356 research outputs found
The evidence for unusual gravity from the large-scale structure of the Universe
Under the assumption that General Relativity (GR) correctly describes the
phenomenology of our Universe, astronomical observations provide compelling
evidence that (1) the dynamics of cosmic structure is dominated by dark matter
(DM), an exotic matter mostly made of hypothetical elementary particles, and
(2) the expansion of the Universe is currently accelerating because of the
presence of a positive cosmological constant Lambda. The DM particles have not
yet been detected and there is no theoretical justification for the tiny
positive Lambda implied by observations. Therefore, over the last decade, the
search for extended or alternative theories of gravity has flourished.Comment: Invited review to appear in the Proceedings of the 1st AFI symposium
"From the Vacuum to the Universe", Innsbruck, Austria, October 2007, to be
published by the Innsbruck University Press, ed. by S.D. Bass, F. Schallhart
and B. Tasse
Wide field imaging of distant clusters
Wide field imaging is key to understanding the build-up of distant clusters
and their galaxy population. By focusing on the so far unexplored outskirts of
clusters, where infalling galaxies first hit the cluster potential and the hot
intracluster medium, we can help separate cosmological field galaxy evolution
from that driven by environment. I present a selection of recent advancements
in this area, with particular emphasis on Hubble Space Telescope wide field
imaging, for its superior capability to deliver galaxy morphologies and precise
shear maps of distant clusters.Comment: Invited contribution. To appear in "Outskirts of galaxy clusters:
intense life in the suburbs", A. Diaferio et al. eds. 7 pages, 5 figures.
Refereed versio
Topology and Geometry of the CfA2 Redshift Survey
We analyse the redshift space topology and geometry of the nearby Universe by
computing the Minkowski functionals of the Updated Zwicky Catalogue (UZC). The
UZC contains the redshifts of almost 20,000 galaxies, is 96% complete to the
limiting magnitude m_Zw=15.5 and includes the Center for Astrophysics (CfA)
Redshift Survey (CfA2). From the UZC we can extract volume limited samples
reaching a depth of 70 hMpc before sparse sampling dominates. We quantify the
shape of the large-scale galaxy distribution by deriving measures of planarity
and filamentarity from the Minkowski functionals. The nearby Universe shows a
large degree of planarity and a small degree of filamentarity. This quantifies
the sheet-like structure of the Great Wall which dominates the northern region
(CfA2N) of the UZC. We compare these results with redshift space mock
catalogues constructed from high resolution N-body simulations of two Cold Dark
Matter models with either a decaying massive neutrino (tauCDM) or a non-zero
cosmological constant (LambdaCDM). We use semi-analytic modelling to form and
evolve galaxies in these dark matter-only simulations. We are thus able, for
the first time, to compile redshift space mock catalogues which contain
galaxies, along with their observable properties, rather than dark matter
particles alone. In both models the large scale galaxy distribution is less
coherent than the observed distribution, especially with regard to the large
degree of planarity of the real survey. However, given the small volume of the
region studied, this disagreement can still be a result of cosmic variance.Comment: 14 pages including 10 figures. Accepted for publication in Monthly
Notice
Conformal gravity: light deflection revisited and the galactic rotation curve failure
We show how Conformal Gravity (CG) has to satisfy a fine-tuning condition to
describe the rotation curves of disk galaxies without the aid of dark matter.
Interpreting CG as a gauge natural theory yields conservation laws and their
associated superpotentials without ambiguities. We consider the light
deflection of a point-like lens and impose that the two Schwarzschild-like
metrics with and without the lens are identical at infinite distances from the
lens. The energy conservation law implies that the parameter in the
linear term of the metric has to vanish, otherwise the two metrics are
physically inaccessible from each other. This linear term is responsible to
mimic the role of dark matter in disk galaxies and gravitational lensing
systems. Our analysis shows that removing the need of dark matter with CG thus
relies on a fine-tuning condition on . We also illustrate why the
results of previous investigations of gravitational lensing in CG largely
disagree. These discrepancies derive from the erroneous use of the deflection
angle definition adopted in General Relativity, where the vacuum solution is
asymptotically flat, unlike CG. In addition, the lens mass is identified with
various combinations of the metric parameters. However, these identifications
are arbitrary, because the mass is not a conformally invariant quantity, unlike
the conserved charge associated to the energy conservation law. Based on this
conservation law and by removing the fine-tuning condition on , i.e. by
setting , the energy difference between the metric with the
point-like lens and the metric without it defines a conformally invariant
quantity that can in principle be used for (1) a proper derivation of light
deflection in CG, and (2) the identification of the lens mass with a function
of the parameters and of the Schwarzschild-like metric.Comment: 16 pages, 1 figure. Revised version according to the referees
comments. The results reported in the original version remain unchange
Mass estimation in the outer regions of galaxy clusters
We present a technique for estimating the mass in the outskirts of galaxy
clusters where the usual assumption of dynamical equilibrium is not valid. The
method assumes that clusters form through hierarchical clustering and requires
only galaxy redshifts and positions on the sky. We apply the method to
dissipationless cosmological N-body simulations where galaxies form and evolve
according to semi-analytic modelling. The method recovers the actual cluster
mass profile within a factor of two to several megaparsecs from the cluster
centre. This error originates from projection effects, sparse sampling, and
contamination by foreground and background galaxies. In the absence of velocity
biases, this method can provide an estimate of the mass-to-light ratio on
scales ~1-10 Mpc/h where this quantity is still poorly known.Comment: 14 pages, 7 figures, MN LaTeX style, MNRAS, in pres
Deep spectroscopy in nearby galaxy clusters: III Orbital structure of galaxies in Abell 85
Galaxies in clusters are strongly affected by their environment. They evolve
according to several physical mechanisms that are active in clusters. Their
efficiency can strongly depend on the orbital configuration of the galaxies.
Our aim is to analyse the orbits of the galaxies in the cluster Abell 85, based
on the study of the galaxy velocity anisotropy parameter. We have solved the
Jeans equation under the assumption that the galaxies in A85 are collisionless
objects, within the spherically symmetric gravitational potential of the
virialized cluster. The mass of the cluster was estimated with X-ray and
caustic analyses. We find that the anisotropy profile of the full galaxy
population in A85 is an increasing monotonic function of the distance from the
cluster centre: on average, galaxies in the central region (r/r200 < 0.3) are
on isotropic orbits, while galaxies in the outer regions are on radial orbits.
We also find that the orbital properties of the galaxies strongly depend on
their stellar colour. In particular, blue galaxies are on less radial orbits
than red galaxies. The different families of cluster galaxies considered here
have the pseudo phase-space density profiles Q(r) and Qr(r) consistent with the
profiles expected in virialized dark matter halos in -body simulations. This
result suggests that the galaxies in A85 have reached dynamical equilibrium
within the cluster potential. Our results indicate that the origin of the blue
and red colour of the different galaxy populations is the different orbital
shape rather than the accretion time.Comment: 15 pages, 15 figures. Accepted for publication at MNRA
Thermodynamical properties of the ICM from hydrodynamical simulations
Modern hydrodynamical simulations offer nowadays a powerful means to trace
the evolution of the X-ray properties of the intra-cluster medium (ICM) during
the cosmological history of the hierarchical build up of galaxy clusters. In
this paper we review the current status of these simulations and how their
predictions fare in reproducing the most recent X-ray observations of clusters.
After briefly discussing the shortcomings of the self-similar model, based on
assuming that gravity only drives the evolution of the ICM, we discuss how the
processes of gas cooling and non-gravitational heating are expected to bring
model predictions into better agreement with observational data. We then
present results from the hydrodynamical simulations, performed by different
groups, and how they compare with observational data. As terms of comparison,
we use X-ray scaling relations between mass, luminosity, temperature and
pressure, as well as the profiles of temperature and entropy. The results of
this comparison can be summarised as follows: (a) simulations, which include
gas cooling, star formation and supernova feedback, are generally successful in
reproducing the X-ray properties of the ICM outside the core regions; (b)
simulations generally fail in reproducing the observed ``cool core'' structure,
in that they have serious difficulties in regulating overcooling, thereby
producing steep negative central temperature profiles. This discrepancy calls
for the need of introducing other physical processes, such as energy feedback
from active galactic nuclei, which should compensate the radiative losses of
the gas with high density, low entropy and short cooling time, which is
observed to reside in the innermost regions of galaxy clusters.Comment: 26 pages, 12 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 13; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
The Stellar Populations of Low-redshift Clusters
We present some preliminary results from an on-going study of the evolution
of stellar populations in rich clusters of galaxies. This sample contains core
line-strength measurements from 183 galaxies with b_J <= 19.5 from four
clusters with ~0.04. Using predictions from stellar population models to
compare with our measured line strengths we can derive relative
luminosity-weighted mean ages and metallicities for the stellar populations in
each of our clusters. We also investigate the Mgb'-sigma and Hbeta_G'-sigma
scaling relations. We find that, consistent with previous results, Mgb' is
correlated with sigma, the likely explanation being that larger galaxies are
better at retaining their heavier elements due to their larger potentials.
Hbeta', on the other hand, we find to be anti-correlated with sigma. This
result implies that the stellar populations in larger galaxies are older than
in smaller galaxies.Comment: 3 pages, 2 figures, to appear in the Proceedings of IAU Colloquium
195: "Outskirts of Galaxy Clusters: intense life in the suburbs", Torino
Italy, March 12-16 200
Galaxy Cluster Formation from the Large-scale Structure: A Case Study of the Abell 2125 Complex at z=0.247
The structure of the universe is believed to have formed by clustering
hierarchically from small to large scales. Much of this evolution occurs very
slowly but at a few special times more, rapid, violent activity may occur as
major subunits collide at high velocities. The Abell 2125 complex (z=0.247)
appears to be undergoing such an event as shown by modeling of the optical
velocity field and by the detection with the VLA of an unusually large number
of associated radio active galaxies. We present an 80 ksec Chandra imaging of
Abell 2125, together with extensive complementary multi-wavelength data. We
show direct evidence for galaxy transformation and destruction during the
cluster formation. The Chandra data unambiguously separate the X-ray
contributions from discrete sources and large-scale diffuse gas in the Abell
2125 complex, which consists of various merging clusters/groups of galaxies and
low-surface brightness emission. This enables us to study processes affecting
galaxy evolution during this special time from scales of Mpc down to a few kpc.
The overall level of activity plus the special time for the cluster-cluster
merger suggests that an important phase of galaxy evolution can take place
during such events.Comment: 4 pages plus 4 figures. To appear in proceedings of IAU CColloquium
No. 195: Outskirts of Galaxy Clusters: Intense Life in the Suburbs. A high
resolution version may be found at
http://www.astro.umass.edu/~wqd/papers/iau195.ps.g
Clusters and Groups of Galaxies in the Simulated Local Universe
We compare the properties of galaxy groups extracted from the Updated Zwicky
Catalogue (UZC) with those of groups extracted from N-body simulations of the
local Universe, in a LambdaCDM and a tauCDM cosmology. In the simulations, the
initial conditions of the dark matter density field are set to reproduce the
present time distribution of the galaxies within 80 Mpc/h from the Milky Way.
These initial conditions minimize the uncertainty originated by cosmic
variance, which has affected previous analyses of this small volume of the
Universe. The simulations also model the evolution of the photometric
properties of the galaxy population with semi-analytic prescriptions. The
models yield a galaxy luminosity function sensibly different from that of the
UZC and are unable to reproduce the distribution of groups and their luminosity
content. The discrepancy between the model and the UZC reduces substantially,
if we redistribute the luminosity among the galaxies in the simulation
according to the UZC luminosity function while preserving the galaxy luminosity
rank. The modified LambdaCDM model provides the best match to the UZC: the
abundances of groups by harmonic radius, velocity dispersion, mass and
luminosity are consistent with observations. We find that this model also
reproduces the halo occupation number of groups and clusters. However, the
large-scale distribution of groups is marginally consistent with the UZC and
the redshift-space correlation function of galaxies on scales larger than 6
Mpc/h is still more than 3-sigma smaller than observed. We conclude that
reproducing the properties of the observed groups certainly requires a more
sophisticated treatment of galaxy formation, and possibly an improvement of the
dark matter model.Comment: 20 pages, 18 figures, accepted by MNRAS. Minor revisions according to
referee's comments. Conclusions unchange
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