Extremely Large Telescope

Abstract:

We compare deep Magellan spectroscopy of 26 groups at 0.3 ≤ z ≤ 0.55, selected from the Canadian Network for Observational Cosmology 2 field survey, with a large sample of nearby groups from the 2PIGG catalogue. We find that the fraction of group galaxies with significant [O II]λ3727 emission (≥5 Å) increases strongly with redshift, from ∼29 per cent in 2dFGRS to ∼58 per cent in CNOC2, for all galaxies brighter than ∼M* + 1.75. This trend is parallel to the evolution of field galaxies, where the equivalent fraction of emission-line galaxies increases from ∼53 to ∼75 per cent. The fraction of emission-line galaxies in groups is lower than in the field, across the full redshift range, indicating that the history of star formation in groups is influenced by their environment. We show that the evolution required to explain the data is inconsistent with a quiescent model of galaxy evolution; instead, discrete events in which galaxies cease forming stars (truncation events) are required. We constrain the probability of truncation (P trunc) and find that a high value is required in a simple evolutionary scenario neglecting galaxy mergers (Ptrunc ≳ 0.3 Gyr-1). However, without assuming significant density evolution, Ptrunc is not required to be larger in groups than in the field, suggesting that the environmental dependence of star formation was embedded at redshifts z ≳ 0.45. © 2005 RAS.

Abstract:

We investigate a spectroscopic sample of 48 early-type galaxies in the rich cluster Abell 2390 at z = 0.23 and 48 early-type galaxies from a previously published survey of Abell 2218 at z = 0.18. The spectroscopic data of A 2390 are based on multi-object spectroscopy using the multi-object spectrograph for Calar Alto at the 3.5-m telescope on Calar Alto Observatory and are complemented by ground-based imaging using the 5.1-m Hale telescope and Hubble Space Telescope (HST) observations in the F555W and F814W filters. Our investigation spans a broad range in luminosity (-20.5 ≥ Mr ≥ -23.0) and a rather wide field of view of 1.53 h70-1 × 1.53 h70-1 Mpc2. As the A 2218 and A2390 samples are very similar, we can combine them and analyse a total number of 96 early-type (E+S0) galaxies at z ∼ 0.2. Using the ground-based data only, we construct the Faber-Jackson relation (FJR) for all 96 E+S0 galaxies and detect a modest luminosity evolution with respect to the local reference. The average offset from the local FJR in the Gunn r band is Δ ̄Mr = 0.32 ± 0.22 mag. Similar results are derived for each cluster separately. Less massive galaxies show a trend for a larger evolution than more massive galaxies. HST/WFPC2 surface brightness profile fits were used to derive the structural parameters for a subsample of 34 E+S0 galaxies. We explore the evolution of the Fundamental Plane (FP) in Gunn r, its projections on to the Kormendy relation and the M/L ratios as a function of velocity dispersion. The FP for the cluster galaxies is offset from the local Coma cluster FP. At a fixed effective radius and velocity dispersion our galaxies are brighter than their local counterparts. For the total sample of 34 E+S0 cluster galaxies which enter the FP we deduce only a mild evolution with a zero-point offset of 0.10 ± 0.06, corresponding to a brightening of 0.31 ± 0.18 mag. Elliptical and lenticular galaxies are uniformly distributed along the FP with a similar scatter of 0.1 dex. Within our sample we find little evidence for differences between the populations of elliptical and S0 galaxies. There is a slight trend that lenticulars induce on average a larger evolution of 0.44 ± 0.18 mag than ellipticals with 0.02 ± 0.21 mag. The M/L ratios of our distant cluster galaxies at z = 0.2 are offset by Δlog (M/L r) = -0.12 ± 0.06 dex compared with those of Coma. Our results can be reconciled with a passive evolution of the stellar populations and a high formation redshift for the bulk of the stars in early-type galaxies. However, our findings are also consistent with the hierarchical formation picture for rich clusters, if ellipticals in clusters had their last major merger at high redshift. © 2005 RAS.

Abstract:

We use the 2dF Galaxy Redshift Survey to measure the dependence of the bJ-band galaxy luminosity function on large-scale environment, defined by density contrast in spheres of radius 8h-1 Mpc, and on spectral type, determined from principal component analysis. We find that the galaxy populations at both extremes of density differ significantly from that at the mean density. The population in voids is dominated by late types and shows, relative to the mean, a deficit of galaxies that becomes increasingly pronounced at magnitudes brighter than MbJ, - 5 log10 h ≲ -18.5. In contrast, cluster regions have a relative excess of very bright early-type galaxies with MbJ, - 5 log10 h ≲ -21. Differences in the mid- to faint-end population between environments are significant: at MbJ, -5 log10 h = -18 early- and late-type cluster galaxies show comparable abundances, whereas in voids the late types dominate by almost an order of magnitude. We find that the luminosity functions measured in all density environments, from voids to clusters, can be approximated by Schechter functions with parameters that vary smoothly with local density, but in a fashion that differs strikingly for early- and late-type galaxies. These observed variations, combined with our finding that the faint-end slope of the overall luminosity function depends at most weakly on density environment, may prove to be a significant challenge for models of galaxy formation.