1,669 research outputs found
Velocity bias in a LCDM model
We use N-body simulations to study the velocity bias of dark matter halos,
the difference in the velocity fields of dark matter and halos, in a flat low-
density LCDM model. The high force, 2kpc/h, and mass, 10^9Msun/h, resolution
allows dark matter halos to survive in very dense environments of groups and
clusters making it possible to use halos as galaxy tracers. We find that the
velocity bias pvb measured as a ratio of pairwise velocities of the halos to
that of the dark matter evolves with time and depends on scale. At high
redshifts (z ~5) halos move generally faster than the dark matter almost on all
scales: pvb(r)~1.2, r>0.5Mpc/h. At later moments the bias decreases and gets
below unity on scales less than r=5Mpc/h: pvb(r)~(0.6-0.8) at z=0. We find that
the evolution of the pairwise velocity bias follows and probably is defined by
the spatial antibias of the dark matter halos at small scales. One-point
velocity bias b_v, defined as the ratio of the rms velocities of halos and dark
matter, provides a more direct measure of the difference in velocities because
it is less sensitive to the spatial bias. We analyze b_v in clusters of
galaxies and find that halos are ``hotter'' than the dark matter: b_v=(1.2-1.3)
for r=(0.2-0.8)r_vir, where r_vir is the virial radius. At larger radii, b_v
decreases and approaches unity at r=(1-2)r_vir. We argue that dynamical
friction may be responsible for this small positive velocity bias b_v>1 found
in the central parts of clusters. We do not find significant difference in the
velocity anisotropy of halos and the dark matter. The dark matter the velocity
anisotropy can be approximated as beta(x)=0.15 +2x/(x^2+4), where x is measured
in units of the virial radius.Comment: 13 pages, Latex, AASTeXv5 and natbi
GALAXY DYNAMICS IN CLUSTERS
We use high resolution simulations to study the formation and distribution of
galaxies within a cluster which forms hierarchically. We follow both dark
matter and baryonic gas which is subject to thermal pressure, shocks and
radiative cooling. Galaxy formation is identified with the dissipative collapse
of the gas into cold, compact knots. We examine two extreme representations of
galaxies during subsequent cluster evolution --- one purely gaseous and the
other purely stellar. The results are quite sensitive to this choice.
Gas-galaxies merge efficiently with a dominant central object while
star-galaxies merge less frequently. Thus, simulations in which galaxies remain
gaseous appear to suffer an ``overmerging'' problem, but this problem is much
less severe if the gas is allowed to turn into stars. We compare the kinematics
of the galaxy population in these two representations to that of dark halos and
of the underlying dark matter distribution. Galaxies in the stellar
representation are positively biased (\ie over-represented in the cluster) both
by number and by mass fraction. Both representations predict the galaxies to be
more centrally concentrated than the dark matter, whereas the dark halo
population is more extended. A modest velocity bias also exists in both
representations, with the largest effect, , found for the more massive star-galaxies. Phase diagrams show that the
galaxy population has a substantial net inflow in the gas representation, while
in the stellar case it is roughly in hydrostatic equilibrium. Virial mass
estimators can underestimate the true cluster mass by up to a factor of 5. The
discrepancy is largest if only the most massive galaxies are used, reflecting
significant mass segregation.Comment: 30 pages, self-unpacking (via uufiles) postscript file without
figures. Eighteen figures (and slick color version of figure 3) and entire
paper available at ftp://oahu.physics.lsa.umich.edu/groups/astro/fews Total
size of paper with figures is ~9.0 Mb uncompressed. Submitted to Ap.J
Gravitational Lensing as a Probe of Quintessence
A large number of cosmological studies now suggest that roughly two-thirds of
the critical energy density of the Universe exists in a component with negative
pressure. If the equation of state of such an energy component varies with
time, it should in principle be possible to identify such a variation using
cosmological probes over a wide range in redshift. Proper detection of any time
variation, however, requires cosmological probes beyond the currently studied
range in redshift of 0.1 to 1. We extend our analysis to gravitational
lensing statistics at high redshift and suggest that a reliable sample of
lensed sources, out to a redshift of 5, can be used to constrain the
variation of the equation of state, provided that both the redshift
distribution of lensed sources and the selection function involved with the
lensed source discovery process are known. An exciting opportunity to catalog
an adequate sample of lensed sources (quasars) to probe quintessence is now
available with the ongoing Sloan Digital Sky Survey. Writing , we study the expected accuracy to which the equation of state
today and its rate of change can simultaneously be
constrained. Such a determination can rule out some missing-energy candidates,
such as classes of quintessence models or a cosmological constant.Comment: Accepted for publication in ApJ Letters (4 pages, including 4
figures
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
Optimizing Observational Strategy for Future Fgas Constraints
The Planck cluster catalog is expected to contain of order a thousand galaxy
clusters, both newly discovered and previously known, detected through the
Sunyaev-Zeldovich effect over the redshift range 0 < z < 1. Follow-up X-ray
observations of a dynamically relaxed sub-sample of newly discovered Planck
clusters will improve constraints on the dark energy equation-of-state found
through measurement of the cluster gas mass fraction fgas. In view of follow-up
campaigns with XMM-Newton and Chandra, we determine the optimal redshift
distribution of a cluster sample to most tightly constrain the dark energy
equation of state. The distribution is non-trivial even for the standard w0-wa
parameterization. We then determine how much the combination of expected data
from the Planck satellite and fgas data will be able to constrain the dark
energy equation-of-state. Our analysis employs a Markov Chain Monte Carlo
method as well as a Fisher Matrix analysis. We find that these upcoming data
will be able to improve the figure-of-merit by at least a factor two.Comment: 11 pages, 8 figure
Group-cluster merging and the formation of starburst galaxies
A significant fraction of clusters of galaxies are observed to have
substructure, which implies that merging between clusters and subclusters is a
rather common physical process of cluster formation.
It still remains unclear how cluster merging affects the evolution of cluster
member galaxies.
We report the results of numerical simulations, which show the dynamical
evolution of a gas-rich late-type spiral in a merger between a small group of
galaxies and a cluster. The simulations demonstrate that time-dependent tidal
gravitational field of the merging excites non-axisymmetric structure of the
galaxy, subsequently drives efficient transfer of gas to the central region,
and finally triggers a secondary starburst.
This result provides not only a new mechanism of starbursts but also a close
physical relationship between the emergence of starburst galaxies and the
formation of substructure in clusters. We accordingly interpret post-starburst
galaxies located near substructure of the Coma cluster as one observational
example indicating the global tidal effects of group-cluster merging.
Our numerical results furthermore suggest a causal link between the observed
excess of blue galaxies in distant clusters and cluster virialization process
through hierarchical merging of subclusters.Comment: 5 pages 3 color figures, ApJL in pres
Unequal-mass galaxy mergers and the creation of cluster S0 galaxies
It is a longstanding and remarkable problem when and how red S0 galaxies were
formed in clusters of galaxies.
We here propose that the major mechanism for the S0 creation is galaxy
merging between two spirals with unequal mass.
Our numerical simulations demonstrate that galaxy merging exhausts a large
amount of interstellar medium of two gas-rich spirals owing to the moderately
enhanced star formation, and subsequently transforms the two into one gas-poor
S0 galaxy with structure and kinematics strikingly similar to the observed
ones.
This secondary S0 formation with enhanced star formation explains a smaller
fraction of S0 population recently observed in some distant clusters of
galaxies.
Unequal-mass galaxy mergers thus provide an evolutionary link between a
larger number of blue spirals observed in intermediate redshift clusters and
red S0s prevalent in the present-day ones.Comment: 14 pages 3 figures (ps file), ApJL in pres
Constraining q_0 with Cluster Gas Mass Fractions: A Feasibility Study
As the largest gravitationally bound objects in the universe, clusters of
galaxies may contain a fair sample of the baryonic mass fraction of the
universe. Since the gas mass fraction from the hot ICM is believed to be
constant in time, the value of the cosmological deceleration parameter
can be determined by comparing the calculated gas mass fraction in nearby and
distant clusters (Pen 1997). To test the potential of this method, we compare
the gas fractions derived for a sample of luminous (erg
s), nearby clusters with those calculated for eight luminous, distant
() clusters using ASCA and ROSAT observations. For consistency,
we evaluate the gas mass fraction at a fixed physical radius of 1
Mpc (assuming ). We find a best fit value of with -0.47 <
q_0 < 0.67 at 95% confidence. We also determine the gas fraction using the
method of Evrard, Metzler, & Navarro (1997) to find the total mass within
, the radius where the mean overdensity of matter is 500 times the
critical density. In simulations, this method reduces the scatter in the
determination of gravitational mass without biasing the mean. We find that it
also reduces the scatter in actual observations for nearby clusters, but not as
much as simulations suggest. Using this method, the best fit value is with -0.50 < q_0 < 0.64. The excellent agreement between these two
methods suggests that this may be a useful technique for determining . The
constraints on should improve as more distant clusters are studied and
precise temperature profiles are measured to large radii.Comment: 8 pages, 4 figures, uses emulateapj.sty, onecolfloat.st
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
COSMOS: A Hybrid N-Body/Hydrodynamics Code for Cosmological Problems
We describe a new hybrid N-body/hydrodynamical code based on the
particle-mesh (PM) method and the piecewise-parabolic method (PPM) for use in
solving problems related to the evolution of large-scale structure, galaxy
clusters, and individual galaxies. The code, named COSMOS, possesses several
new features which distinguish it from other PM-PPM codes. In particular, to
solve the Poisson equation we have written a new multigrid solver which can
determine the gravitational potential of isolated matter distributions and
which properly takes into account the finite-volume discretization required by
PPM. All components of the code are constructed to work with a nonuniform mesh,
preserving second-order spatial differences. The PPM code uses vacuum boundary
conditions for isolated problems, preventing inflows when appropriate. The PM
code uses a second-order variable-timestep time integration scheme. Radiative
cooling and cosmological expansion terms are included. COSMOS has been
implemented for parallel computers using the Parallel Virtual Machine (PVM)
library, and it features a modular design which simplifies the addition of new
physics and the configuration of the code for different types of problems. We
discuss the equations solved by COSMOS and describe the algorithms used, with
emphasis on these features. We also discuss the results of tests we have
performed to establish that COSMOS works and to determine its range of
validity.Comment: 43 pages, 14 figures, submitted to ApJS and revised according to
referee's comment
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