-on mergers, the electron
temperature is significantly lower than the plasma mean one especially in the
post-shock regions in the expanding phase. When the systems have relatively
large angular momentum, double-peak structures in the X-ray images can survive
even after the most contracting epoch. Morphological features in both X-ray
images and electron temperature distribution characteristic to off-center
mergers are seriously affected by the viewing angle. When the clusters are
observed nearly along the collision axis, the distribution of galaxies'
line-of-sight (LOS) velocities is a good indicator of mergers. In the
contracting phase, an negative kurtosis and a large skewness are expected for
nearly equal mass collisions and rather different mass ones, respectively. To
obtain statistically significant results, about 1000 galaxies' LOS velocities
are required. For nearby clusters (), large redshift surveys such as
2dF will enable us to study merger dynamics.Comment: 21 pages, 7 figures. Accepted for publication in Ap
Clustering of dark matter halos on the light-cone: scale-, time- and mass-dependence of the halo biasing in the Hubble volume simulations
We develop a phenomenological model to predict the clustering of dark matter
halos on the light-cone by combining several existing theoretical models.
Assuming that the velocity field of halos on large scales is approximated by
linear theory, we propose an empirical prescription of a scale-, mass-, and
time-dependence of halo biasing. We test our model against the Hubble Volume
-body simulation and examine its validity and limitations. We find a good
agreement in two-point correlation functions of dark matter halos between the
phenomenological model predictions and measurements from the simulation for
Mpc both in the real and redshift spaces. Although calibrated on the
mass scale of groups and clusters and for redshifts up to , the model
is quite general and can be applied to a wider range of astrophysical objects,
such as galaxies and quasars, if the relation between dark halos and visible
objects is specified.Comment: 5 pages, 2 figures, ApJL accepted. New references adde
Getting the Measure of the Flatness Problem
The problem of estimating cosmological parameters such as from noisy
or incomplete data is an example of an inverse problem and, as such, generally
requires a probablistic approach. We adopt the Bayesian interpretation of
probability for such problems and stress the connection between probability and
information which this approach makes explicit.
This connection is important even when information is ``minimal'' or, in
other words, when we need to argue from a state of maximum ignorance. We use
the transformation group method of Jaynes to assign minimally--informative
prior probability measure for cosmological parameters in the simple example of
a dust Friedman model, showing that the usual statements of the cosmological
flatness problem are based on an inappropriate choice of prior. We further
demonstrate that, in the framework of a classical cosmological model, there is
no flatness problem.Comment: 11 pages, submitted to Classical and Quantum Gravity, Tex source
file, no figur
Accuracy of Mesh Based Cosmological Hydrocodes: Tests and Corrections
We perform a variety of tests to determine the numerical resolution of the
cosmological TVD eulerian code developed by Ryu et al (1993). Tests include
512^3 and 256^3 simulations of a Pk=k^{-1} spectrum to check for
self-similarity and comparison of results with those from higher resolution SPH
and grid-based calculations (Frenk et al 1998). We conclude that in regions
where density gradients are not produced by shocks the code degrades resolution
with a Gaussian smoothing (radius) length of 1.7 cells. At shock caused
gradients (for which the code was designed) the smoothing length is 1.1 cells.
Finally, for \beta model fit clusters, we can approximately correct numerical
resolution by the transformation R^2_{core}\to R^2_{core}-(C\Delta l)^2, where
\Delta l is the cell size and C=1.1-1.7. When we use these corrections on our
previously published computations for the SCDM and \Lambda CDM models we find
luminosity weighted, zero redshift, X-ray cluster core radii of (210\pm 86,
280\pm 67)h^{-1}kpc, respectively, which are marginally consistent with
observed (Jones & Forman 1992) values of 50-200h^{-1}kpc. Using the corrected
core radii, the COBE normalized SCDM model predicts the number of bright
L_x>10^{43}erg/s clusters too high by a factor of \sim 20 and the \Lambda CDM
model is consistent with observations.Comment: ApJ in press (1999
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