Stellar populations in superclusters of galaxies

A catalogue of superclusters of galaxies is used to investigate the influence of the supercluster environment on galaxy populations, considering galaxies brighter than M$_r<$-21+5$\log$ h. Empirical spectral synthesis techniques are applied to obtain the stellar population properties of galaxies which belong to superclusters and representative values of stellar population parameters are attributed to each supercluster. We show that richer superclusters present denser environments and older stellar populations. The galaxy populations of superclusters classified as filaments and pancakes are statistically similar, indicating that the morphology of superclusters does not have a significative influence on the stellar populations. Clusters of galaxies within superclusters are also examined in order to evaluate the influence of the supercluster environment on their galaxy properties. Our results suggest that the environment affects galaxy properties but its influence should operate on scales of groups and clusters, more than on the scale of superclusters.Comment: 7 pages, 4 figures; accepted to MNRA

How Stochastic is the Relative Bias Between Galaxy Types?

Examining the nature of the relative clustering of different galaxy types can help tell us how galaxies formed. To measure this relative clustering, I perform a joint counts-in-cells analysis of galaxies of different spectral types in the Las Campanas Redshift Survey (LCRS). I develop a maximum-likelihood technique to fit for the relationship between the density fields of early- and late-type galaxies. This technique can directly measure nonlinearity and stochasticity in the biasing relation. At high significance, a small amount of stochasticity is measured, corresponding to a correlation coefficient of about 0.87 on scales corresponding to 15 Mpc/h spheres. A large proportion of this signal appears to derive from errors in the selection function, and a more realistic estimate finds a correlation coefficient of about 0.95. These selection function errors probably account for the large stochasticity measured by Tegmark & Bromley (1999), and may have affected measurements of very large-scale structure in the LCRS. Analysis of the data and of mock catalogs shows that the peculiar geometry, variable flux limits, and central surface-brightness selection effects of the LCRS do not seem to cause the effect.Comment: 38 pages, 14 figures. Submitted to Apj. Modified from a chapter of my Ph.D. Thesis at Princeton University, available at http://www-astro-theory.fnal.gov/Personal/blanton/thesis/index.htm

AUTOMATED MORPHOLOGICAL CLASSIFICATION OF APM GALAXIES BY SUPERVISED ARTIFICIAL NEURAL NETWORKS

We train Artificial Neural Networks to classify galaxies based solely on the morphology of the galaxy images as they appear on blue survey plates. The images are reduced and morphological features such as bulge size and the number of arms are extracted, all in a fully automated manner. The galaxy sample was first classified by 6 independent experts. We use several definitions for the mean type of each galaxy, based on those classifications. We then train and test the network on these features. We find that the rms error of the network classifications, as compared with the mean types of the expert classifications, is 1.8 Revised Hubble Types. This is comparable to the overall rms dispersion between the experts. This result is robust and almost completely independent of the network architecture used.Comment: The full paper contains 25 pages, and includes 22 figures. It is available at ftp://ftp.ast.cam.ac.uk/pub/hn/apm2.ps . The table in the appendix is available on request from [email protected]. Mon. Not. R. Astr. Soc., in pres

A moving cold front in the intergalactic medium of A3667

We present results from a Chandra observation of the central region of the galaxy cluster A3667, with emphasis on the prominent sharp X-ray brightness edge spanning 0.5 Mpc near the cluster core. Our temperature map shows large-scale nonuniformities characteristic of the ongoing merger, in agreement with earlier ASCA results. The brightness edge turns out to be a boundary of a large cool gas cloud moving through the hot ambient gas, very similar to the "cold fronts" discovered by Chandra in A2142. The higher quality of the A3667 data allows the direct determination of the cloud velocity. At the leading edge of the cloud, the gas density abruptly increases by a factor of 3.9+-0.8, while the temperature decreases by a factor of 1.9+-0.2 (from 7.7 keV to 4.1 keV). The ratio of the gas pressures inside and outside the front shows that the cloud moves through the ambient gas at near-sonic velocity, M=1+-0.2 or v=1400+-300 km/s. In front of the cloud, we observe the compression of the ambient gas with an amplitude expected for such a velocity. A smaller surface brightness discontinuity is observed further ahead, ~350 kpc in front of the cloud. We suggest that it corresponds to a weak bow shock, implying that the cloud velocity may be slightly supersonic. Given all the evidence, the cold front appears to delineate the remnant of a cool subcluster that recently has merged with A3667. The cold front is remarkably sharp. The upper limit on its width, 3.5 arcsec or 5 kpc, is several times smaller than the Coulomb mean free path. This is a direct observation of suppression of the transport processes in the intergalactic medium, most likely by magnetic fields.Comment: Submitted to ApJ. 9 pages with embedded color figures, uses emulateapj5. Postscript with higher quality figures is available at http://hea-www.harvard.edu/~alexey/a3667-hydro.ps.g

Semi-empirical analysis of Sloan Digital Sky Survey galaxies: II. The bimodality of the galaxy population revisited

We revisit the bimodal distribution of the galaxy population commonly seen in the local universe. Here we address the bimodality observed in galaxy properties in terms of spectral synthesis products, such as mean stellar ages and stellar masses, derived from the application of this powerful method to a volume-limited sample, with magnitude limit cutoff M_r = -20.5, containing about 50 thousand luminous galaxies from the SDSS Data Release 2. In addition, galaxies are classified according to their emission line properties in three distinct spectral classes: star-forming galaxies, with young stellar populations; passive galaxies, dominated by old stellar populations; and, hosts of active nuclei, which comprise a mix of young and old stellar populations. We show that the extremes of the distribution of some galaxy properties, essentially galaxy colours, 4000 A break index, and mean stellar ages, are associated to star-forming galaxies at one side, and passive galaxies at another. We find that the mean light-weighted stellar age of galaxies is the direct responsible for the bimodality seen in the galaxy population. The stellar mass, in this view, has an additional role since most of the star-forming galaxies present in the local universe are low-mass galaxies. Our results also give support to the existence of a 'downsizing' in galaxy formation, where massive galaxies seen nowadays have stellar populations formed at early times.Comment: 18 pages, 19 figures, accepted for publication in MNRA

Bayesian `Hyper-Parameters' Approach to Joint Estimation: The Hubble Constant from CMB Measurements

Recently several studies have jointly analysed data from different cosmological probes with the motivation of estimating cosmological parameters. Here we generalise this procedure to take into account the relative weights of various probes. This is done by including in the joint \chi^2 function a set of `Hyper-Parameters', which are dealt with using Bayesian considerations. The resulting algorithm (in the case of uniform priors on the log of the Hyper-Parameters) is very simple: instead of minimising \sum \chi_j^2 (where \chi_j^2 is per data set j) we propose to minimise \sum N_j \ln (\chi_j^2) (where N_j is the number of data points per data set j). We illustrate the method by estimating the Hubble constant H_0 from different sets of recent CMB experiments (including Saskatoon, Python V, MSAM1, TOCO and Boomerang).Comment: submitted to MNRAS, 6 pages, Latex, with 3 figures embedde