arXiv subjects

Richard Ellis

Publications and source records attributed to Richard Ellis.

77 records · Page 5Linked to original sources

arXiv1999

A Spectroscopic Catalog of 10 Distant Rich Clusters of Galaxies

We present spectroscopic observations of galaxies in the fields of 10 distant clusters for which we have previously presented deep imaging with WFPC2 on board the HST. 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, redshifts, spectral line strengths and classifications for 657 galaxies in the fields of the 10 clusters. The catalog comprises 424 cluster members across the 10 clusters and 233 field galaxies, with detailed morphological information from our WFPC2 images for 204 of the cluster galaxies and 71 in the field. We illustrate some basic properties of the catalog, including correlations between the morphological and spectral properties of our large sample of cluster galaxies. A direct comparison of the spectral properties of the high redshift cluster and field populations suggest that the phenomenon of strong Balmer lines in otherwise passive galaxies (commonly called E+A, but renamed here as the k+a class) shows an order-of-magnitude increase in the rich cluster environment, compared to a more modest increase in the field population. This suggests that the process or processes involved in producing k+a galaxies are either substantially more effective in the cluster environment or that this environment prolongs the visibility of this phase. A more detailed analysis and modeling of these data will be presented in Poggianti et al. (1998).

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arXiv1997

The physical parameters of the evolving population of faint galaxies

The excess numbers of blue galaxies at faint magnitudes is a subject of much controversy. Recent Hubble Space Telescope results has revealed a plethora of galaxies with peculiar morphologies tentatively identified as the evolving population. We report the results of optical spectroscopy and near-infrared photometry of a sample of faint \HST galaxies from the Medium Deep Survey to ascertain the physical properties of the faint morphological populations. We find four principal results: Firstly that the population of objects classified as `peculiar' are intrinsically luminous in the optical ($M_B\sim -19$). Secondly these systems tend to be strong sources of [OII] line luminosity. Thirdly the optical-infrared colours of the faint population (a) confirm the presence of a population of compact blue galaxies and (b) show the stellar populations of Irregular/Peculiar galaxies encompass a wide range in age. Finally a surface-brightness comparison with the local galaxy sample of Frei et al shows that these objects are not of anomalously low surface brightness, rather we find that all morphological classes have evolved to a higher surface brightness at higher-redshifts ($z>0.3$).

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arXiv1997

Spectroscopic Confirmation of Redshifts Predicted by Gravitational Lensing

We present deep spectroscopic measurements of 18 distant field galaxies identified as gravitationally-lensed arcs in a Hubble Space Telescope image of the cluster Abell 2218. Redshifts of these objects were predicted by Kneib et al. (1996) using a lensing analysis constrained by the properties of two bright arcs of known redshift and other multiply-imaged sources. The new spectroscopic identifications were obtained using long exposures with the LDSS-2 spectrograph on the William Herschel Telescope and demonstrate the capability of that instrument to new limits, $R\simeq$24; the lensing magnification implies true source magnitudes as faint as $R\simeq$25. Statistically, our measured redshifts are in excellent agreement with those predicted from Kneib et al.'s lensing analysis which gives considerable support to the redshift distribution derived by the lensing inversion method for the more numerous and fainter arclets extending to $R\simeq 25.5$. We explore the remaining uncertainties arising from both the mass distribution in the central regions of Abell 2218 and the inversion method itself, and conclude that the mean redshift of the faint field population at $R\simeq$25.5 ($B\sim 26$-27) is low, $ $=0.8-1. We discuss this result in the context of redshift distributions estimated from multi-colour photometry. Although such comparisons are not straightforward, we suggest that photometric techniques may achieve a reasonable level of agreement particularly when they include near-infrared photometry with discriminatory capabilities in the $1<z<$2 range.

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arXiv1996

Evolution of Normal Galaxies: HST Morphologies and Deep Spectroscopy

I review progress in understanding the evolution of normal field and cluster galaxies through the combination of HST imaging and ground-based spectroscopy. These data suggest that the bulk of the star formation producing the present-day galaxy population occurred at accessible redshifts, $z<$2. Furthermore, a surprising amount of the detailed processing that shaped the Hubble sequence and morphology-density relation occurred surprisingly recently. The stage is thus set for a concerted attack on these questions with the present generation of 8-10 metre large telescopes. An important step forward will be the development of efficient survey techniques for the systematic exploration of the $z>$1 Universe. Some possible approaches are briefly discussed.

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arXiv1995

The morphological identification of the rapidly evolving population of faint galaxies

The excess numbers of blue galaxies at faint magnitudes are a long-standing cosmological puzzle. We present new number-magnitude counts as a function of galactic morphology from the first deep fields of the Cycle 4 Hubble Space Telescope {\it Medium Deep Survey} project. From a sample of 301 galaxies we define counts for elliptical, spiral and irregular/peculiar galaxies to $I=22$. We find two principal results. Firstly the elliptical and spiral galaxy counts both follow the predictions of high-normalisation no-evolution models at all magnitudes, indicating that regular Hubble types evolve only slowly to $z\sim 0.5$. Secondly we find that irregular/peculiar galaxies, including multiple-peaked, possibly merging, objects, have a very steep number-magnitude relation and greatly exceed predictions based on proportions in local surveys. These systems make up half the total counts by $I=22$ and imply the rapidly-evolving component of the faint galaxy population has been identified.

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