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Showing papers by "Warrick J. Couch published in 2000"


Journal ArticleDOI
TL;DR: The morphological types of galaxies in nine distant clusters in the redshift range 0.1 z 0.25 are derived from very good seeing images taken at the NOT and the La Silla-Danish telescopes, with all galaxies at MV < -20 and within the central ~1 Mpc2 area being classified.
Abstract: The morphological types of galaxies in nine clusters in the redshift range 0.1 z 0.25 are derived from very good seeing images taken at the NOT and the La Silla-Danish telescopes, with all galaxies at MV < -20 and within the central ~1 Mpc2 area being classified. With the purpose of investigating the evolution of the fraction of different morphological types with redshift, we compare our results with the morphological content of nine distant clusters studied by the MORPHS group, five clusters observed with HST/WFPC2 at redshift z = 0.2-0.3, and Dressler's large sample of nearby clusters. After having checked the reliability of our morphological classification both in an absolute sense and relative to the MORPHS scheme, we analyze the relative occurrence of elliptical, S0, and spiral galaxies as a function of the cluster properties and redshift. We find a large intrinsic scatter in the S0/E ratio, mostly related to the cluster morphology. In particular, in our cluster sample, clusters with a high concentration of ellipticals display a low S0/E ratio and, vice versa, low concentration clusters have a high S0/E. At the same time, the trend of the morphological fractions (%E's, %S0's, %Sp's) and of the S0/E and S0/Sp ratios with redshift clearly points to a morphological evolution: as the redshift decreases, the S0 population tends to grow at the expense of the spiral population, whereas the frequency of E's remains almost constant. We also analyze the morphology-density (MD) relation in our clusters and find that—similarly to higher redshift clusters—a good MD relation exists in the high-concentration clusters, while it is absent in the less concentrated clusters. Finally, the comparison of the MD relation in our clusters with that of the MORPHS sample suggests that the transformation of spirals into S0 galaxies becomes more efficient with decreasing local density.

321 citations


Journal ArticleDOI
TL;DR: The results of a wide-field survey for H-alpha emitting galaxies in the cluster AC114 at z = 0.32 were presented in this paper, showing that only ~10% of these galaxies were found to be H-α emitting cluster members.
Abstract: 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.

90 citations


Journal ArticleDOI
TL;DR: In this article, the authors combine morphological classifications from deep Hubble Space Telescope (HST) imaging of a sample of three clusters at z = 0.31 and a further nine clusters at 0.37-0.56 with existing spectroscopic observations of their elliptical (E) and S0 populations, to study the relative spectral properties of these two galaxy types.
Abstract: We combine morphological classifications from deep Hubble Space Telescope (HST) imaging of a sample of three clusters at z = 0.31 and a further nine clusters at z = 0.37-0.56 with existing spectroscopic observations of their elliptical (E) and S0 populations, in order to study the relative spectral properties of these two galaxy types. We have also used spectroscopic and imaging data in the Coma Cluster as a present-day example of a rich cluster environment with which to compare our data at higher redshift. These data span the range in which strong evolution is claimed in the proportion of S0 galaxies within rich clusters. Techniques have recently been developed to analyze the strengths of absorption lines in the spectra of local, passive galaxies, to separate the effects of age and metallicity in the spectra and hence date the ages of the most recent substantial star formation episode in these galaxies. We show that the spectroscopic data in the distant clusters is of sufficient quality to allow us to apply these techniques and use them to determine the relative ages for the E and S0 populations in these distant clusters. We then compare the "ages" for each type in order to search for the signature of the recent formation of the bulk of the S0 population. We find no statistically significant difference between the luminosity-weighted ages of the E and S0 galaxies in these clusters. We translate this into a limit such that no more than half of the galaxies in the clusters at z = 0.31 have undergone a burst of star formation (>11% by mass) in the 1 Gyr prior to the observations. Our results, in conjunction with other work, suggest that the progenitors of the S0 galaxies in rich clusters are mostly early-type spirals that, through interactions with the cluster environment, have had their star formation truncated. This indicates a relatively unspectacular origin for the missing S0 population.

51 citations


Journal ArticleDOI
TL;DR: In this article, the authors combine the 2MASS extended source catalogue and the 2dFGRS to produce an IR selected galaxy catalogue with 17,173 measured redshifts and use this extensive dataset to estimate the J and K-band galaxy luminosity functions.
Abstract: We combine the 2MASS extended source catalogue and the 2dFGRS to produce an IR selected galaxy catalogue with 17,173 measured redshifts. We use this extensive dataset to estimate the J and K-band galaxy luminosity functions. The LFs are fairly well fit by Schechter functions with J: M*-5log h= -22.36+/-0.02, alpha= -0.93+/-0.04, Phi=0.0104+/-0.0016 h^3/Mpc^3 and K: M*-5log h= -23.44+/-0.03, alpha=-0.96+/-0.05, Phi=0.0108+/-0.0016 h^3/Mpc^3 (2MASS Kron magnitudes). These parameters assume a cosmological model with Omega=0.3 and Lambda=0.7. With datasets of this size, systematic rather than random errors are the dominant source of uncertainty in the determination of the LF. We carry out a careful investigation of possible systematic effects in our data. The surface brightness distribution of the sample shows no evidence that significant numbers of low surface brightness or compact galaxies are missed by the survey. We estimate the present-day distributions of B-K and J-K colours as a function of absolute magnitude and use models of the galaxy stellar populations, constrained by the observed optical and infrared colours, to infer the galaxy stellar mass function. Integrated over all galaxy masses, this yields a total mass fraction in stars (in units of the critical mass density) of Omega_*.h= (1.6+/-0.24)/10^3 for a Kennicutt IMF and Omega_*.h= (2.9+/-0.43)/10^3 for a Salpeter IMF. These values agree with those inferred from observational estimates of the star formation history of the universe provided that dust extinction corrections are modest.

2 citations


Book ChapterDOI
TL;DR: In this article, the first results of a 3D search for galaxy clusters based on known 2D compilations were presented, where 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.
Abstract: 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.