arXiv subjects

Warrick J. Couch

Publications and source records attributed to Warrick J. Couch.

At least 109 records · Page 6Linked to original sources

arXiv2001

Galaxy threshing and the formation of ultra-compact dwarf galaxies

Recent spectroscopic and morphological observational studies of galaxies around NGC 1399 in the Fornax Cluster (Drinkwater et al. 2000b) have discovered several `ultra-compact dwarf' galaxies with intrinsic sizes of $\sim$ 100 pc and absolute $B$ band magnitudes ranging from -13 to -11 mag. In order to elucidate the origin of these enigmatic objects, we perform numerical simulations on the dynamical evolution of nucleated dwarf galaxies orbiting NGC 1399 and suffering from its strong tidal gravitational field. Adopting a plausible scaling relation for dwarf galaxies, we find that the outer stellar components of a nucleated dwarf are totally removed. This is due to them being tidally stripped over the course of several passages past the central region of NGC 1399. The nucleus, however, manages to survive. We also find that the size and luminosity of the remnant are similar to those observed for ultra-compact dwarf galaxies, if the simulated precursor nucleated dwarf has a mass of $\sim$ $10^8$ $M_{\odot}$. These results suggest that ultra-compact dwarf galaxies could have previously been more luminous dwarf spheroidal or elliptical galaxies with rather compact nuclei.

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arXiv2001

Spectroscopic evolution of dusty starburst galaxies

By using a one-zone chemical and spectrophotometric evolution model of a disk galaxy undergoing a dusty starburst, we investigate, numerically, the optical spectroscopic properties in order to explore galaxy evolution in distant clusters. We adopt an assumption that the degree of dust extinction (represented by $A_V$) depends on the ages of starburst populations in such a way that younger stars have larger $A_V$ (originally referred to as selective dust extinction by Poggianti & Wu 2000). In particular, we investigate how the time evolution of the equivalent widths of [OII]$λ$3727 and H$δ$ is controlled by the adopted age dependence. This leads to three main results: (1) If a young stellar population (with the age of $\sim$ $10^6$ yr) is more heavily obscured by dust than an old one ($>$ $10^8$ yr), the galaxy can show an ``e(a)'' spectrum characterized by strong H$δ$ absorption and relatively modest [OII] emission. (2) A dusty starburst galaxy with an e(a) spectrum can evolve into a poststarburst galaxy with an a+k (or k+a) spectrum 0.2 Gyr after the starburst and then into a passive one with a k-type spectrum 1 Gyr after the starburst. This result clearly demonstrates an evolutionary link between galaxies with different spectral classes (i.e., e(b), e(a), a+k, k+a, and k). (3) A dusty starburst galaxy can show an a+k or k+a spectrum even in the dusty starburst phase if the age-dependence of dust extinction is rather weak, i.e., if young starburst populations with different ages ($\le$ $10^7$ yr) are uniformly obscured by dust.

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arXiv2001

Tidal truncation of gas replenishment and global suppression of galactic star formation in distant clusters

Recent spectroscopic observations of galaxies in distant clusters have revealed that the rate of star formation in star-forming galaxies is significantly suppressed with respect to their counterparts in the field at a similar redshift. It is, however, highly uncertain which physical processes are responsible for this suppression. We present the results of a numerical investigation of how the global tidal field of a cluster dynamically influences the reservoir of halo gas surrounding a disk galaxy as it falls into the cluster from the surrounding field. We find that the tidal field of the cluster efficiently removes the halo gas from the galaxy, thereby halting its accretion onto the disk, and thus the fueling of star formation within. This effectively truncates the galaxy's star formation. We also find that this tidal truncation does not depend very strongly on the orbit of the disk with respect to the center of the cluster. These results suggest that the global tidal field of clusters is capable of causing a widespread and uniform suppression of star formation in galaxies accreted by the cluster. In light of these results, we discuss the importance of this tidal truncation of the gas supply in the formation of S0 galaxies in clusters.

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arXiv2000

Formation and evolution of E+A galaxies in dusty starburst galaxies

The formation and evolution of the ``E+A'' (also named ``k+a'' and ``a+k'' types by Dressler et al. 1999) galaxies found in significant numbers in the cores of intermediate redshift clusters has been extensively discussed by many authors. In this paper, we model the spectral, dynamical and morphological evolution of a prime candidate for producing this spectral signature: a dusty starburst associated with a major galaxy merger. We show that as this system evolves dynamically, its spectral type changes from and ``e(a)'' type (exhibiting strong H$δ$ absorption and modest [OII] emission -- the identifying features of local dusty starburst galaxies) to a k+a type and then finally to a passive ``k'' type. This result shows that galaxies with an e(a) spectral type can be precursors to the k+a systems and that dynamical evolution greatly controls the spectral evolution in these merger cases. Our simulations also show that a merger with very high infrared luminosity ($L_{\rm IR}$ $>$ $10^{11}$ $L_{\odot}$) is more likely to show an e(a) spectrum, which implies that spectral types can be correlated with infrared fluxes in dusty starburst galaxies. Based on these results, we discuss the origin of the evolution of k+a/a+k galaxies in distant clusters and the role merging is likely to have.

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arXiv2000

A low global star formation rate in the rich galaxy cluster AC114 at z=0.32

We present the results of a wide-field survey for H-alpha emitting galaxies in the cluster AC114 at z=0.32. Spectra centred on H-alpha at the cluster redshift have been obtained for 586 galaxies to I~22 out to a radius around 2Mpc. At most, only ~10% of these were found to be H-alpha-emitting cluster members. These objects are predominantly blue and of late-type spiral morphology, consistent with them hosting star formation. However, ~65% of the cluster members classified morphologically as spirals (with HST), have no detectable H-alpha emission; star-formation and morphological evolution in cluster galaxies appear to be largely decoupled. Changes in the H-alpha detection rate and the strength of H-alpha emission with environment are found to be weak within the region studied. Star formation within the cluster members is also found to be strongly and uniformly suppressed, with the rates inferred from the H-alpha emission not exceeding 4Mo/yr, and AC114's H-alpha luminosity function being an order of magnitude below that observed for field galaxies at the same redshift. None of the galaxies detected have the high star formation rates associated with `starburst' galaxies; however, this may still be reconciliable with the known (8+/-3%) fraction of `post-starburst' galaxies within AC114, given the poorly determined but short lifetimes of starbursts and the possibility that much of the associated star formation is obscured by dust.

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arXiv2000

Clusters of Galaxies in the 2dF Galaxy Redshift Survey

The 2dF Galaxy Redshift Survey has obtained 135,000 redshifts for galaxies in two broad strips. Here we present the first results of a 3-dimensional search for galaxy clusters based on known 2-dimensional compilations. We derive new redshifts and velocity dispersions for clusters, assess the level of contamination in the sample, analyze the accuracy of photometric redshift estimates and study the space distribution of clusters.

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arXiv2000

Evolution of Galaxy morphologies in Clusters

We have studied the evolution of galaxian morphologies from ground-based, good-seeing images of 9 clusters at z=0.09-0.25. The comparison of our data with those relative to higher redshift clusters (Dressler et al. 1997) allowed us to trace for the first time the evolution of the morphological mix at a look-back time of 2-4 Gyr, finding a dependence of the observed evolutionary trends on the cluster properties.

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arXiv1999

Constraints on the evolution of S0 galaxies in rich clusters at moderate redshifts

We combine morphological classifications from deep HST imaging of a sample of 3 clusters at z=0.31 and a further 9 clusters at z=0.37-0.56 with existing spectroscopic observations of their E and S0 populations, to study the relative spectral properties of these two galaxy types. We have also used similar data for the Coma cluster as a present day bench-mark with which to compare our data at higher redshift. The data span the range where strong evolution is claimed in the proportion of S0 galaxies within rich clusters. Techniques have been recently developed to analyse the strengths of absorption lines in the spectra of local, passive galaxies to separate the effects of age & metallicity and hence date the ages of the most recent, substantial star formation episode in these galaxies. We have applied these techniques to the co-added spectra of the distant cluster galaxies to determine the relative ages of the E and S0 populations. We find no statistically significant difference between the luminosity-weighted ages of the E and S0 galaxies in these clusters. The precision of this measurement would indicate that any recent star formation in the cluster S0s at z=0.31 could have involved bursts of no more than ~11% (by mass) in strength in no more than half of these galaxies. Our results, in conjunction with other work, suggest that the progenitors of S0 galaxies in rich clusters are mostly early-type spirals who, through interactions with the cluster environment, have had their star formation truncated. This indicates a relatively unspectacular origin for the missing S0 population at high redshift.

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arXiv1998

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.

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arXiv1998

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.

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arXiv1997

Evolution since z = 0.5 of the Morphology-Density relation for Clusters of Galaxies

Using traditional morphological classifications of galaxies in 10 intermediate-redshift (z~0.5) clusters observed with WFPC-2 on the Hubble Space Telescope, we derive relations between morphology and local galaxy density similar to that found by Dressler for low-redshift clusters. Taken collectively, the `morphology-density' relationship, M-D, for these more distant, presumably younger clusters is qualitatively similar to that found for the local sample, but a detailed comparison shows two substantial differences: (1) For the clusters in our sample, the M-D relation is strong in centrally concentrated ``regular'' clusters, those with a strong correlation of radius and surface density, but nearly absent for clusters that are less concentrated and irregular, in contrast to the situation for low redshift clusters where a strong relation has been found for both. (2) In every cluster the fraction of elliptical galaxies is as large or larger than in low-redshift clusters, but the S0 fraction is 2-3 times smaller, with a proportional increase of the spiral fraction. Straightforward, though probably not unique, interpretations of these observations are (1) morphological segregation proceeds hierarchically, affecting richer, denser groups of galaxies earlier, and (2) the formation of elliptical galaxies predates the formation of rich clusters, and occurs instead in the loose-group phase or even earlier, but S0's are generated in large numbers only after cluster virialization.

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arXiv1997

A Dynamical Study of Optically Selected Distant Clusters

We present a programme of spectroscopic observations of galaxies in a sample of optically-selected clusters taken from the catalogue of Couch et al (1991). Previous ROSAT observations of these clusters have shown them to have lower X-ray luminosities, given their optical richness, than might be expected on the basis of local samples. In the present paper we extend this work by determining velocity dispersions of a subsample of the clusters. We confirm the dynamical reality of all but one of the original sample, and find velocity dispersions comparable with present-day clusters of equivalent comoving space density. Thus, in the context of the $L_X-σ$ relation for present-day clusters, there is evidence for a higher velocity dispersion at fixed X-ray luminosity. A key question is whether the high velocity dispersions are indicative of the gravitational potential. If they are, the X-ray luminosities measured in Bower et al., 1994 (Paper I), would then imply an implausibly low efficiency of X-ray generation. Alternatively, the discrepancy could be explained if the clusters were systems of lower virial temperature, in which the apparent velocity dispersion is inflated by an infalling, unrelaxed halo. This might result either from an increase with redshift in the infall rate for clusters, or from the preferential selection of clusters embedded in filaments oriented along the line of sight. Since clusters with similar properties can be found in local optically selected catalogues, we suggest that the latter explanation is more likely.

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arXiv1997

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 which have been observed with HST. We demonstrate the robustness of using 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*+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.

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arXiv1996

A Catalog of Morphological Types in 10 Distant Rich Clusters of Galaxies

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.

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arXiv1996

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

We present weak lensing results for 12 distant clusters determined from images obtained with the refurbished HST. We detect the signature of gravitational lensing in 11 of the 12 clusters; the clusters span nearly an order of magnitude in lensing strength. The sample thus provides an excellent database for correlating direct mass estimates from lensing with indirect ones which rely on baryonic tracers. We examine the correlation between the cluster X-ray luminosities and the mean gravitational shear strengths and develop a model which predicts the relationship expected from the properties of local clusters. After allowing for various observational effects, we find that the predicted correlation is a reasonable match to the available data, indicating that there has been little evolution in the X-ray luminosity-central mass relationship between z=0.4 and now. We discuss the implications of this result in the context of the evolution of the X-ray luminosity function found by earlier workers. The comparison between shear amplitudes and velocity dispersions, estimated from a modest sample of members, reveals a discrepancy in the sense that these velocity dispersions are typically over-estimated by factors of ~50%. This supports earlier suggestions that high dispersions measured for distant clusters may be seriously affected by both unidentified substructure and outliers. Combining our lensing masses with morphologically-based luminosity estimates, we determine mass/light ratios in solar units of M/L_V=180+/-160 h for the entire population and 620+/-250 h for the spheroidal galaxies where the evolutionary effects can be best treated. We argue that this provides an upper bound to the local cluster M/L corresponding to Omega~0.4.

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arXiv1996

The Faint End of the Galaxy Luminosity Function In Moderate Redshift Clusters

We present deep two-colour photometry of two rich clusters at z=0.18, A665 and A1689. We use these data to construct number counts as a function of magnitude in the two fields. By combining these counts with similar observations from a large area field survey we subtract the field contamination statistically to produce luminosity functions for the two clusters. Great care has been taken to achieve agreement between the photometry of these two samples. The cluster data are complete to a limiting magnitude of I=22.5 or an absolute magnitude in the cluster of I= -18.0 ($M^\star+5$). The luminosity functions of both clusters are well described by a Gaussian function for the bright galaxies, combined with a Schechter function at the faint end, similar to that required to fit the luminosity function in local clusters. The slope at the faint end of the Schechter function in both clusters is extremely steep in V, alpha ~ -2. A shallower slope is seen to the limit of the I data, indicating that the cluster population is rapidly blueing as we reach fainter. The excellent agreement between the form of the luminosity function in our two distant clusters, as well as agreement with the luminosity function given by Driver et al (1994) for a single z=0.21 cluster, indicates that this faint blue population is a general constituent of distant clusters. We compare our results with those from studies of local clusters. Depending upon the degree of fading (or disruption) of these faint blue galaxies,we tentatively identify their remnants with the low surface brightness dwarf galaxies which are the dominant population in local clusters. We discuss the possible role of this population as the source of most of the X-ray gas in rich clusters.

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arXiv1996

The inferred redshift distribution of the faint blue excess

We infer the redshift distribution of the faint blue galaxy excess (FBE) at B=23.5 by subtracting the predicted distribution of giant/normal galaxies from the observed N(z) distribution for all types. This is possible because of the recent deep {\it Hubble Space Telescope} (HST) WFPC2 morphological number counts which have convincingly demonstrated that little evolution of the giant population is seen to B=26.0. The mean redshift of the FBE at B=23.5 is found to be _{FBE}=0.40 +/- 0.07 with upper and lower quartiles defined by z_{0.75}=0.58 +/- 0.05 and z_{0.25}=0.28 +/- 0.05, respectively. We compare this inferred FBE N(z) distribution to the predictions from three generic faint galaxy models: dwarf dominated (no evolution), pure luminosity evolution, and evolving dwarfs. The inferred FBE N(z) distribution strongly supports a hybrid evolving dwarf--rich model wherein a large population of dwarfs present at z=0.5 has subsequently faded to obscurity. The total integrated number density of dwarfs (down to M_{B}=-11) is estimated to be a factor of 20 times greater than that of E---Sc galaxies and the estimated fading to be 1.0 < Δm < 1.4 mags. Thus, the dwarf population is estimated to be responsible for ~30% of the luminosity density locally, rising to ~57% at z=0.5.

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arXiv1995

HST Observations of Giant Arcs: High Resolution Imaging Of Distant Field Galaxies.

We present HST imaging of eight spectroscopically-confirmed giant arcs, pairs and arclets. These objects have all been extensively studied from the ground and we demonstrate the unique advantages of HST imaging in the study of such features by a critical comparison of our data with the previous observations. In particular we present new estimates of the core radii of two clusters (Cl0024+16, A370) determined from lensed features which are identifiable in our HST images. Although our HST observations include both pre- and post-refurbishment images, the depth of the exposures guarantees that the majority of the arcs are detected with diffraction-limited resolution. A number of the objects in our sample are multiply-imaged and we illustrate the ease of identification of such features when working at high resolution. We discuss the morphological and scale information on these distant field galaxies in the light of HST studies of lower redshift samples. We conclude that the dominant population of star-forming galaxies at z=1 is a factor of 1.5-2 times smaller than the similar group in the local field. This implies either a considerable evolution in the sizes of star-forming galaxies within the last $\sim$10 Gyrs or a shift in the relative space densities of massive and dwarf star-forming systems over the same timescale.

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