Showing papers by "Warrick J. Couch published in 1998"

Morphological Studies of the Galaxy Populations in Distant “Butcher-Oemler” Clusters with the Hubble Space Telescope. II. AC 103, AC 118, and AC 114 at z = 0.31*

Abstract: 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 seen in these environments at earlier epochs. Deep, high-resolution imagery has been obtained of three rich clusters, AC 103, AC 118, and AC 114 at z = 0.31, through the R (F702W) filter of the Wide Field Planetary Camera (WFPC2) of the Hubble Space Telescope (HST). For AC 103 and AC 118, single three-orbit pointings covering a central ~0.5 × 0.5 h-1 Mpc area have been obtained. For AC 114, a mosaic of four six-orbit pointings has provided coverage of a central ~1.2 × 0.7 h-1 Mpc area, for the first time allowing a study extending to the outer regions of a more distant cluster. Morphological classification has been conducted visually using these images, with Hubble types plus evidence of dynamical interactions and/or structural abnormalities recorded for all galaxies down to R702 = 22.25 in AC 103 and AC 118 and R702 = 23.00 in AC 114. New spectroscopy limited at K' ≤ 18 has also been acquired, providing within our WFPC2 images a total sample of 129 confirmed cluster members, of which 117 have secure star formation classifications (e.g., starburst, poststarburst, Hδ strong). Our study reveals that the mix of Hubble types within the distant clusters is, overall, significantly different from that seen in the same high-density environments nearby: there are proportionally more spiral galaxies and fewer E/S0 galaxies at these earlier epochs. Within the core (r 200 h-1 kpc) regions of the three clusters, spirals covering the full Sa-Sdm/Irr range are present in numbers up to a factor of ~4 higher than that seen in present-day clusters, the highest fraction being observed in the least massive cluster within our sample, AC 103. Only in the virialized core of our most massive, regular cluster, AC 114, do we see morphological fractions approaching those of the present epoch. However, outside the core of this cluster the spiral fraction rises to ~60%: 3 times above the present-day level. Dynamical interactions are also widespread throughout the populations of all three clusters, with ~20% of the members, on average, showing morphological evidence of this phenomenon. The various subsets of "active" cluster members show emergent morphological trends. The majority of blue galaxies either undergoing a starburst or seen shortly (<0.5 Gyr) thereafter are systems involved in major mergers. These galaxies, however, are generally of modest luminosity (L ~ L* + 1 mag) even in this brightened phase; in their faded state they appear destined to become dwarfs, too faint to be included in magnitude-limited spectroscopic samples such as ours. Cluster members with ongoing star formation typical of present-day spirals are mostly normal Sb-Sdm/Irr Hubble types. Galaxies identified as having completed their last major episode of star formation 1-2 Gyr prior to the epoch of observation are conspicuous by their commonality in morphology, all being early-type (S0-Sb) disk systems. The red Hδ-strong objects, interpreted from their colors and spectra as being the remnants of secondary star formation in old dormant systems, have morphologies consistent with this picture, being a mixture of normal E and S0 galaxies. In combination, these results point to cluster galaxy evolution being driven by not one but several different physical processes. The only conspicuous one is galaxy-galaxy interactions and merging. However, our study reveals many galaxies where this process could not have been responsible for altering the course of their star formation activity. Furthermore, the normal disk morphology of the majority of these previously active galaxies indicates the operation of process(es) that halt star formation but leave the basic disk structure intact and largely unperturbed. This tends to favor mechanisms that affect the gas supply (e.g., ram-pressure stripping, galaxy infall) rather than processes like "galaxy harassment," where we simply do not see the population of severely distorted spirals expected if its operation is widespread.

Cosmology from Type Ia supernovae

Abstract: This presentation reports on first evidence for a low-mass-density/positive-cosmological-constant universe that will expand forever, based on observations of a set of 40 high-redshift supernovae. The experimental strategy, data sets, and analysis techniques are described. More extensive analyses of these results with some additional methods and data are presented in the more recent LBNL report #41801 (Perlmutter et al., 1998; accepted for publication in Ap.J.), astro-ph/9812133 . This Lawrence Berkeley National Laboratory reprint is a reduction of a poster presentation from the Cosmology Display Session #85 on 9 January 1998 at the American Astronomical Society meeting in Washington D.C. It is also available on the World Wide Web at http://supernova.LBL.gov/ This work has also been referenced in the literature by the pre-meeting abstract citation: Perlmutter et al., B.A.A.S., volume 29, page 1351 (1997).

Morphological Number Counts and Redshift Distributions to I<26 from the Hubble Deep Field: Implications for the Evolution of Ellipticals, Spirals, and Irregulars

Abstract: We combine the photometric redshift data of Fern?ndez-Soto et al. with the morphological data of Odewahn et al. for all galaxies with I 24. The breadth of the redshift distribution at faint magnitudes implies strong clustering or an extended epoch of formation commencing at z>3. 2.?Spiral galaxies are present in numbers consistent with zero-evolution predictions to I=22. Beyond this magnitude some net positive evolution is required. Although the number counts are consistent with the passive-evolution predictions to I=26.0, the redshift distributions favor number and luminosity evolution, although few obvious mergers are seen (possibly classified as irregulars). We note that beyond z~2 very few ordered spirals are seen suggesting a formation epoch of spiral galaxies at z~1.5-2. 3.?There is no obvious explanation for the late-type/irregular class, and this category requires further subdivision. While a small fraction of the population lies at low redshift (i.e., true irregulars), the majority lie at redshifts 1 1.5 mergers are frequent and, taken in conjunction with the absence of normal spirals at z>2, the logical inference is that they represent the progenitors of normal spirals that form via hierarchical merging.

Morphological number-count and redshift distributions to I < 26 from the Hubble Deep Field: Implications for the evolution of Ellipticals, Spirals and Irregulars

Abstract: We combine the photometric redshift data of Fernandez-Soto et al. (1997) with the morphological data of Odewahn et al. (1996) for all galaxies with I 24. (2) Spiral galaxies are present in numbers consistent with zero- evolution predictions to I = 22. Beyond this magnitude some net- positive evolution is required. Although the number-counts are consistent with the passive-evolution predictions to I=26.0 the redshift distributions favor number AND luminosity evolution. (3) There is no obvious explanation for the late-type/irregular class and this category requires further subdivision. While a small fraction of the population lies at low redshift (i.e. true irregulars), the majority lie at redshifts, 1 1.5 mergers are frequent and, taken in conjunction with the absence of normal spirals at z > 2, the logical inference is that they represent the progenitors of normal spirals forming via hierarchical merging.

The Luminosity Distribution in Galaxy Clusters: A Dwarf Population-Density Relation?

Abstract: Recent work suggests that rich clusters of galaxies commonly have large populations of dwarf (i.e., low-luminosity) members, that is, their luminosity function turns up to a steep slope at the faint end. This population, or more particularly the relative numbers of dwarfs to giants, appears to be very similar for clusters of similar morphology but may vary between cluster types. We have previously suggested that dwarfs may be more common in less compact, spiral-rich clusters. Similarly, we have found evidence for population gradients across clusters, in that the dwarf population appears more spatially extended. In the present Letter, we summarize the current evidence and propose, in an analogy to the well-known morphology-density relation, that what we are seeing is a dwarf population-density relation: dwarfs are more common in lower density environments. Finally, we discuss recent semianalytic models of galaxy formation in the hierarchical clustering picture, which may give clues as to the origin of our proposed relation.

Luminosity distributions within rich clusters - III. A comparative study of seven Abell/ACO clusters

Abstract: ABSTRA C T We recover the luminosity distributions over a wide range of absolute magnitude (π24:5 < MR < π16:5) 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# DGR #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 & 0:56 Mpc). Outside these regions we find the DGR to be strongly anticorrelated 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 indicates that the dwarf galaxies are less centrally clustered than the giants, and they 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.

New Constraints on the Luminosity Evolution of Spheroidal Galaxies in Distant Clusters

Abstract: We investigate various probes of luminosity evolution in the rich cluster environment, concentrating in particular on the spheroidal (E/S0) galaxies, using a newly constructed catalog of morphologically classified Hubble Space Telescope (HST) images of galaxies observed in the cores of 13 clusters with redshifts 0.17 ≤ z ≤ 0.56. An important distinction of this study compared to earlier work is the availability for a substantial subset of our HST fields of new near-infrared ground-based photometry, which we have used to select and study the various populations. We find no significant change in the characteristic luminosity, M*K, of the spheroidal populations at redshifts of 0.31 and 0.56. As a more sensitive probe of luminosity evolution, we analyze the surface photometry of our HST-classified ellipticals by deriving effective metric radii, Re, and mean effective surface brightnesses, μe. At the standard condition corresponding to Re = 1 kpc, we find convincing evidence of evolutionary brightening in both rest frame B and K light, consistent with model predictions based on the passive evolution of stellar populations.

Luminosity distributions within rich clusters — II. Demonstration and verification via simulation

Abstract: 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 to be presented in Paper III. Based on our intended CCD field-of-view (6.5 × 6.5 arcmin2) and a 1σ detection limit of 26 mag arcsec−2, we conclude that the luminosity distribution can be robustly determined over a wide range of absolute magnitude (−23 < MR < −16) provided: (a) the cluster has an Abell richness 1.5 or greater; (b) the redshift of the cluster lies in the range 0.1 < z < 0.3; (c) the seeing is better than FWHM 1.25 arcsec, and (d) the photometric zero-points are accurate to within Δm = ± 0.12. If these conditions are not met, then the recovered luminosity distribution is unreliable. Finally, although the method clearly has limitations, within these limitations the technique represents an extremely promising probe of galaxy evolution and environmental dependences.

The Las Campanas/AAT Rich Cluster Survey

Abstract: Some unsolved cosmological questions remain in relation to the formation of structure in the universe. One way of addressing such questions is to use rich galaxy clusters as tracers of the growth of large-scale structure. To date, studies of rich clusters of galaxies have concentrated on systems generally at either high redshift or in the local universe. The properties of clusters and their constituent galaxies at these extrema are becoming well understood. In particular, it is becoming clear that rich clusters have undergone considerable evolution both dynamically and in their galaxy populations over the last 5{8 Gyr. We are undertaking a detailed study of rich clusters of galaxies in the range 0¢ 05 < z < 0¢ 15. Our results will be directly comparable to those of previous studies both at high and low redshift and, for the flrst time, provide continuous coverage across this important and unexplored transitory epoch in terms of galaxy evolution and structure growth.

Cosmology from Type Ia Supernovae

Abstract: This presentation reports on first evidence for a low-mass-density/positive-cosmological-constant universe that will expand forever, based on observations of a set of 40 high-redshift supernovae. The experimental strategy, data sets, and analysis techniques are described. More extensive analyses of these results with some additional methods and data are presented in the more recent LBNL report #41801 (Perlmutter et al., 1998; accepted for publication in Ap.J.), astro-ph/9812133 . This Lawrence Berkeley National Laboratory reprint is a reduction of a poster presentation from the Cosmology Display Session #85 on 9 January 1998 at the American Astronomical Society meeting in Washington D.C. It is also available on the World Wide Web at this http URL This work has also been referenced in the literature by the pre-meeting abstract citation: Perlmutter et al., B.A.A.S., volume 29, page 1351 (1997).

A Statistical Comparison of Line Strength Variations in Coma and Cluster Galaxies at z ~ 0.3

Abstract: We present a statistical comparison between spectral line strength variations in Coma galaxies and galaxies in three rich clusters at z ~ 0.3. Using a principal component analysis, we have transformed the observable quantities, the line strengths, into new mutually orthogonal axes and found two specific results. First, more independent parameters are required to account for the line strength variations in the distant cluster data than in the Coma data, and second, line strengths which are not correlated in the distant cluster data are strongly coupled in the Coma data. These results suggest that galaxies in clusters have been homogenised such that most of the information from the conditions present at their formation has been destroyed. Hence, it may be possible that the present-day homogeneity of early-type galaxy properties, e.g. the fundamental plane relation, does not require a single formation scenario, but that a variety of formation scenarios for different galaxies could still yield the observed behaviour of nearby galaxies.

Environmental Effects on the Faint End of the Luminosity Function

Abstract: Recent studies have demonstrated that many galaxy clusters have luminosity functions (LFs) which are steep at the faint end However, it is equally clear that not all clusters have identical LFs In this paper we explore whether the variation in LF shape correlates with other cluster or environmental properties