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

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&

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

Environmental Effects on the UV Upturn in Local Clusters of Galaxies

We explore the dependence of UV upturn colours in early type cluster galaxies on the properties of their parent clusters (such as velocity dispersion and X-ray luminosity) and on the positions and kinematics of galaxies within them. We use a sample of 24 nearby clusters with highly complete spectroscopy and optical/infrared data to select a suitable sample of red sequence galaxies, whose FUV and NUV magnitudes we measure from archival GALEX data. Our results show that the UV upturn colour has no dependence on cluster properties and has the same range in all clusters. There is also no dependence on the projected position within clusters or on line-of-sight velocity. Therefore, our conclusion is that the UV upturn phenomenon is an intrinsic feature of cluster early type galaxies, irrespective of their cluster environment.Comment: 8 pages. Accepted for publication MNRA

Luminosity Distributions within Rich Clusters - II: Demonstration and Verification via Simulation

We present detailed simulations of long exposure CCD images. The simulations are used to explore the validity of the statistical method for reconstructing the luminosity distribution of galaxies within a rich cluster i.e. by the subtraction of field number-counts from those of a sight-line through the cluster. In particular we use the simulations to establish the reliability of our observational data presented in Paper 3. Based on our intended CCD field-of-view (6.5 by 6.5 arcmins) and a 1-sigma detection limit of 26 mags per sq arcsecond, we conclude that the luminosity distribution can be robustly determined over a wide range of absolute magnitude (-23 < M_{R} < -16) provided: (a) the cluster has an Abell richness 1.5 or greater, (b) the cluster's redshift lies in the range 0.1 < z < 0.3, (c) the seeing is better than FWHM 1.25'' and (d) the photometric zero points are accurate to within Delta m = \pm 0.12. If these conditions are not met then the recovered luminosity distribution is unreliable and potentially grossly miss-leading. Finally although the method clearly has limitations, within these limitations the technique represents an extremely promising probe of galaxy evolution and environmental dependencies.Comment: 24 pages, 8 figures accepted for publication in MNRAS also available from http://star-www.st-and.ac.uk/~spd3/bib.htm

Is the luminosity distribution of field galaxies really flat ?

Recent observations of the galaxy population within rich clusters have found a characteristic luminosity distribution described by a flat (alpha = -1.0) Schechter function which exhibits an upturn at faint absolute magnitudes (B Mag = -18). Here we discuss whether such a form for the field luminosity distribution is ruled out by local and/or faint magnitude limited redshift surveys (MLRS). Our conclusions are that existing redshift surveys provide little constraints on the volume-density distribution of field galaxies faintwards of B Mag = -18. The local MLRS suffer from poor statistics over inhomogeneous volumes, while the faint MLRS are ambiguous because of the unknown nature of the ``faint blue excess'' and the ``normalization'' problem. Adopting a functional form similar to that seen in rich clusters we find that the maximum allowable faint end slope, based on the Mt Stromlo-APM redshift survey, is $\alpha \approx -1.8$ faintwards of B Mag = -18.0 (Ho = 50 km/s/Mpc^{3}).Comment: To appear in The Astrophysical Journal (Sept 20th, 1996). 27 pages including 4 figures (gzipped uuencoded

Morphological evolution of cluster red sequence galaxies in the past 9 Gyr

Galaxies arrive on the red sequences of clusters at high redshift (z>1) once their star formation is quenched and evolve passively thereafter. However, we have previously found that cluster red sequence galaxies (CRSGs) undergo significant morphological evolution subsequent to the cessation of star formation, at some point in the past 9-10~Gyr. Through a detailed study of a large sample of cluster red sequence galaxies spanning 0.2<z<1.4 we elucidate the details of this evolution. Below $z \sim 0.5-0.6$ (in the last 5-6 Gyr) there is little or no morphological evolution in the population as a whole, unlike in the previous 4-5 Gyrs. Over this earlier time (i) disk-like systems with S{\'e}rsic n < 2 progressively disappear, as (ii) the range of their axial ratios similarly decreases, removing the most elongated systems (those consistent with thin disks seen at an appreciable inclination angle), and (iii) radial colour gradients (bluer outwards) decrease in an absolute sense from significant age-related gradients to a residual level consistent with the metallicity-induced gradients seen in low redshift cluster members. The distribution of their effective radii shows some evidence of evolution, consistent with growth of {\it at most} a factor <1.5 between $z\sim 1.4$ and $z \sim 0.5$, significantly less than for comparable field galaxies, while the distribution of their central (<1kpc) bulge surface densities shows no evolution at least at z<1. A simple model involving the fading and thickening of a disk component after comparatively recent quenching (after $z\sim 1.5$) around an otherwise passively evolving older spheroid component is consistent with all of these findings