Extremely Large Telescope

Abstract:

Accurately determining the star formation history in NGC 4038/4039 -- ``The Antennae'' is hampered by extinction. We therefore used near infrared images obtained with ISAAC at the VLT and with SOFI at the NTT to determine the recent star formation history in this merger. In combination with archival HST data, we determined ages, extinction and other parameters for single star clusters, and properties of the cluster population as a whole. About 70% of the K_s-band detected star clusters with masses >= 10^5 M_sun are younger than 10 Myrs (approximately an e-folding time for cluster ages), which we interpret as evidence for rapid dissolution but not free expansion. The total mass of K-band selected clusters is about 5-10x10^8 M_sun and represents about 3-6% of the total molecular gas. This takes into account only the detected clusters and in view of the rapid dissolution means that this is only a lower limit to the total mass of stars produced in clusters during the burst. Studies of cluster formation in other galaxies recently suggested short cluster dissolution timescales, too, which means that star formation rates may have been severely underestimated in the past. Extinction is strongly variable and very high in some regions, but around A_V=1.3 mag on average. Even though most clusters are detected at least in I-band, only the information about individual cluster ages and extinction allows to avoid uncertainties of orders of magnitude in star formation rate estimates determined from optical fluxes. From the distribution of individual cluster extinction vs. age, which is significantly higher for clusters below 8-9 Myr than for older clusters, we infer that this is the time by which a typical cluster blows free of its native dust cocoon.

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.