Ultra-compact dwarf galaxies in galaxy clusters

We review the evidence that the ultra-compact dwarf (UCD) galaxies we recently discovered in the Fornax Cluster form a new, previously unknown class of galaxies and we discuss possible scenarios for their formation. We then present recent results that UCDs are also present in the Virgo Cluster, and that there is a much larger than expected population of fainter UCDs in the Fornax Cluster. The size and properties of this population may lead us to revise our original 'galaxy threshing' hypothesis for the formation of UCDs.

Diffuse optical light in galaxy clusters. I. Abell 3888

We are undertaking a program to measure the characteristics of the intracluster light ( ICL; total flux, profile, color, and substructure) in a sample of 10 galaxy clusters with a range of cluster mass, morphology, and redshift. We present here the methods and results for the first cluster in that sample, A3888. We have identified an ICL component in A3888 in V and r that contains 13% +/- 5% of the total cluster light and extends to 700 h(70)(-1) kpc (similar to 0.3r(200)) from the center of the cluster. The ICL color in our smallest radial bin is V - r 0.3 +/- 0.1, similar to the central cluster elliptical galaxies. The ICL is redder than the galaxies at 400 h(70)(-1) kpc < r < 700 h(70)(-1) kpc, although the uncertainty in any one radial bin is high. Based on a comparison of V - r color with simple stellar models, the ICL contains a component that formed more than 7 Gyr ago ( at z less than 1) with a high-metallicity ( 1.0 Z(circle dot) < Z(ICL) less than or similar to 2.5 Z(circle dot)) and a more centralized component that contains stars formed within the past 5 Gyr ( at z similar to 1). The profile of the ICL can be roughly fitted by a shallow exponential in the outer regions and a steeper exponential in the central region. We also find a concentration of diffuse light around a small group of galaxies 1.4 h(70)(-1) Mpc from the center of the cluster. In addition, we find three low surface brightness features near the cluster center that are blue ( V - r 0.0) and contain a total flux of 0.1M*. Based on these observations and X-ray and galaxy morphology, we suggest that this cluster is entering a phase of significant merging of galaxy groups in the core, whereupon we expect the ICL fraction to grow significantly with the formation of a cD galaxy, as well as the infall of groups.

The surface brightness and colour-magnitude relations for Fornax cluster galaxies

We present BVI photometry of 190 galaxies in the central 4 x 3 deg(2) region of the Fornax cluster observed with the Michigan Curtis Schmidt Telescope. Results from the Fornax Cluster Spectroscopic Survey (FCSS) and the Flair-II Fornax Surveys have been used to confirm the membership status of galaxies in the Fornax Cluster Catalogue (FCC). In our catalogue of 213 member galaxies, 92 (43 per cent) have confirmed radial velocities. In this paper, we investigate the surface brightness-magnitude relation for Fornax cluster galaxies. Particular attention is given to the sample of cluster dwarfs and the newly discovered ultracompact dwarf galaxies (UCDs) from the FCSS. We examine the reliability of the surface brightness-magnitude relation as a method for determining cluster membership and find that at surface brightnesses fainter than 22 mag arcsec(-2), it fails in its ability to distinguish between cluster members and barely resolved background galaxies. Cluster members exhibit a strong surface brightness-magnitude relation. Both elliptical (E) galaxies and dwarf elliptical (dE) galaxies increase in surface brightness as luminosity decreases. The UCDs lie off the locus of the relation. B-V and V-I colours are determined for a sample of 113 cluster galaxies and the colour-magnitude relation is explored for each morphological type. The UCDs lie off the locus of the colour-magnitude relation. Their mean V - I colours (similar to1.09) are similar to those of globular clusters associated with NGC 1399. The location of the UCDs on both surface brightness and colour-magnitude plots supports the 'galaxy threshing' model for infalling nucleated dwarf elliptical (dE, N) galaxies.

Galaxy threshing and ultra-compact dwarfs in the Fornax cluster

We have discovered a new type of galaxy in the Fornax Cluster: 'ultra-compact' dwarfs (UCDs). The UCDs are unresolved in ground-based imaging and have spectra typical of old stellar systems. Although the UCDs resemble overgrown globular clusters, based on VLT UVES echelle spectroscopy, they appear to be dynamically distinct systems with higher internal velocity dispersions and M/L ratios for a given luminosity than Milky Way or M31 globulars. Our preferred explanation for their origin is that they are the remnant nuclei of dwarf elliptical galaxies which have been tidally stripped, or 'threshed' by repeated encounters with the central cluster galaxy, NGC1399. If correct, then tidal stripping of nucleated dwarfs to form UCDs may, over a Hubble time, be an important source of the plentiful globular cluster population in the halo of NGC1399, and, by implication, other cD galaxies. In this picture, the dwarf elliptical halo contents, up to 99% of the original dwarf luminosity, contribute a significant fraction of the populations of intergalactic stars, globulars, and gas in galaxy clusters.

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 ultracompact dwarf (UCD) galaxies or nuclear clusters (NCs) - form and evolve. By adopting a formation scenario in which 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 among 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.

A comparison of surface brightness profiles for ultracompact dwarfs and dwarf elliptical nuclei: implications for the “threshing” scenario

Using imaging from the Hubble Space Telescope, we derive surface brightness profiles for ultracompact dwarfs in the Fornax Cluster and for the nuclei of dwarf elliptical galaxies in the Virgo Cluster. Ultracompact dwarfs are more extended and have higher surface brightnesses than typical dwarf nuclei, while the luminosities, colors, and sizes of the nuclei are closer to those of Galactic globular clusters. This calls into question the production of ultracompact dwarfs via threshing, whereby the lower surface brightness envelope of a dwarf elliptical galaxy is removed by tidal processes, leaving behind a bare nucleus. Threshing may still be a viable model if the relatively bright Fornax ultracompact dwarfs considered here are descended from dwarf elliptical galaxies whose nuclei are at the upper end of their luminosity and size distributions.

Galaxy threshing and the origin of ultra-compact dwarf galaxies in the Fornax cluster

A recent all-object spectroscopic survey centred on the Fornax cluster of galaxies has discovered a population of subluminous and extremely compact members, called 'ultra-compact dwarf' (UCD) galaxies. In order to clarify the origin of these objects, we have used self-consistent numerical simulations to study the dynamical evolution a nucleated dwarf galaxy would undergo if orbiting the centre of the Fornax cluster and suffering from its strong tidal gravitational field. We find that the outer stellar components of a nucleated dwarf are removed by the strong tidal field of the cluster, whereas the nucleus manages to survive as a result of its initially compact nature. The developed naked nucleus is found to have physical properties (e. g. size and mass) similar to those observed for UCDs. We also find that although this formation process does not have a strong dependence on the initial total luminosity of the nucleated dwarf, it does depend on the radial density profile of the dark halo in the sense that UCDs are less likely to be formed from dwarfs embedded in dark matter haloes with central 'cuspy' density profiles. Our simulations also suggest that very massive and compact stellar systems can be rapidly and efficiently formed in the central regions of dwarfs through the merging of smaller star clusters. We provide some theoretical predictions on the total number and radial number density profile of UCDs in a cluster and their dependencies on cluster masses.

On a resampling approach for tests on the number of clusters with mixture model-based clustering of tissue samples

We consider the problem of assessing the number of clusters in a limited number of tissue samples containing gene expressions for possibly several thousands of genes. It is proposed to use a normal mixture model-based approach to the clustering of the tissue samples. One advantage of this approach is that the question on the number of clusters in the data can be formulated in terms of a test on the smallest number of components in the mixture model compatible with the data. This test can be carried out on the basis of the likelihood ratio test statistic, using resampling to assess its null distribution. The effectiveness of this approach is demonstrated on simulated data and on some microarray datasets, as considered previously in the bioinformatics literature. (C) 2004 Elsevier Inc. All rights reserved.

Respiratory gene clusters of Metallosphaera sedula - differential expression and transcriptional organization

Metallosphaera sedula is a thermoacidophilic Crenarchaeon which is capable of leaching metals from sulfidic ores. The authors have investigated the presence and expression of genes encoding respiratory complexes in this organism when grown heterotrophically or chemolithotrophically on either sulfur or pyrite. The presence of three gene clusters, encoding two terminal oxidase complexes, the quinol oxidase SoxABCD and the SoxM oxidase supercomplex, and a gene cluster encoding a high-potential cytochrome b and components of a bc(1) complex analogue (cbsBA-soxL2N gene cluster) was established. Expression studies showed that the soxM gene was expressed to high levels during heterotrophic growth of M. sedula on yeast extract, while the soxABCD mRNA was most abundant in cells grown on sulfur. Reduced-minus-oxidized difference spectra of cell membranes showed cytochrome-related peaks that correspond to published spectra of Sulfolobus-type terminal oxidase complexes. In pyrite-grown cells, expression levels of the two monitored oxidase gene clusters were reduced by a factor of 10-12 relative to maximal expression levels, although spectra of membranes clearly contained oxidase-associated haems, suggesting the presence of additional gene clusters encoding terminal oxidases in M. sedula. Pyrite- and sulfur-grown cells contained high levels of the cbsA transcript, which encodes a membrane-bound cytochrome b with a possible role in iron oxidation or chemolithotrophy. The cbsA gene is not co-transcribed with the soxL2N genes, and therefore does not appear to be an integral part of this bc(1) complex analogue. The data show for the first time the differential expression of the Sulfolobus-type terminal oxidase gene clusters in a Crenarchaeon in response to changing growth modes.

Analysis of the acute postoperative pain experience following oral surgery: identification of 'unaffected', 'disabled' and 'depressed, anxious and disabled' patient clusters

Background: Pain is defined as both a sensory and an emotional experience. Acute postoperative tooth extraction pain is assessed and treated as a physiological (sensory) pain while chronic pain is a biopsychosocial problem. The purpose of this study was to assess whether psychological and social changes Occur in the acute pain state. Methods: A biopsychosocial pain questionnaire was completed by 438 subjects (165 males, 273 females) with acute postoperative pain at 24 hours following the surgical extraction of teeth and compared with 273 subjects (78 males, 195 females) with chronic orofacial pain. Statistical methods used a k-means cluster analysis. Results: Three clusters were identified in the acute pain group: 'unaffected', 'disabled' and 'depressed, anxious and disabled'. Psychosocial effects showed 24.8 per cent feeling 'distress/suffering' and 15.1 per cent 'sad and depressed'. Females reported higher pain intensity and more distress, depression and inadequate medication for pain relief (p

Clusters of internally primed transcripts reveal novel long noncoding RNAs

Non- protein- coding RNAs ( ncRNAs) are increasingly being recognized as having important regulatory roles. Although much recent attention has focused on tiny 22- to 25- nucleotide microRNAs, several functional ncRNAs are orders of magnitude larger in size. Examples of such macro ncRNAs include Xist and Air, which in mouse are 18 and 108 kilobases ( Kb), respectively. We surveyed the 102,801 FANTOM3 mouse cDNA clones and found that Air and Xist were present not as single, full- length transcripts but as a cluster of multiple, shorter cDNAs, which were unspliced, had little coding potential, and were most likely primed from internal adenine- rich regions within longer parental transcripts. We therefore conducted a genome- wide search for regional clusters of such cDNAs to find novel macro ncRNA candidates. Sixty- six regions were identified, each of which mapped outside known protein- coding loci and which had a mean length of 92 Kb. We detected several known long ncRNAs within these regions, supporting the basic rationale of our approach. In silico analysis showed that many regions had evidence of imprinting and/ or antisense transcription. These regions were significantly associated with microRNAs and transcripts from the central nervous system. We selected eight novel regions for experimental validation by northern blot and RT- PCR and found that the majority represent previously unrecognized noncoding transcripts that are at least 10 Kb in size and predominantly localized in the nucleus. Taken together, the data not only identify multiple new ncRNAs but also suggest the existence of many more macro ncRNAs like Xist and Air.

Redshifts and velocity dispersions of galaxy clusters in the horologium-reticulum supercluster

We present 118 new optical redshifts for galaxies in 12 clusters in the Horologium-Reticulum supercluster (HRS) of galaxies. For 76 galaxies, the data were obtained with the Dual Beam Spectrograph on the 2.3 m telescope of the Australian National University at Siding Spring Observatory. After combining 42 previously unpublished redshifts with our new sample, we determine mean redshifts and velocity dispersions for 13 clusters in which previous observational data were sparse. In 6 of the 13 clusters, the newly determined mean redshifts differ by more than 750 km s(-1) from the published values. In three clusters, A3047, A3109, and A3120, the redshift data indicate the presence of multiple components along the line of sight. The new cluster redshifts, when combined with other reliable mean redshifts for clusters in the HRS, are found to be distinctly bimodal. Furthermore, the two redshift components are consistent with the bimodal redshift distribution found for the intercluster galaxies in the HRS by Fleenor and coworkers.

Ross River virus disease clusters and spatial relationship with mosquito biting exposure in Redland Shire, southern Queensland, Australia

The spatial heterogeneity in the risk of Ross River virus (family Togaviridae, genus Alphavirus, RRV) disease, the most common mosquito-borne disease in Australia, was examined in Redland Shire in southern Queensland, Australia. Disease cases, complaints from residents of intense mosquito biting exposure, and human population data were mapped using a geographic information system. Surface maps of RRV disease age-sex standardized morbidity ratios and mosquito biting complaint morbidity ratios were created. To determine whether there was significant spatial variation in disease and complaint patterns, a spatial scan analysis method was used to test whether the number of cases and complaints was distributed according to underlying population at risk. Several noncontiguous areas in proximity to productive saline water habitats of Aedes vigilax (Skuse), a recognized vector of RRV, had higher than expected numbers of RRV disease cases and complaints. Disease rates in human populations in areas which had high numbers of adult Ae. vigilax in carbon dioxide- and octenol-baited light traps were up to 2.9 times those in areas that rarely had high numbers of mosquitoes. It was estimated that targeted control of adult Ae. vigilax in these high-risk areas could potentially reduce the RRV disease incidence by an average of 13.6%. Spatial correlation was found between RRV disease risk and complaints from residents of mosquito biting. Based on historical patterns of RRV transmission throughout Redland Shire and estimated future human population growth in areas with higher than average RRV disease incidence, it was estimated that RRV incidence rates will increase by 8% between 2001 and 2021. The use of arbitrary administrative areas that ranged in size from 4.6 to 318.3 km2, has the potential to mask any small scale heterogeneity in disease patterns. With the availability of georeferenced data sets and high-resolution imagery, it is becoming more feasible to undertake spatial analyses at relatively small scales.

The Las Campanas/Anglo-Australian Telescope Rich Cluster Survey - III. Spectroscopic studies of X-ray bright galaxy clusters at z similar to 0.1

We present the analysis of the spectroscopic and photometric catalogues of 11 X-ray luminous clusters at 0.07 < z < 0.16 from the Las Campanas/Anglo-Australian Telescope Rich Cluster Survey. Our spectroscopic data set consists of over 1600 galaxy cluster members, of which two-thirds are outside r(200). These spectra allow us to assign cluster membership using a detailed mass model and expand on our previous work on the cluster colour-magnitude relation ( CMR) where membership was inferred statistically. We confirm that the modal colours of galaxies on the CMR become progressively bluer with increasing radius d( B - R)/dr(p) = - 0.011 +/- 0.003 and with decreasing local galaxy density d( B - R)/dlog ( Sigma)= - 0.062 +/- 0.009. Interpreted as an age effect, we hypothesize that these trends in galaxy colour should be reflected in mean H delta equivalent width. We confirm that passive galaxies in the cluster increase in Hd line strength as dH delta/dr(p) = 0.35 +/- 0.06. Therefore, those galaxies in the cluster outskirts may have younger luminosity-weighted stellar populations; up to 3 Gyr younger than those in the cluster centre assuming d( B - R)/dt = 0.03 mag per Gyr. A variation of star formation rate, as measured by [ O II]lambda 3727 angstrom, with increasing local density of the environment is discernible and is shown to be in broad agreement with previous studies from the 2dF Galaxy Redshift Survey and the Sloan Digital Sky Survey. We divide our spectra into a variety of types based upon the MORPHs classification scheme. We find that clusters at z similar to 0.1 are less active than their higher-redshift analogues: about 60 per cent of the cluster galaxy population is non-star forming, with a further 20 per cent in the post-starburst class and 20 per cent in the currently active class, demonstrating that evolution is visible within the past 2 - 3 Gyr. We also investigate unusual populations of blue and very red non-star forming galaxies and we suggest that the former are likely to be the progenitors of galaxies which will lie on the CMR, while the colours of the latter possibly reflect dust reddening. We show that 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 similar to 0.1 and examine the broad differences between the two populations. Individually, the clusters show significant variation in their galaxy populations which we suggest reflects their recent infall histories.