2,427 research outputs found
Dark Energy and the Hubble Age
I point out that an effective upper limit of approximately 20 Gyr (for a
Hubble constant of 72 km/s/Mpc) or alternatively on the -independent
quantity , exists on the age of the Universe, essentially
independent of the unknown equation of state of the dominant dark energy
component in the Universe. Unless astrophysical constraints on the age of the
Universe can convincingly reduce the upper limit to below this value no useful
lower limit on the equation of state parameter for this component can be
obtained. Direct dating by stars does not provide a useful constraint, but
model-dependent cosmological limits from supernovae and the CMB observations
may. For a constant value of , a bound Comment: 4 pages, submitted to Ap. J. Lett (analytic asymptotic upper bound
now added
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
Gravitational Lensing Limits on the Average Redshift of Submillimeter Sources
The submillimeter universe has now been explored with the Submillimeter
Common User Bolometer Array (SCUBA) camera on the James Clerk Maxwell
Telescope, and a claim has been made to the presence of a new population of
optically unidentified starforming galaxies at high redshifts (z \gtrsim 3).
Such a population dramatically alters current views on the star formation
history of the universe as well as galaxy formation and evolution. Recently,
new radio identifications of the Hubble Deep Field submm sources have led to
the suggestion that some of these sources are at low redshifts, however, submm
source redshift distribution is still not well determined. Here, we present an
upper limit to the average redshift by comparing the expected number of
gravitationally lensed submm sources due to foreground cluster potentials to
current observed statistics of such lensed sources. The upper limit depends on
the cosmological parameters, and at the 68% confidence level, < 3.1, 4.8,
5.2, or 8.0 for (Omega,Lambda) values of (0.3,0.7), (0.5,0.5), (0.3,0.0) or
(1.0,0.0) respectively. These upper limits are consistent with redshift
distribution for 850 micron sources implied by starformation history models
based on measured background radiation at far-infrared and submm wavelengths.Comment: Accepted for publication in ApJ Letters (4 pages, including 1 table
High redshift X-ray galaxy clusters. II. The L_X-T relationship revisited
In this paper we re-visit the observational relation between X-ray luminosity
and temperature for high-z galaxy clusters and compare it with the local L_X-T
and with theoretical models. To these ends we use a sample of 17 clusters
extracted from the Chandra archive supplemented with additional clusters from
the literature, either observed by Chandra or XMM-Newton, to form a final
sample of 39 high redshift (0.25 < z < 1.3) objects. Different statistical
approaches are adopted to analyze the L_X-T relation. The slope of the L_X-T
relation of high redshift clusters is steeper than expected from the
self-similar model predictions and steeper, even though still compatible within
the errors, than the local L_X-T slope. The distant cluster L_X-T relation
shows a significant evolution with respect to the local Universe: high-z
clusters are more luminous than the local ones by a factor ~2 at any given
temperature. The evolution with redshift of the L_X-T relation cannot be
described by a single power law nor by the evolution predicted by the
self-similar model. We find a strong evolution, similar or stronger than the
self-similar model, from z = 0 to z <0.3 followed by a much weaker, if any,
evolution at higher redshift. The weaker evolution is compatible with
non-gravitational models of structure formation. According to us a
statistically significant sample of nearby clusters (z < 0.25) should be
observed with the current available X-ray telescopes to completely exclude
observational effects due to different generation detectors and to understand
this novel result.Comment: 14 pages, 10 postscript figures. Accepted for publication in
Astronomy & Astrophysics. Corrected typo
Red Sequence Cluster Finding in the Millennium Simulation
We investigate halo mass selection properties of red-sequence cluster finders
using galaxy populations of the Millennium Simulation (MS). A clear red
sequence exists for MS galaxies in massive halos at redshifts z < 1, and we use
this knowledge to inform a cluster-finding algorithm applied to 500 Mpc/h
projections of the simulated volume. At low redshift (z=0.4), we find that 90%
of the clusters found have galaxy membership dominated by a single, real-space
halo, and that 10% are blended systems for which no single halo contributes a
majority of a cluster's membership. At z=1, the fraction of blends increases to
22%, as weaker redshift evolution in observed color extends the comoving length
probed by a fixed range of color. Other factors contributing to the increased
blending at high-z include broadening of the red sequence and confusion from a
larger number of intermediate mass halos hosting bright red galaxies of
magnitude similar to those in higher mass halos. Our method produces catalogs
of cluster candidates whose halo mass selection function, p(M|\Ngal,z), is
characterized by a bimodal log-normal model with a dominant component that
reproduces well the real-space distribution, and a redshift-dependent tail that
is broader and displaced by a factor ~2 lower in mass. We discuss implications
for X-ray properties of optically selected clusters and offer ideas for
improving both mock catalogs and cluster-finding in future surveys.Comment: final version to appear in MNRAS. Appendix added on purity and
completeness, small shift in red sequence due to correcting an error in
finding i
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
Self-similarity of clusters of galaxies and the L_X-T relation
In this paper based on ROSAT/PSPC data we investigate the emission measure
profiles of a sample of hot clusters of galaxies (kT>3.5keV) in order to
explain the differences between observed and theoretically predicted L_X-T
relation. Looking at the form of the emission measure profiles as well as their
normalizations we find clear indication that indeed the profiles have similar
shapes once scaled to the virial radius, however, the normalization of the
profiles shows a strong temperature dependence. We introduce a M_gas-T relation
with the dependence M_gas propto T^1.94. This relationship explains the
observed L_X-T relation and reduces the scatter in the scaled profiles by a
factor of 2 when compared to the classical scaling. We interpret this finding
as strong indication that the M_gas-T relation in clusters deviates from
classical scaling.Comment: 4 pages including 4 figures, accepted for publication in A&A Letter
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
European coastal state competitive marine research funding programmes: The MarinERA Reference Manual (2008): A description of European marine research funding programmes and implementation procedures
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
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