Ray M. Sharples

The Homogeneity of Spheroidal Populations in Distant Clusters

The small scatter observed for the (U-V) colors of spheroidal galaxies in nearby clusters of galaxies provides a powerful constraint on the history of star formation in dense environments. However, with local data alone, it is not possible to separate models where galaxies assembled synchronously over redshifts 0 < z < 1 from ones where galaxies formed stochastically at much earlier times. Here we attempt to resolve this ambiguity via high-precision rest-frame UV-optical photometry of a large sample of morphologically selected spheroidal galaxies in three z ~ 0.54 clusters that have been observed with the Hubble Space Telescope (HST). We demonstrate the robustness of using the HST to conduct the morphological separation of spheroidal and disk galaxies at this redshift and use our new data to repeat the analysis conducted locally at a significant look-back time. We find a small scatter (<0.1 mag rms) for galaxies classed as Es and E/S0s, both internally within each of the three clusters and externally from cluster to cluster. We do not find any trend for the scatter to increase with decreasing luminosity down to L~L*V+3, other than can be accounted for by observational error. Neither is there evidence for a distinction between the scatter observed for galaxies classified as ellipticals and S0. Our result provides a new constraint on the star formation history of cluster spheroidals prior to z 0.5 confirming and considerably strengthening the earlier conclusions. Most of the star formation in the elliptical galaxies in dense clusters was completed before z 3 in conventional cosmologies. Although we cannot rule out the continued production of some ellipticals, our results do indicate an era of initial star formation consistent with the population of star-forming galaxies recently detected beyond z 3.

Hubble Space Telescope Observations of the Lensing Cluster Abell 2218

We present a striking new Hubble Space Telescope (HST) observation of the rich cluster Abell 2218 taken with the Wide-Field and Planetary Camera (WFPC2). HSTs restored image quality reveals a sizeable number of gravitationally-lensed features in this cluster, significantly more than had been identified using ground-based telescopes. The brightest arcs are resolved by HST and show internal features enabling us to identify multiply-imaged examples, confirming and improving the mass models of the cluster determined from ground-based observations. Although weak lensing has been detected statistically in this and other clusters from ground-based data, the superlative resolution of HST enables us to individually identify weakly distorted images more reliably than hitherto, with important consequences for their redshift determination. Using an improved mass model for the cluster calibrated with available spectroscopy for the brightest arcs, we demonstrate how inversion of the lensing model can be used to yield the redshift distribution of

Morphological studies of the galaxy populations in distant 'Butcher-Oemler' clusters with HST. 1: AC 114 AT Z = 0.31 and Abell 370 at Z = 0.37

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On the formation of globular cluster systems in a hierarchical universe

80 faint arclets to

The Formation and Evolution of Candidate Young Globular Clusters in NGC 3256

R\simeq25

A Catalog of morphological types in 10 distant rich clusters of galaxies

. We present a new formalism for estimating the uncertainties in this inversion method and review prospects for interpreting our results and verifying the predicted redshifts.

A Wide-Field K-band Survey: The Luminosity Function of Galaxies

We present new results of a program to study the detailed morphologies of galaxies in intermediate redshift clusters and hence understand the physical origin of the enhanced star formation in these environments at earlier epochs. Deep, high resolution imagery has been obtained of 3 rich clusters, AC103,AC118 & AC114 at z=0.31, using the WFPC2 on HST. For AC103 & AC118, single pointings covering a central 0.5x0.5Mpc have been obtained; for AC114, 4 pointings covering a 1.2x0.7Mpc area have been obtained, allowing the more outer regions of a distant cluster to be studied. Hubble types plus evidence for dynamical interactions and/or structural abnormalities have been determined visually for all galaxies down to R=22.25 in AC103 & AC118 and R=23.0 in AC114.We find the numbers of spirals (Sa-Sdm) in our clusters to be up to 4x higher than that seen in present-day clusters; only in the virialised core of our most massive regular cluster, AC114, do we see morphological fractions approaching those of the present epoch. Dynamical interactions are also widespread throughout the clusters with ~20% of members showing morphological evidence of this phenomenon. The highest incidence is amongst those blue members either undergoing a starburst or seen <0.5Gyr thereafter with the majority being involved in major mergers. These galaxies, however, are of modest luminosity (L~L*+1mag), destined to become dwarfs once they fade. Cluster members with ongoing star formation typical of nearby spirals are largely late Sb-Sdm Hubble types. Galaxies whose star formation ended 1-2Gyr prior to the epoch of observation are all normal early-type (S0-Sb) disk systems. The red H-delta strong galaxies are a mixture of E & S0 galaxies, consistent with them being old dormant systems which have undergone a secondary star formation episode.

A Comparison of Direct and Indirect Mass Estimates for Distant Clusters of Galaxies

ABSTRA C T We have investigated the formation of globular cluster (GC) systems in the fiducial semianalytic model of galaxy formation of Cole et al., by assuming that GCs are formed at high redshifts ﷿z . 5fi in protogalactic fragments, and during the subsequent gas-rich merging of these fragments. Under these assumptions we have simulated the GC systems of 450 elliptical galaxies, and find that the majority (93 per cent) are intrinsically bimodal in metallicity. We find that, in the mean, the metal-rich GC subpopulations are younger than the metal-poor GC subpopulations, with ages of 9 and 12 Gyr respectively, and that the mean ages of the metalrich GCs are dependent upon host galaxy luminosity and environment (halo circular velocity), whereas the metal-poor GCs are not. We find that the continued gaseous merging of the protogalactic fragments leads to significant age-structure amongst the metal-rich GCs. These GCs exhibit a large age-range (5 to 12 Gyr), which increases for low-luminosity galaxies, and for galaxies in low circular velocity haloes. Moreover, the metal-rich GCs associated with low-luminosity field and/or group ellipticals are , 2 Gyr younger than the metal-rich GCs in luminous cluster ellipticals. We find that the total GC populations scale with host galaxy luminosity as NGC / L 1:25 V;gal , a result in agreement with observations of luminous elliptical galaxies. This scaling is due to a systematic increase in the M/L ratios of the galaxy haloes with luminosity for LV;gal . L * galaxies in the model. A comparison between the luminosity growth of the model ellipticals and their GC formation indicates that mergers do not significantly affect SN at z , 2. We find the mean colours of the both the metal-rich and metal-poor GCs exhibit only a weak dependence upon host galaxy luminosity, a result consistent with contemporary observations. We conclude that gaseous merging, the bulk of which occurs at 1 # z # 4i n ourLCDM model, leads to the formation of the metalrich peak of the GC systems of elliptical galaxies. We suggest that the formation and subsequent truncation of the metal-poor GCs in the protogalactic fragments is closely related to the star formation rate in these fragments, which may have been significantly higher at very early times.

Evidence for Wide-spread Active Galactic Nucleus-driven Outflows in the Most Massive z 1-2 Star-forming Galaxies

We present images of the recent galaxy merger NGC 3256 obtained with the Wide Field Planetary Camera 2 of the Hubble Space Telescope in B and I filters. We show that there is a large population of more than 1000 compact, bright, blue objects in this galaxy within the 7 kpc × 7 kpc region studied. These objects have sizes, colors, and luminosities like those expected for young Galactic globular clusters, with ages ranging from a few to several hundred megayears. On this basis, we identify at least some fraction of the compact, bright, blue objects in NGC 3256 as young globular clusters. The young cluster system makes up a significant fraction of the total luminosity of the galaxy within the region studied—15%–20% in B and half that in I, indicating a high efficiency of cluster formation on a galaxy-wide scale. In order to determine the properties of this young cluster system, the selection effects in size, color, and luminosity are carefully modeled. We find that the intrinsic color distribution is broad and there is no significant trend of color with magnitude. The combination of the broad range of observed colors and the lack of a trend of redder colors at fainter magnitudes cannot be fitted solely by a broad age distribution and/or differential reddening, although the latter is clearly present. The observations can be accounted for by either the preferential depletion/destruction of lower mass clusters as they age or a very young age (20 Myr) for the cluster population, comparable to or less than the dynamical time of the region in which the clusters are observed. We also find that the luminosity function of the young cluster system can be roughly fitted by a power law with an exponent of -1.8, with tentative evidence that it flattens at faint magnitudes. The clusters are compact in size, with typical estimated half-light radii of 5–10 pc, but there is no obvious cutoff for larger radii and only a shallow trend of size with luminosity. We discuss the implications of these results for models of the formation and dynamical evolution of globular clusters, as well as for interpretation of the properties of older globular cluster systems.

Luminosity and stellar mass functions of discs and spheroids in the SDSS and the supermassive black hole mass function

We present catalogs of objects detected in deep images of 11 fields in 10 distant clusters obtained using WFPC-2 on board the Hubble Space Telescope. The clusters span the redshift range z = 0.37-0.56 and are the subject of a detailed ground- and space-based study to investigate the evolution of galaxies as a function of environment and epoch. The data presented here include positions, photometry and basic morphological information on ~9000 objects in the fields of the 10 clusters. For a brighter subset of 1857 objects in these areas, we provide more detailed morphological information.