Environment and galaxy evolution at intermediate redshift in the CNOC2 survey

The systematic variation of galaxy colors and types with clustering environment could either be the result of local conditions at formation or subsequent environmental effects as larger scale structures draw together galaxies whose stellar mass is largely in place. Below redshift 0.7 galaxy luminosities (k-corrected and evolution compensated) are relatively invariant, whereas galaxy star formation rates, as reflected in their colors, are a "transient" property that have a wide range for a given luminosity. The relations between these galaxy properties and the clustering properties are key statistics for understanding the forces driving late-time galaxy evolution. At z0.4 the comoving galaxy correlation length, r0, measured in the CNOC2 sample is strongly color dependent, rising from 2 h-1 Mpc to nearly 10 h-1 Mpc as the volume-limited subsamples range from blue to red. The luminosity dependence of r0 at z0.4 is weak below L* in the R band, although there is an upturn at high luminosity, where its interpretation depends on separating it from the r0-color relation. In the B band there is a slow, smooth increase of r0 with luminosity, at least partially related to the color dependence. Study of the evolution of galaxies within groups, which create much of the strongly nonlinear correlation signal, allows a physical investigation of the source of these relations. The dominant effect of the group environment on star formation is seen in the radial gradient of the mean galaxy colors, which on the average become redder than the field toward the group centers. The color differentiation begins around the dynamical radius of virialization of the groups. The redder-than-field trend applies to groups with a line-of-sight velocity dispersion, 1>150 km s-1. There is an indication, somewhat statistically insecure, that the high-luminosity galaxies in groups with 1150 km s-1 groups is very low, whereas for 1<150 km s-1 about 25% of the galaxies will merge in 0.5 Gyr. We conclude that the higher velocity dispersion groups largely act to suppress star formation relative to the less clustered field, leading to "embalmed" galaxies. On the other hand, the low velocity dispersion groups are prime sites of both strong merging and enhanced star formation that leads to the formation of some new massive galaxies at intermediate redshifts. The tidal fields within the groups appear to be a strong candidate for the physical source of the reduction of star formation in group galaxies relative to field. Tides operate effectively at all velocity dispersions to remove gas-rich companions and low-density gas in galactic halos. We find a close resemblance of the color-dependent galaxy luminosity function evolution in the field and groups, suggesting that the clustering-dependent star formation reduction mechanism is important for the evolution of field galaxies as a whole

Galaxy groups at intermediate redshift

Galaxy groups likely to be virialized are identified within the CNOC2 intermediate-redshift galaxy survey. The resulting groups have a median velocity dispersion, 1200 km s-1. The virial masstolight ratios, using k-corrected and evolution-compensated luminosities, have medians in the range of 150250 h M/L, depending on group definition details. The numbervelocity dispersion relation at 1200 km s-1 is in agreement with the low-mass extrapolation of the cluster-normalized Press-Schechter model. Lower velocity dispersion groups are deficient relative to the Press-Schechter model. The two-point group-group autocorrelation function has r0=6.8±0.3 h-1 Mpc, which is much larger than the correlations of individual galaxies, but about as expected from biased clustering. The mean number density of galaxies around group centers falls nearly as a power law with r-2.5 and has no well-defined core. The projected velocity dispersion of galaxies around group centers is either flat or slowly rising outward. The combination of a steeper than isothermal density profile and the outward rising velocity dispersion implies that the mass-to-light ratio of groups rises with radius if the velocity ellipsoid is isotropic but could be nearly constant if the galaxy orbits are nearly circular. Such strong tangential anisotropy is not supported by other evidence. Although the implication of a rising M/L must be viewed with caution, it could naturally arise through dynamical friction acting on the galaxies in a background of "classical" collisionless dark matter

The Galaxy Correlation Function in the CNOC2 Redshift Survey: Dependence on Color, Luminosity, and Redshift

We examine how the spatial correlation function of galaxies from the CNOC2 Field Galaxy Redshift Survey depends on galaxy color, luminosity and redshift. The projected correlation function w_p is determined for volume-limited samples of objects with 0.12 < z < 0.51 and evolution-compensated Rc absolute magnitudes M < -20, over the comoving projected separation range 0.04 Mpc/h < r_p < 10 Mpc/h. Our sample consists of 2937 galaxies which are classified as being either early- or late-type objects according to their spectral energy distribution (SED), determined from UBVRcIc photometry. For simplicity, galaxy SEDs are classified independently of redshift: our classification scheme therefore does not take into account the colour evolution of galaxies.Comment: 37 pages, 14 figures. To appear in The Astrophysical Journa

The CNOC2 Field Galaxy Redshift Survey. I. The Survey and the Catalog for the Patch CNOC 0223+00

The Canadian Network for Observational Cosmology (CNOC2) Field Galaxy Redshift Survey is a spectroscopic/photometric survey of faint galaxies over 1.5 square degrees of sky with a nominal spectroscopic limit of R_c=21.5 mag. The primary goals of the survey are to investigate the evolution of galaxy clustering and galaxy populations over the redshift range of approximately 0.1 to 0.6. The survey area contains four widely separated patches on the sky with a total spectroscopic sample of over 6000 redshifts and a photometric sample of over 40,000 galaxies with 5-color photometry. We describe the survey and observational strategies, multi-object spectroscopy mask design procedure, and data reduction techniques for creating the spectroscopic-photometric catalogs. We also discuss the derivations of various statistical weights for the redshift sample which allow it to be used as a complete sample. As the initial release of the survey data, we present the data set and some statistics for the Patch CNOC0223+00.Comment: 28 pages, AAS-Latex version 5.0, 15 eps figures. Accepted for publication by the Astrophysical Journal Supplemen

Weak-Lensing Study of Low-Mass Galaxy Groups: Implications for Omegam

We report on the first measurement of the average mass and mass-to-light ratio of galaxy groups by analyzing the weak-lensing signal induced by these systems. The groups, which have velocity dispersions of 50-400 km s(-1), have been selected from the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2). This survey allows the identification of a large number of groups with redshifts ranging from z = 0.12 to 0.55, ideal for a weak-lensing analysis of their mass distribution. For our analysis we use a sample of 50 groups that are selected on the basis of a careful dynamical analysis of group candidates. We detect a signal at the 99% confidence limit. The best-fit singular isothermal sphere model yields an Einstein radius. r(E) = 0".72 +/- 0".29. This corresponds to a velocity dispersion of [sigma (2)](1/2) = 274(-59)(+48) km s(-1) (using photometric redshift distributions for the source galaxies), which is in good agreement with the dynamical estimate. Under the assumption that the light traces the mass, we find an average mass-to-light ratio of 191 +/- 83 h (M)./L-B. in the rest-frame B band. Unlike dynamical estimates, this result is insensitive to problems associated with determining group membership. After correction of the observed mass-to-light ratio for luminosity evolution to z = 0, we find 254 +/- 110 h M./L-B., lower than what is found for rich clusters. We use the observed mass-to-light ratio to estimate the matter density of the universe, for which we find Omega (m) = 0.19 +/- 0.10 (Omega (Lambda) = 0), in good agreement with other recent estimates. For a closed universe (Omega (m) + Omega (Lambda) = 1), we obtain Omega (m) = 0.13 +/- 0.07

Results on Galaxy Evolution from the CNOC2 Field Galaxy Redshift Survey

The CNOC2 Field Galaxy Redshift Survey presently contains some 5000 galaxy redshifts, plus extensive UBgRI photometry, and is the largest galaxy sample at moderate redshifts 0.1 < z < 0.6. Here we present some preliminary results on the galaxy luminosity function (LF) and its redshift evolution, using a sample of R < 21.5 CNOC2 galaxies, subdivided into early, intermediate, and late types based on their B-R colors relative to non-evolving galaxy models. We find a significant steepening in the faint-end slope alpha of the LF as one proceeds from early to late types. Also, for all galaxy types we find a rate of M* evolution consistent with that from passively evolving galaxy models. Finally, late-type galaxies show positive density evolution with redshift, in contrast to negative or no density evolution for earlier types

The CNOC2 Field Galaxy Redshift Survey

Fundamental to the understanding of the universe is the evolution of structures, from galaxies to clusters of galaxies to large-scale sheets and filaments of galaxies and voids. The CNOC2 (Canadian Network for Observational Cosmology) Field Galaxy Redshift Survey is the first large redshift survey of faint galaxies carried out with the explicit goal of investigating the evolution of large scale structure. This survey also provides the largest redshift and photometric data set currently available for the study of galaxy population and evolution at the moderate redshift range between 0.1 and 0.6. In this paper, the authors describe the scope and technique of the survey, its status, and some preliminary results