Are disappearing dwarfs just lying low ?

Recent redshift surveys have shown that the excess galaxies seen in faint galaxy number counts (above those expected given the local galaxy luminosity function) are not evolved giants at high redshifts, but low to moderate luminosity objects at more modest redshifts. This has led to the suggestion that there was once an additional population of dwarf galaxies which has since disappeared, ie. there is non-conservation of galaxy number. Here we investigate the possibility that these disappearing dwarfs have actually evolved to become the population of very low surface brightness galaxies which is now being detected in nearby clusters.Comment: 12 pages, 7 figures. Figures available from http://www.phys.unsw.edu.au/~spd/bib.htm

Dynamics of Three Agent Games

We study the dynamics and resulting score distribution of three-agent games where after each competition a single agent wins and scores a point. A single competition is described by a triplet of numbers $p$, $t$ and $q$ denoting the probabilities that the team with the highest, middle or lowest accumulated score wins. We study the full family of solutions in the regime, where the number of agents and competitions is large, which can be regarded as a hydrodynamic limit. Depending on the parameter values $(p,q,t)$, we find six qualitatively different asymptotic score distributions and we also provide a qualitative understanding of these results. We checked our analytical results against numerical simulations of the microscopic model and find these to be in excellent agreement. The three agent game can be regarded as a social model where a player can be favored or disfavored for advancement, based on his/her accumulated score. It is also possible to decide the outcome of a three agent game through a mini tournament of two-a gent competitions among the participating players and it turns out that the resulting possible score distributions are a subset of those obtained for the general three agent-games. We discuss how one can add a steady and democratic decline rate to the model and present a simple geometric construction that allows one to write down the corresponding score evolution equations for $n$-agent games

Discovery of an isolated compact elliptical galaxy in the field

We present the discovery of an isolated compact elliptical (cE) galaxy, found during a search of SDSS DR7 for cEs, and for which we obtained WHT/ACAM imaging. It is ~900 kpc distant from its nearest neighbour, has an effective r-band radius of ~500 pc and a B-band mean surface brightness within its effective radius of 19.75 mag/arcsec. Serendipitous deep SuprimeCam imaging shows that there is no underlying disk. Its isolated position suggests that there is an alternative channel to the stripping scenario for the formation of compact ellipticals. We also report analysis of recent deeper imaging of the previous candidate free-flying cE, which shows that it is, in fact, a normal dwarf elliptical (dE). Hence the new cE reported here is the first confirmed isolated compact elliptical to be found in the field.Comment: 5 pages, 6 figures. Accepted to MNRA

Luminosity functions of cluster galaxies: The Near-ultraviolet luminosity function at <z>∼0.05<z> \sim 0.05

We derive NUV luminosity functions for 6471 NUV detected galaxies in 28 $0.02 < z < 0.08$ clusters and consider their dependence on cluster properties. We consider optically red and blue galaxies and explore how their NUV LFs vary in several cluster subsamples, selected to best show the influence of environment. Our composite LF is well fit by the Schechter form with $M^*_{NUV}=-18.98 \pm 0.07$ and $\alpha=-1.87 \pm 0.03$ in good agreement with values for the Coma centre and the Shapley supercluster, but with a steeper slope and brighter $L^*$ than in Virgo. The steep slope is due to the contribution of massive quiescent galaxies that are faint in the NUV. There are significant differences in the NUV LFs for clusters having low and high X-ray luminosities and for sparse and dense clusters, though none are particularly well fitted by the Schechter form, making a physical interpretation of the parameters difficult. When splitting clusters into two subsamples by X-ray luminosity, the ratio of low to high NUV luminosity galaxies is higher in the high X-ray luminosity subsample (i.e the luminosity function is steeper across the sampled luminosity range). In subsamples split by surface density, when characterised by Schechter functions the dense clusters have an $M^*$ about a magnitude fainter than that of the sparse clusters and $\alpha$ is steeper ($-1.9$ vs. $-1.6$ respectively). The differences in the data appear to be driven by changes in the LF of blue (star-forming) galaxies. This appears to be related to interactions with the cluster gas [abridged]Comment: Accepted A&

Deep Luminosity Functions and Colour-Magnitude Relations for Cluster Galaxies at 0.2 < z < 0.6

We derive deep $I$ band luminosity functions and colour-magnitude diagrams from HST imaging for eleven $0.2<z<0.6$ clusters observed at various stages of merging, and a comparison sample of five more relaxed clusters at similar redshifts. The characteristic magnitude $M^*$ evolves passively out to $z=0.6$, while the faint end slope of the luminosity function is $\alpha \sim -1$ at all redshifts. Cluster galaxies must have been completely assembled down to $M_I \sim -18$ out to $z=0.6$. We observe tight colour-magnitude relations over a luminosity range of up to 8 magnitudes, consistent with the passive evolution of ancient stellar populations. This is found in all clusters, irrespective of their dynamical status (involved in a collision or not, or even within subclusters for the same object) and suggests that environment does not have a strong influence on galaxy properties. A red sequence luminosity function can be followed to the limits of our photometry: we see no evidence of a weakening of the red sequence to $z=0.6$. The blue galaxy fraction rises with redshift, especially at fainter absolute magnitudes. We observe bright blue galaxies in clusters at $z > 0.4$ that are not encountered locally. Surface brightness selection effects preferentially influence the detectability of faint red galaxies, accounting for claims of evolution at the faint end.Comment: 21 pages. A series of figures for individual clusters (the full sample) will be made available on the MNRAS website. Accepted by MNRA

Luminosity Distributions within Rich Clusters - III: A comparative study of seven Abell/ACO clusters

We recover the luminosity distributions over a wide range of absolute magnitude (-24.5 < M_{R} < -16.5) for a sample of seven rich southern galaxy clusters. We find a large variation in the ratio of dwarf to giant galaxies, DGR: 0.8\le $DGR$\le 3.1. This variation is shown to be inconsistent with a ubiquitous cluster luminosity function. The DGR shows a smaller variation from cluster to cluster in the inner regions (r \ls 0.56 Mpc). Outside these regions we find the DGR to be strongly anti-correlated with the mean local projected galaxy density with the DGR increasing towards lower densities. In addition the DGR in the outer regions shows some correlation with Bautz-Morgan type. Radial analysis of the clusters indicate that the dwarf galaxies are less centrally clustered than the giants and form a significant halo around clusters. We conclude that measurements of the total cluster luminosity distribution based on the inner core alone are likely to be severe underestimates of the dwarf component, the integrated cluster luminosity and the contribution of galaxy masses to the cluster's total mass. Further work is required to quantify this. The observational evidence that the unrelaxed, lower density outer regions of clusters are dwarf-rich, adds credence to the recent evidence and conjecture that the field is a predominantly dwarf rich environment and that the dwarf galaxies are under-represented in measures of the local field luminosity function.Comment: 31 pages including 11 figures. Also available from http://star-www.st-and.ac.uk/~spd3/bib.htm

Morphological evolution in situ: Disk-dominated cluster red sequences at z ~ 1.25

We have carried out a joint photometric and structural analysis of red sequence galaxies in four clusters at a mean redshift of z ~ 1.25 using optical and near-IR HST imaging reaching to at least 3 magnitudes fainter than $M^*$. As expected, the photometry and overall galaxy sizes imply purely passive evolution of stellar populations in red sequence cluster galaxies. However, the morphologies of red sequence cluster galaxies at these redshifts show significant differences to those of local counterparts. Apart from the most massive galaxies, the high redshift red sequence galaxies are significantly diskier than their low redshift analogues. These galaxies also show significant colour gradients, again not present in their low redshift equivalents, most straightforwardly explained by radial age gradients. A clear implication of these findings is that red sequence cluster galaxies originally arrive on the sequence as disk-dominated galaxies whose disks subsequently fade or evolve secularly to end up as high S\'ersic index early-type galaxies (classical S0s or possibly ellipticals) at lower redshift. The apparent lack of growth seen in a comparison of high and low redshift red sequence galaxies implies that any evolution is internal and is unlikely to involve significant mergers. While significant star formation may have ended at high redshift, the cluster red sequence population continues to evolve (morphologically) for several Gyrs thereafter.Comment: Accepted by MNRA

The Fornax Spectroscopic Survey --- Low Surface Brightness Galaxies in Fornax

The Fornax Spectroscopic Survey is a large optical spectroscopic survey of ALL 14,000 objects with 16.5<Bj<19.7 in a 12 sq.deg area of sky centered on the Fornax Cluster. We are using the 400-fibre Two Degree Field spectrograph on the Anglo-Australian Telescope: the multiplex advantage of this system allows us to observe objects conventionally classified as `stars' as well as `galaxies'. This is the only way to minimise selection effects caused by image classification or assessing cluster membership. In this paper we present the first measurements of low surface brightness (LSB) galaxies we have detected both in the Fornax Cluster and among the background field galaxies. The new cluster members include some very low luminosity (M_B approx -11.5 mag) dwarf ellipticals, whereas the background LSB galaxies are luminous (-19.6<M_B<-17.0 mag) disk-like galaxies.Comment: To appear in "The Low Surface Brightness Universe", IAU Coll 171, eds. J.I. Davies et al., A.S.P. Conference Series. 8 pages, LaTex, 6 encapsulated ps-figures, requires paspconf.st

A Topological Characterization Of Knots and Links Arising From Site-Specific Recombination

We develop a topological model of knots and links arising from a single (or multiple processive) round(s) of recombination starting with an unknot, unlink, or (2,m)-torus knot or link substrate. We show that all knotted or linked products fall into a single family, and prove that the size of this family grows linearly with the cube of the minimum number of crossings. Additionally, we prove that the only possible products of an unknot substrate are either clasp knots and links or (2,m)-torus knots and links. Finally, in the (common) case of (2,m)-torus knot or link substrates whose products have minimal crossing number m+1, we prove that the types of products are tightly prescribed, and use this to examine previously uncharacterized experimental data.Comment: 18 pages, 3 tables, 18 figures. See also arXiv:0707.3775v1 for biological evidence for and applications of the model developed her