7,583 research outputs found
The Entropy-Driven X-ray Evolution of Galaxy Clusters
Observations of the evolution of the galaxy cluster X-ray luminosity function
suggest that the entropy of the intra-cluster medium plays a significant role
in determining the development of cluster X-ray properties. I present a
theoretical framework in which the evolution of the entropy of the central
intra-cluster gas is explicitly taken into account. The aim of this work is to
develop a theoretical context within which steadily improving measurements of
the X-ray luminosities and temperatures of distant galaxy clusters can be
interpreted. I discuss the possible range of entropy evolution parameters and
relate these to the physical processes heating and cooling the intra-cluster
medium. The practical application of this work is demonstrated by combining
currently available evolutionary constraints on the X-ray luminosity function
and the luminosity--temperature correlation to determine the best-fitting model
parameters.Comment: 9 pages Tex including 4 postscript figures. To be appear in MNRAS.
minor miss-quote correcte
The Ks-band Luminosity and Stellar Mass Functions of Galaxies in z~1 Clusters
We present the near-infrared (Ks-band) luminosity function of galaxies in two
z~1 cluster candidates, 3C336 and Q1335+28. A third cluster, 3C289, was
observed but found to be contaminated by a foreground system. Our wide field
imaging data reach to Ks=20.5 (5sigma), corresponding to ~M*+2.7 with respect
to the passive evolution. The near-infrared luminosity traces the stellar mass
of a galaxy due to its small sensitivity to the recent star formation history.
Thus the luminosity function can be transformed to the stellar mass function of
galaxies using the colours with only a small correction (factor<2) for
the effects of on-going star formation. The derived stellar mass function spans
a wide range in mass from ~3 x 10^{11}Msun down to ~6 x 10^{9}Msun (set by the
magnitude limit). The form of the mass function is very similar to lower
redshift counterparts such as that from 2MASS/LCRS clusters (Balogh et al.
2001) and the z=0.31 clusters (Barger et al. 1998). This indicates little
evolution of galaxy masses from z=1 to the present-day. Combined with colour
data that suggest star formation is completed early (z>>1) in the cluster core,
it seems that the galaxy formation processes (both star formation and mass
assembly) are strongly accerelated in dense environments and has been largely
completed by z=1. We investigate whether the epoch of mass assembly of massive
cluster galaxies is earlier than that predicted by the hierarchical galaxy
formation models. These models predict the increase of characteristic mass by
more than factor ~3 between z=1 and the present day. This seems incompatible
with our data.Comment: 12 pages, including 12 figures, uses mn.sty and epsf.sty. Accepted
for publication in MNRAS Main Journa
Galaxy bulges and their black holes: a requirement for the quenching of star formation
One of the central features of the last 8 to 10 billion years of cosmic
history has been the emergence of a well-populated red sequence of
non-star-forming galaxies. A number of models of galaxy formation and evolution
have been devised to attempt to explain this behavior. Most current models
require feedback from supermassive black holes (AGN feedback) to quench star
formation in galaxies in the centers of their dark matter halos (central
galaxies). Such models make the strong prediction that all quenched central
galaxies must have a large supermassive black hole (and, by association, a
prominent bulge component). I show using data from the Sloan Digital Sky Survey
that the observations are consistent with this prediction. Over 99.5% of red
sequence galaxies with stellar masses in excess of 10^{10} M_{\sun} have a
prominent bulge component (as defined by having a Sersic index n above 1.5).
Those very rare red sequence central galaxies with little or no bulge (n<1.5)
usually have detectable star formation or AGN activity; the fraction of truly
quenched bulgeless central galaxies is <0.1% of the total red sequence
population. I conclude that a bulge, and by implication a supermassive black
hole, is an absolute requirement for full quenching of star formation in
central galaxies. This is in agreement with the most basic prediction of the
AGN feedback paradigm.Comment: 6 pages, 4 color figures (figure 1 is of slightly degraded quality).
To appear in August 1 edition of the Astrophysical Journa
Galaxy Formation Spanning Cosmic History
Over the past several decades, galaxy formation theory has met with
significant successes. In order to test current theories thoroughly we require
predictions for as yet unprobed regimes. To this end, we describe a new
implementation of the Galform semi-analytic model of galaxy formation. Our
motivation is the success of the model described by Bower et al. in explaining
many aspects of galaxy formation. Despite this success, the Bower et al. model
fails to match some observational constraints and certain aspects of its
physical implementation are not as realistic as we would like. The model
described in this work includes substantially updated physics, taking into
account developments in our understanding over the past decade, and removes
certain limiting assumptions made by this (and most other) semi-analytic
models. This allows it to be exploited reliably in high-redshift and low mass
regimes. Furthermore, we have performed an exhaustive search of model parameter
space to find a particular set of model parameters which produce results in
good agreement with a wide range of observational data (luminosity functions,
galaxy sizes and dynamics, clustering, colours, metal content) over a wide
range of redshifts. This model represents a solid basis on which to perform
calculations of galaxy formation in as yet unprobed regimes.Comment: MNRAS accepted. Extended version (with additional figures and details
of implementation) is available at http://www.galform.or
Reconstructing the History of Star Formation in Rich Cluster Cores
We address the current crucial issues on the formation and evolution of
cluster galaxies: ie., connection between the Butcher-Oemler effect, assembly
of cluster galaxies, truncation of star formation, and the origin of S0
galaxies.
We construct the field corrected colour-magnitude (CM) diagrams for 7 CNOC
clusters (0.23<z<0.43) and Coma, and illustrate the evolution of the complete
cluster population down to the present-day based on the model in which star
formation is truncated when the galaxies infall from the surrounding field.
We show that the blue galaxies are incorporated into the present-day tight CM
relation as they fade and become redder after the truncation, which is possibly
responsible for producing faint S0's (>M*+1). Truncation of star formation is,
however, found to be relatively milder (with a time scale of 1 Gyr) than
suggested by the viorent processes such as ram-pressure stripping and/or
mergers/harassment.
The BO effect is after all found to be a combination of three effects,
namely, increasing field star formation activity, increasing galaxy infall
rate, and the truncatin of star formation after the accretion. Our approach
naturally leads to the history of galaxy assembly and `global' star foramtion
for `cluster' galaxies.Comment: 20 pages, 12 figures, Accepted for Publication in MNRAS (first
submitted on 9th Feb 2000
The Colour-Magnitude Relation as a Constraint on the Formation of Rich Cluster Galaxies
In this paper, we examine the role that the colour-magnitude relation (CMR)
can play in constraining the formation history of rich cluster galaxies.
Firstly, we consider the colour evolution of galaxies after star formation
ceases. We show that the scatter of the CMR places a strong constraint on the
spread in age of the bulk of the stellar population. However, although the bulk
of stars must be formed in a short period, continuing formation of stars in a
fraction of the galaxies is not so strongly constrained.
We examine a model in which star formation occurs over an extended period of
time in most galaxies. An extension of this type of star formation history
allows us to reconcile the small present-day scatter of the CMR with the
observed blue galaxy fractions of intermediate redshift galaxy clusters.
Secondly, the CMR can also be used to constrain the degree of merging between
pre-existing stellar systems. This test relies on the slope of the CMR. We show
that random mergers between galaxies very rapidly remove any well-defined CMR.
However, we prefer to examine the merger process using a self-consistent merger
tree. In such a model there are two effects: massive galaxies preferentially
merge with systems of similar mass; and the rate of mass growth is considerably
smaller than for the random merger case. As a result of both of these effects,
the CMR persists through a larger number of merger steps. The passive evolution
of galaxy colours and their averaging in dissipationless mergers provide
opposing constraints on the formation of cluster galaxies in a hierarchical
model; but at the level of current constraints, a compromise solution appears
possible.Comment: 17 pages, including 11 figures. Accepted for publication in MNRA
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