Showing papers by "Warrick J. Couch published in 2008"

The 2dF-SDSS LRG and QSO Survey: evolution of the clustering of luminous red galaxies since z= 0.6

Abstract: We present an analysis of the small-to-intermediate scale clustering of samples of luminous red galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS) and the 2dF-SDSS LRG and QSO Survey (2SLAQ) survey carefully matched to have the same rest-frame colours and luminosity. We study the spatial two-point autocorrelation function in both redshift space [xi(s)] and real space [xi(r)] of a combined sample of over 10 000 LRGs, which represent the most massive galaxies in the universe with stellar masses > 10(11) h(-1) M-circle dot and space densities similar or equal to 10(-4) h(3) Mpc(-3). We find no significant evolution in the amplitude (r(0)) of the correlation function with redshift, but do see a slight decrease in the slope (gamma) with increasing redshift over 0.19 99.9 per cent significance) with a passive model whereby the LRGs do not merge with one another; a model with a merger rate of 7.5 +/- 2.3 per cent from z = 0.55 to 0.19 (i.e. an average rate of 2.4 per cent Gyr(-1)) provides a better fit to our observations. Our clustering and number density measurements are consistent with the hypothesis that the merged LRGs were originally central galaxies in different haloes which, following the merger of these haloes, merged to create a single brightest cluster galaxy. In addition, we show that the small-scale clustering signal constrains the scatter in halo merger histories. When combined with measurements of the luminosity function, our results suggest that this scatter is sub-Poisson. While this is a generic prediction of hierarchical models, it has not been tested before.

The 2dF-SDSS LRG and QSO survey: evolution of the clustering of luminous red galaxies since z = 0.6

Abstract: We present an analysis of the small-to-intermediate scale clustering of samples of Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey and the 2dF-SDSS LRG and QSO (2SLAQ) survey carefully matched to have the same rest-frame colours and luminosity. We study the spatial two-point auto-correlation function in both redshift-space and real-space of a combined sample of over 10,000 LRGs, which represent the most massive galaxies in the universe with stellar masses > 10^11 h^-1 M_sun and space densities 10^-4 h^-3 Mpc^-3. We find no significant evolution in the amplitude r_0 of the correlation function with redshift, but do see a slight decrease in the slope with increasing redshift over 0.19 99.9% significance) with a passive model whereby the LRGs do not merge with one another; a model with a merger rate of 7.5 +/- 2.3% from z = 0.55 to z = 0.19 (i.e. an average rate of 2.4% Gyr^-1) provides a better fit to our observations. Our clustering and number density measurements are consistent with the hypothesis that the merged LRGs were originally central galaxies in different haloes which, following the merger of these haloes, merged to create a single Brightest Cluster Galaxy. In addition, we show that the small-scale clustering signal constrains the scatter in halo merger histories. When combined with measurements of the luminosity function, our results suggest that this scatter is sub-Poisson. While this is a generic prediction of hierarchical models, it has not been tested before.

WINGS-SPE Spectroscopy in the WIde-field Nearby Galaxy-cluster Survey

Abstract: Aims. We present the results from a comprehensive spectroscopic survey of the WINGS (WIde-field Nearby Galaxy-cluster Survey) clusters, a program called WINGS-SPE. The WINGS-SPE sample consists of 48 clusters, 22 of which are in the southern sky and 26 in the north. The main goals of this spectroscopic survey are: (1) to study the dynamics and kinematics of the WINGS clusters and their constituent galaxies, (2) to explore the link between the spectral properties and the morphological evolution in different density environments and across a wide range in cluster X-ray luminosities and optical properties. Methods. Using multi object fiber fed spectrographs, we observed our sample of WINGS cluster galaxies at an intermediate resolu- tion of 6-9 A and, using a cross-correlation technique, we measured redshifts with a mean accuracy of about 45 km/s. Results. We present redshift measurements for 6137 galaxies and their first analyses. Details of the spectroscopic observations are reported. The WINGS-SPE has about 30% overlap with previously published data sets, allowing us to do both a complete comparison with the literature and to extend the catalogs. Conclusions. Using our redshifts, we calculate the velocity dispersion for all the clusters in the WINGS-SPE sample. We almost trip- licate the number of member galaxies known in each cluster with respect to previous works. We also investigate the X-ray luminosity vs. velocity dispersion relation for our WINGS-SPE clusters, and find it to be consistent with the form Lx proportional to sigma^4.

The luminosity-halo mass relation for brightest cluster galaxies

Abstract: We examine the central galaxy luminosity–host halo mass relation for 54 brightest group galaxies (BGGs) and 92 brightest cluster galaxies (BCGs) at z < 0.1 and present the first measurement of this relation for a sample of known BCGs at . At z < 0.1 we find LK∝M0.24±0.08200 for the BCGs and the early-type BGGs in groups with extended X-ray emission and LK∝M0.11±0.10200 for the BCGs alone. At 0.1 < z < 0.8 we find LK∝M0.28±0.11200. We conclude that there is no evidence for evolution in this relationship between z < 0.1 and z < 0.8: BCG growth appears to still be limited by the time-scale for dynamical friction at these earlier times, not proceeding according to the predictions of present semi-analytic models.

The luminosity -- halo-mass relation for brightest cluster galaxies

Abstract: We examine the central-galaxy luminosity -- host-halo mass relation for 54 Brightest Group Galaxies (BGGs) and 92 Brightest Cluster Galaxies (BCGs) at z<0.1 and present the first measurement of this relation for a sample of known BCGs at 0.1

The Fundamental Plane of Early-Type Galaxies in Nearby Clusters from the WINGS Database

Abstract: By exploting the data of three large surveys (WINGS, NFPS, and SDSS), we analyze the fundamental plane (FP) of early-type galaxies (ETGs) in 59 nearby clusters ( -->0.04 < z < 0.07). We show that the variances of the FP coefficients for our clusters are just marginally consistent with the hypothesis of universality of the FP. We found they are influenced by the distribution of photometric/kinematic properties of galaxies in the particular sample under analysis, suggesting that the FP is actually a bent surface. We also find a strong correlation between the local density and the FP coefficients, while they appear to be poorly correlated with the global properties of clusters. The relation between luminosity and mass of our galaxies, computed by assuming Sersic luminosity profiles, indicates that, for a given mass, the greater the light concentration, the higher the luminosity, while, for a given luminosity, the lower the light concentration, the greater the mass. Moreover, the M/L versus mass relation (again with Sersic profile fitting) turns out to be steeper and broader than that obtained for the Coma Cluster sample with de Vaucouleurs profile fitting. This broadness, together with the FP bending, might reconcile the FP phenomenology with the expectations from the ΛCDM cosmology. We conclude that the claimed universality of the FP of ETGs is still far from being proved and that systematic biases might affect the studies of luminosity evolution of ETGs, since data sets at different redshifts and with different distributions of the photometric/kinematic galaxy properties are compared each other.

The wiggleZ dark energy survey

Abstract: The accelerating expansion of the universe, attributed to "dark energy", has no accepted theoretical explanation. The origin of this phenomenon unambiguously implicates new physics via a novel form of matter exerting negative pressure or an alteration to Einstein's general relativity. These profound consequences have inspired a new generation of cosmological surveys that will measure the influence of dark energy using various techniques. One of the forerunners is the WiggleZ Survey at the Anglo-Australian Telescope, a new large-scale high-redshift galaxy survey that is now 50% complete and scheduled to finish in 2010. The WiggleZ project is aiming to map the cosmic expansion history using delicate features in the galaxy clustering pattern imprinted 13.7 billion years ago. In this article we outline the survey design and context, and predict the science highlights.

The effects of stellar populations on galaxy scaling relations in the 6dF Galaxy Survey

Abstract: We present an analysis of the stellar populations as a function of mass in a sample of ∼7000 galaxies of all morphological and emission types from the 6dF Galaxy Survey. We measure velocity dispersions and Lick indices from the spectra of the central regions of these galaxies, deriving ages and metallicities from the Lick indices using stellar population models. We also derive dynamical masses and dynamical mass-to-light ratios for these galaxies by combining the central velocity dispersions with global photometry in the B, R and K bands from Super-COSMOS and 2MASS. Together, these data allow us to reduce the degeneracies between age, metallicity and star formation burst strength that have limited previous studies. We find that old galaxies exhibit a mass-metallicity relation with logarithmic slope d[Fe/H]/d logM ≈ 0.25, while young galaxies show slopes consistent with zero. When we account for the effects of the mass-metallicity relation, we obtain a single, consistent relation between mass-to-light ratio and mass for old galaxies in all passbands, M/L α M 0.15 . As we have accounted for stellar population effects, this remaining variation in the mass-to-light ratio with mass (the residual 'tilt' of the Fundamental Plane) must have a dynamical origin. However, we demonstrate that any simple trend between mass-to-light ratio and mass or luminosity is inconsistent with the observations, and that a more complex relationship must exist. We find that the central regions of galaxies of all masses often exhibit young stellar populations. However, it is only in the lowest mass galaxies studied (∼ 10 10 M ⊙ ) that these populations are evident in the global photometry. In higher mass galaxies, young central populations have decreasing influence on the global photometry, with there being no discernible impact in galaxies more massive than ∼2 x 10 11 M ⊙ . We conclude that the young stellar populations detected in spectroscopic studies are generally centrally concentrated, and that there is an upper limit on the mass of star-forming events in massive galaxies. These results have important ramifications for mass-to-light ratios estimated from photometric observations.

Abell 1201: The anatomy of a cold front cluster from combined optical and X-ray data

Abstract: We present a combined X-ray and optical analysis of the cold front cluster Abell 1201 using archival Chandra, data and multi-object spectroscopy taken with the 3.9m Anglo Australian and 6.5m Multiple Mirror Telescopes. This paper represents the first in a series presenting a study of a sample of cold front clusters selected from the Chandra, archives with the aim of relating cold fronts to merger activity, understanding the dynamics of mergers and their effect on the cluster constituents. The Chandra X-ray imagery of Abell 1201 reveals two conspicuous surface brightness discontinuities, which are shown to be cold fronts, and a remnant core structure. Temperature maps reveal a complex multi-phase temperature structure with regions of hot gas interspersed with fingers of cold gas. Our optical analysis is based on a sample of 321 confirmed members, whose mean redshift is z=0.1673 +/- 0.0002 and velocity dispersion is 778 +/- 36 km/s. We search for dynamical substructure and find clear evidence for multiple localized velocity substructures coincident with over-densities in the galaxy surface density. Most notably, we find structure coincident with the remnant X-ray core. Despite the clear evidence for dynamical activity, we find the peculiar velocity distribution does not deviate significantly from Gaussian. We apply two-body dynamical analyses in order to assess which of the substructures are bound, and thus dynamically important in terms of the cluster merger history. We propose that the cold fronts in Abell 1201 are a consequence of its merger with a smaller subunit, which has induced gas motions that gave rise to `sloshing' cold fronts. Abell 1201 illustrates the value of combining multi-wavelength data and multiple substructure detection techniques when attempting to ascertain the dynamical state of a cluster.

Substructure in the cold front cluster Abell 3667

Abstract: We present evidence for the existence of significant substructure in the cold front cluster Abell 3667 based on multi-object spectroscopy taken with the 3.9m Anglo Australian Telescope. This paper is the second in a series analyzing the relationship between cold fronts observed in Chandra X-ray images and merger activity observed at optical wavelengths. We have obtained 910 galaxy redshifts in the field of Abell 3667 out to 3.5 Mpc, of which 550 are confirmed cluster members, more than doubling the number of spectroscopically confirmed members previously available and probing some 3 mag down the luminosity function. From this sample, we derive a cluster redshift of z=0.0553 +/-0.0002 and velocity dispersion of 1056 +/- 38 km/s and use a number of statistical tests to search for substructure. We find significant evidence for substructure in the spatial distribution of member galaxies and also in the localized velocity distributions and, in spite of this evidence, find the global velocity distribution does not deviate significantly from a Gaussian. Using combined spatial and velocity information, we find the cluster can be separated into two major structures, with roughly equal velocity dispersions, but offset in peculiar velocity from each other by ~500 km/s, and a number of minor substructures. We propose two scenarios which explain the radio and X-ray observations. Our data show the cold front is directly related to cluster merger activity, and also highlights the extent of optical data required to unambiguously detect the presence of substructure.

The effects of stellar populations on galaxy scaling relations in the 6dF Galaxy Survey

Abstract: We present an analysis of the stellar populations in a sample of ~7000 galaxies from the 6dF Galaxy Survey (6dFGS). We derive ages and metallicities using stellar population models. We also derive dynamical masses and dynamical mass-to-light ratios by combining central velocity dispersions with global photometry in B, R and K bands. Together, these data allow to reduce the degeneracies between age, metallicity and star formation burst-strength that have limited previous studies. We find old galaxies exhibit a mass-metallicity relation with slope d[Fe/H]/dlogM = 0.25, while young galaxies show slopes consistent with zero. When we account for the effects of the mass-metallicity relation, we obtain a single, consistent relation between mass-to-light ratio and mass for old galaxies in all passbands. As we have accounted for stellar population effects, this relation must have a dynamical origin. However, we demonstrate that any simple trend between mass-to-light-ratio and mass or luminosity is inconsistent with the observations, and that a more complex relationship must exist. We find the central regions of galaxies often exhibit young stellar populations. However it is only in the lowest-mass galaxies (~10^{10} M$_{\odot}$) that these populations are evident in the global photometry. In higher-mass galaxies, young central populations have decreasing influence on the global photometry, with there being no discernible impact in galaxies more massive than ~2x10^{11} M$_{\odot}$. We conclude that the young stellar populations detected in spectroscopic studies are generally centrally concentrated, and that there is an upper limit on the mass of star-forming events in massive galaxies. These results have ramifications for mass-to-light ratios estimated from photometric observations.

The 2dF-SDSS LRG and QSO Survey: The spectroscopic QSO catalogue

Abstract: We present the final spectroscopic QSO catalogue from the 2dF-SDSS LRG and QSO (2SLAQ) Survey. This is a deep, 18