arXiv subjects

Warrick J. Couch

Publications and source records attributed to Warrick J. Couch.

At least 91 records · Page 5Linked to original sources

arXiv2005

The Las Campanas/AAT Rich Cluster Survey III: Spectroscopic Studies of X-ray Bright Galaxy Clusters at z~0.1

[abridged] We present the analysis of the spectroscopic and photometric catalogues of 11 X-ray luminous clusters at z=0.07-0.16 from the Las Campanas / Anglo-Australian Telescope Rich Cluster Survey. Our spectroscopic dataset consists of over 1600 galaxy cluster members, of which two thirds are outside r_200. We assign cluster membership using a detailed mass model and expand on our previous work on the cluster colour-magnitude relation where membership was inferred statistically. We confirm that the modal colours of galaxies on the colour magnitude relation become progressively bluer with increasing radius and decreasing local galaxy density. Interpreted as an age effect, we hypothesize that these trends in galaxy colour should be reflected in mean Hdelta equivalent width. We confirm that passive galaxies in the cluster increase in Hdelta line strength as dHdelta / d r_p = 0.35 +/- 0.06. A variation of star formation rate, as measured by [OII], with increasing local density of the environment is discernible and is shown to be in broad agreement with previous studies from 2dFGRS and SDSS. We find that clusters at z~0.1 are less active than their higher redshift analogues. We also investigate unusual populations of blue and very red nonstarforming galaxies and we suggest that the former are likely to be the progenitors of galaxies which will lie on the colour-magnitude relation, while the colours of the latter possibly reflect dust reddening. The cluster galaxies at large radii consist of both backsplash ones and those that are infalling to the cluster for the first time. We make a comparison to the field population at z~0.1 and examine broad differences between the two populations. Individually, the clusters show significant variation in their galaxy populations which reflects their recent infall histories.

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arXiv2005

The 2dF Galaxy Redshift Survey: correlation with the ROSAT-ESO Flux Limited X-ray (REFLEX) galaxy cluster survey

The ROSAT-ESO Flux Limited X-ray (REFLEX) galaxy cluster survey and the 2dF Galaxy Redshift Survey (2dFGRS) respectively comprise the largest, homogeneous X-ray selected cluster catalogue and completed galaxy redshift survey. In this work we combine these two outstanding datasets in order to study the effect of the large-scale cluster environment, as traced by X-ray luminosity, on the properties of the cluster member galaxies. We measure the LX-sigma relation from the correlated dataset and find it to be consistent with recent results found in the literature. Using a sample of 19 clusters with LX>=0.36*10^44 erg s^-1 in the (0.1-2.4 keV) band, and 49 clusters with lower X-ray luminosity, we find that the fraction of early spectral type (eta<=-1.4), passively-evolving galaxies is significantly higher in the high-LX sample within R200. We extend the investigation to include composite bJ cluster luminosity functions, and find that the characteristic magnitude of the Schechter-function fit to the early-type luminosity function is fainter for the high-LX sample compared to the low-LX sample (Delta M*=0.58+/-0.14). This seems to be driven by a deficit of such galaxies with M_bJ ~ -21. In contrast, we find no significant differences between the luminosity functions of star-forming, late-type galaxies. We believe these results are consistent with a scenario in which the high-LX clusters are more dynamically evolved systems than the low-LX clusters.

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arXiv2004

Discovery of a Large-scale Wall in the Direction of Abell 22

We report on the discovery of a large-scale wall in the direction of Abell 22. Using photometric and spectroscopic data from the Las Campanas Observatory and Anglo-Australian Telescope Rich Cluster Survey, Abell 22 is found to exhibit a highly unusual and striking redshift distribution. We show that Abell 22 exhibits a foreground wall-like structure by examining the galaxy distributions in both redshift space and on the colour-magnitude plane. A search for other galaxies and clusters in the nearby region using the 2dF Galaxy Redshift Survey database suggests that the wall-like structure is a significant large-scale, non-virialized filament which runs between two other Abell clusters either side of Abell 22. The filament stretches over at least >40 Mpc in length and 10 Mpc in width at the redshift of Abell 22.

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arXiv2004

Cluster cannibalism and scaling relations of galactic stellar nuclei

Recently, very massive compact stellar systems have been discovered in the intracluster regions of galaxy clusters and in the nuclear regions of late-type disk galaxies. It is unclear how these compact stellar systems -- known as ``ultra-compact dwarf'' (UCD) galaxies or ``nuclear clusters'' (NCs) -- form and evolve. By adopting a formation scenario where these stellar systems are the product of multiple merging of star clusters in the central regions of galaxies, we investigate, numerically, their physical properties. We find that physical correlations between velocity dispersion, luminosity, effective radius, and average surface brightness in the stellar merger remnants are quite different from those observed in globular clusters. We also find that the remnants have triaxial shapes with or without figure rotation, and these shapes and their kinematics depend strongly on the initial number and distribution of the progenitor clusters. These specific predictions can be compared with the corresponding results of ongoing and future observations of UCDs and NCs, thereby providing a better understanding of the origin of these enigmatic objects.

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arXiv2004

The dwarf galaxy population in Abell 2218

We present results from a deep photometric study of the rich galaxy cluster Abell 2218 (z=0.18) based on archival HST WFPC2 F606W images. These have been used to derive the luminosity function to extremely faint limits (M_{F606W}=-13.2 mag, mu_{0}=24.7 mag arcsec^{-2}) over a wide field of view (1.3 h^{-2} Mpc^2). We find the faint-end slope of the luminosity function to vary with environment within the cluster, going from alpha=-1.23\pm0.13 within the projected central core of the cluster (100 < r < 300 h^{-1} kpc) to alpha=-1.49\pm 0.06 outside this radius (300 < r < 750 h^{-1} kpc). We infer that the core is 'dwarf depleted', and further quantify this by studying the ratio of 'dwarf' to 'giant' galaxies and its dependency as a function of cluster-centric radius and local galaxy density. We find that this ratio varies strongly with both quantities, and that the dwarf galaxy population in A2218 has a more extended distribution than the giant galaxy population.

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arXiv2004

The Stellar Populations of Low-redshift Clusters

We present some preliminary results from an on-going study of the evolution of stellar populations in rich clusters of galaxies. This sample contains core line-strength measurements from 183 galaxies with b_J <= 19.5 from four clusters with ~0.04. Using predictions from stellar population models to compare with our measured line strengths we can derive relative luminosity-weighted mean ages and metallicities for the stellar populations in each of our clusters. We also investigate the Mgb'-sigma and Hbeta_G'-sigma scaling relations. We find that, consistent with previous results, Mgb' is correlated with sigma, the likely explanation being that larger galaxies are better at retaining their heavier elements due to their larger potentials. Hbeta', on the other hand, we find to be anti-correlated with sigma. This result implies that the stellar populations in larger galaxies are older than in smaller galaxies.

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arXiv2003

Formation of star clusters in the LMC and SMC. I. Preliminary results on cluster formation from colliding gas clouds

We demonstrate that single and binary star clusters can be formed during cloud-cloud collisions triggered by the tidal interaction between the Large and Small Magellanic clouds. We run two different sets of self-consistent numerical simulations which show that compact, bound star clusters can be formed within the centers of two colliding clouds due to strong gaseous shocks, compression, and dissipation, providing the clouds have moderately large relative velocities (10-60 km s$^{-1}$). The impact parameter determines whether the two colliding clouds become a single or a binary cluster. The star formation efficiency in the colliding clouds is dependent upon the initial ratio of the relative velocity of the clouds to the sound speed of the gas. Based on these results, we discuss the observed larger fraction of binary clusters, and star clusters with high ellipticity, in the Magellanic clouds.

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arXiv2003

Recycling of ghost galaxies: The origin of giant HI ring around NGC 1533

We propose that the giant HI ring recently discovered by HIPASS for S0 galaxy NGC 1533 is formed by unequal-mass merging between gas-rich LSB (low surface brightness: ``ghost'') galaxies and HSB disks. The NGC 1533 progenitor HSB spiral is transformed into a barred S0 during merging and the outer HI gas disk of the LSB is transformed into the giant HI ring. We also discuss two different possibilities for the origin of isolated star-forming regions (``ELdot'' objects) in the giant gas ring.

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arXiv2003

Formation of globular clusters in galaxy mergers

Our numerical simulations first demonstrate that the pressure of ISM in a major merger becomes so high ($>$ $10^5$ $\rm k_{\rm B}$ K $\rm cm^{-3}$) that GMCs in the merger can collapse to form globular clusters (GCs) within a few Myr. The star formation efficiency within a GMC in galaxy mergers can rise up from a few percent to $\sim$ 80 percent, depending on the shapes and the temperature of the GMC. This implosive GC formation due to external high pressure of warm/hot ISM can be more efficient in the tidal tails or the central regions of mergers. The developed clusters have King-like profile with the effective radius of a few pc. The structural, kinematical, and chemical properties of these GC systems can depend on orbital and chemical properties of major mergers.

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arXiv2003

Starbursts from strong compression of galactic molecular clouds due to the high pressure of the intracluster medium

We demonstrate that the high pressure of the hot intracluster medium (ICM) can trigger the collapse of molecular clouds in a spiral galaxy, leading to a burst of star formation in the clouds. Our hydrodynamical simulations show that the high gaseous (ram pressure and static thermal) pressure of the ICM strongly compresses a self-gravitating gas cloud within a short time scale ($\sim$ $10^{7}$ yr), dramatically increasing the central gas density, and consequently causing efficient star formation within the cloud. The stars developed in the cloud form a compact, gravitationally bound, star cluster. The star formation efficiency within such a cloud is found to depend on the temperature and the density of the ICM and the relative velocity of the galaxy with respect to it. Based on these results, we discuss the origin of starburst/poststarburst populations observed in distant clusters, the enhancement of star formation for galaxies in merging clusters, and the isolated compact HII regions recently discovered in the Virgo cluster.

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arXiv2002

Passive spiral formation from halo gas starvation: Gradual transformation into S0s

Recent spectroscopic and high resolution $HST$-imaging observations have revealed significant numbers of ``passive'' spiral galaxies in distant clusters, with all the morphological hallmarks of a spiral galaxy (in particular, spiral arm structure), but with weak or absent star formation. Exactly how such spiral galaxies formed and whether they are the progenitors of present-day S0 galaxies is unclear. Based on analytic arguments and numerical simulations of the hydrodynamical evolution of a spiral galaxy's halo gas (which is a likely candidate for the source of gas replenishment for star formation in spirals), we show that the origin of passive spirals may well be associated with halo gas stripping. Such stripping results mainly from the hydrodynamical interaction between the halo gas and the hot intracluster gas. Our numerical simulations demonstrate that even if a spiral orbits a cluster with a pericenter distance $\sim$ 3 times larger than the cluster core radius, $\sim$ 80 % of the halo gas is stripped within a few Gyr and, accordingly, cannot be accreted by the spiral. Furthermore, our study demonstrates that this dramatic decline in the gaseous infall rate leads to a steady increase in the $Q$ parameter for the disk, with the spiral arm structure, although persisting, becoming less pronounced as the star formation rate gradually decreases. These results suggest that passive spirals formed in this way, gradually evolve into red cluster S0s.

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arXiv2002

An H-alpha survey of the rich cluster A1689

(Abridged) We present results of an H-alpha survey in the rich cluster A1689 at z=0.18, using the LDSS++ spectrograph on the AAT. We obtained spectra covering redshifted H-alpha for 522 galaxies brighter than I=22.5, covering a field of 8.7'X8.7'. We detect H-alpha emission in 46 of these galaxies; accounting for selection effects due to sampling and cluster membership, we determine that 24% of cluster members brighter than M_R=-16.5+5log(h) are detected with H-alpha flux greater than 4h^{-2}X 10^{38} ergs/s. This corresponds to a limiting star formation rate of 0.008 h^{-2} Msun/yr, assuming 1 magnitude of dust extinction. From a Hubble Space Telescope mosaic covering 7.5' X 10.0', we determine morphologies for 199 galaxies brighter than I=21, and find that 20% of the cluster members are of type Sa or later. More than 90% of cluster spirals show H-alpha emission, compared with less than 10% of E and S0 galaxies. The cluster H-alpha luminosity function has a low normalisation relative to the z~0.2 field, by ~50%, after accounting for the different fraction of spiral galaxies in the two environments. When compared with local field galaxies, this suggests that star formation activity is suppressed in early-type cluster galaxies, relative to their field counterparts. Our sample includes 29 galaxies previously observed with ISOCAM at 6 and 15 microns. We detect all 15-micron sources at H-alpha, so there is no evidence for any star formation completely hidden at H-alpha. Comparing the 15-micron and H-alpha fluxes, we find evidence that some mid-infrared-detected galaxies could be obscured by as much as 3 magnitudes of extinction at H-alpha, although this depends on the largely unknown contribution from any AGN-heated dust to the mid-infrared flux.

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arXiv2002

Spectrophotometric evolution of spiral galaxies with truncated star formation: An evolutionary link between spirals and S0s in distant clusters

A one-zone chemo-spectro-photometric model is used to investigate the time evolution of disk galaxies whose star formation is truncated, and to determine the dependence of this evolution on the previous star formation history and the truncation epoch. Truncated spirals show red colors $\sim 1$ Gyr after truncation, and evolve spectrally from an e(b) type, down through the e(a), a+k, and k+a classes, to finally become a k type. The exact behavior in this phase depends on the truncation epoch and the star formation history prior to truncation. For example, earlier type disks show redder colors and do not show a+k-type spectra after truncation of star formation. We also discuss a possible evolutionary link between the k-type galaxies with spiral morphology found in distant clusters, and present-day S0s, by investigating whether truncated spirals reproduce the infrared color-magnitude relation of Coma galaxies. We suggest that only less luminous, later-type disk galaxies whose star formation is truncated at intermediate and high redshifts can reproduce the red $I-K$ colors observed for S0s in the Coma cluster.

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arXiv2001

The Las Campanas/AAT Rich Cluster Survey II: The Environmental Dependance of Galaxy Colours in Clusters at z~0.1

(Abridged) We present a photometric investigation of the variation in galaxy colour with environment in 11 X-ray luminous clusters at 0.07<z<0.16 taken from the Las Campanas/AAT Rich Cluster Survey. We study the properties of the galaxy populations in individual clusters and take advantage of the homogeneity of the sample to combine the clusters together to investigate weaker trends in the composite sample. We find that modal colours of galaxies lying on the colour-magnitude relation in the clusters become bluer by d(B-R)/dr_p=-0.022 \pm 0.004 from the cluster core out to a projected radius of r_p=6 Mpc; further out in radius than any previous study.....

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arXiv2001

The Las Campanas/AAT Rich Cluster Survey I: Precision and Reliability of the Photometric Catalogue

The Las Campanas Observatory and Anglo--Australian Telescope Rich Cluster Survey (LARCS) is a panoramic imaging and spectroscopic survey of an X-ray luminosity-selected sample of 21 clusters of galaxies at 0.07<z<0.16. CCD imaging was obtained in B and R of typically 2-degree wide regions centred on the 21 clusters, and the galaxy sample selected from the imaging is being used for an on-going spectroscopic survey of the clusters with the 2dF spectrograph on the Anglo-Australian Telescope. This paper presents the reduction of the imaging data and the photometric analysis used in the survey. Based on an overlapping area of 12.3 square degrees, we compare the CCD-based LARCS catalogue with the photographic-based galaxy catalogue used for the input to the 2dF Galaxy Redshift Survey (2dFGRS) from the APM, to the completeness of the GRS/APM catalogue, b_J=19.45. This comparison confirms the reliability of the photometry across our mosaics and between the clusters in our survey. This comparison also provides useful information about the properties of the GRS/APM. The stellar contamination in the GRS/APM galaxy catalogue is confirmed to be around 5-10 percent, as originally estimated. However, using the superior sensitivity and spatial resolution in the LARCS survey, we find evidence for four distinct populations of galaxies that are systematically omitted from the GRS/APM catalogue. The characteristics of the `missing' galaxy populations are described, reasons for their absence examined and the impact they will have on the conclusions drawn from the 2dF Galaxy Redshift Survey are discussed.

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arXiv2001

The nature of the dwarf population in Abell 868

We present the results of a study of the morphology of the dwarf galaxy population in Abell 868, a rich, intermediate redshift (z=0.154) cluster which has a galaxy luminosity function with a steep faint-end slope (alpha=-1.26 +/- 0.05). A statistical background subtraction method is employed to study the B-R colour distribution of the cluster galaxies. This distribution suggests that the galaxies contributing to the faint-end of the measured cluster LF can be split into three populations: dIrrs with B-R<1.4; dEs with 1.4 2.5. The remvoal of the contribution of the background gEs from the counts only marginally lessens the faint-end slope (alpha=-1.22 +/- 0.16). However, the removal of the contribution of the dIrrs from the counts produces a flat LF (alpha=-0.91 +/- 0.16). The dEs and the dIrrs have similar spatial distributions within the cluster except that the dIrrs appear to be totally absent within a central projected radius of about 0.2 Mpc (Ho=75 km/s /Mpc). The number density of both dEs and dIrrs appear to fall off beyond a projected radius of about 0.35 Mpc. We suggest that the dE and dIrr populations of A868 have been associated with the cluster for similar timescales but that evolutionary processes such as `galaxy harassment' tend to fade the dIrr galaxies while having much less effect on the dE galaxies. The harassement would be expected to have the greatest effect on dwarfs residing in the central parts of the cluster.

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arXiv2001

A new formation model for M32: A threshed early-type spiral?

The origin of the closest compact elliptical galaxy (cE) M32 is a longstanding problem of galaxy formation in the Local Group. Our N-body/SPH simulations suggest a new scenario in which the strong tidal field of M31 can transform a spiral galaxy into a compact elliptical. As a low luminosity spiral galaxy plunges into the central region of M31, most of the outer stellar and gaseous components of its disk are dramatically stripped due to M31's tidal field The central bulge component, on the other hand, is just weakly influenced by the tidal field owing to its compact configuration, and retains its morphology. M31's strong tidal field also induces rapid gas transfer to the central region, triggers a nuclear starburst, and consequently forms the central high density and more metal-rich stellar populations with relatively young ages. Thus, in this scenario, M32 was previously the bulge of a spiral tidally interacting with M31 several Gyr ago. Furthermore, we suggest cEs like M32 are rare, the result of both the rather narrow parameter space for tidal interactions which morphologically transform spirals into cEs and the very short time scale ($<$ a few $10^9$ yr) for cEs to be swallowed.

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arXiv2001

Potential formation sites of super star clusters in ultra-luminous infrared galaxies

Recent observational results on high spatial resolution images of ultra-luminous infrared galaxies (ULIGs) have revealed very luminous, young, compact, and heavily obscured super star clusters in their central regions, suggested to be formed by gas-rich major mergers. By using stellar and gaseous numerical simulations of galaxy mergers, we firstly demonstrate that the central regions of ULIGs are the most promising formation sites of super star clusters owing to the rather high gaseous pressure of the interstellar medium. Based on simple analytical arguments, we secondly discuss the possibility that super star clusters in an ULIG can be efficiently transferred into the nuclear region owing to dynamical friction and consequently merge with one another to form a single compact stellar nucleus with a seed massive black hole. We thus suggest that multiple merging between super star clusters formed by nuclear starbursts in the central regions of ULIGs can result in the formation of massive black holes.

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