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

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

Ram Pressure Stripping of Spiral Galaxies in Clusters

We use 3-dimensional SPH/N-BODY simulations to study ram pressure stripping of gas from spiral galaxies orbiting in clusters. We find that the analytic expectation of Gunn & Gott (1972) relating the gravitational restoring force provided by the disk to the ram pressure force, provides a good approximation to the radius that gas will be stripped from a galaxy. However, at small radii it is also important to consider the potential provided by the bulge component. A spiral galaxy passing through the core of a rich cluster such as Coma, will have its gaseous disk truncated to $\sim 4$ kpc, thus losing $\sim 80$ of its diffuse gas mass. The timescale for this to occur is a fraction of a crossing time $\sim 10^7$ years. Galaxies orbiting within poorer clusters, or inclined to the direction of motion through the intra-cluster medium will lose significantly less gas. We conclude that ram-pressure alone is insufficient to account for the rapid and widespread truncation of star-formation observed in cluster galaxies, or the morphological transformation of Sab's to S0's that is necessary to explain the Butcher-Oemler effect.Comment: 8 pages, 7 figures, to be published in MNRAS. Levels added/corrected on figures 3, 4 and

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

A Bayesian Classifier for Photometric Redshifts: Identification of high redshift clusters

Photometric redshift classifiers provide a means of estimating galaxy redshifts from observations using a small number of broad-band filters. However, the accuracy with which redshifts can be determined is sensitive to the star formation history of the galaxy, for example the effects of age, metallicity and on-going star formation. We present a photometric classifier that explicitly takes into account the degeneracies implied by these variations, based on the flexible stellar population synthesis code of Kodama & Arimoto. The situation is encouraging since many of the variations in stellar populations introduce colour changes that are degenerate. We use a Bayesian inversion scheme to estimate the likely range of redshifts compatible with the observed colours. When applied to existing multi-band photometry for Abell 370, most of the cluster members are correctly recovered with little field contamination. The inverter is focussed on the recovery of a wide variety of galaxy populations in distant (z~1) clusters from broad band colours covering the 4000 angstrom break. It is found that this can be achieved with impressive accuracy ($|\Delta z| < 0.1$), allowing detailed investigation into the evolution of cluster galaxies with little selection bias.Comment: 18 pages, including 15 figures, Accepted for publication in MNRA