We have assembled a catalogue of relative ages, metallicities and abundance ratios for about 150 local galaxies in field, group and cluster environments. The galaxies span morphological types from cD and ellipticals, to late‐type spirals. Ages and metallicities were estimated from high‐quality published spectral line indices using Worthey & Ottaviani (1997) single stellar population evolutionary models. The identification of galaxy age as a fourth parameter in the fundamental plane (Forbes, Ponman & Brown 1998) is confirmed by our larger sample of ages. We investigate trends between age and metallicity, and with other physical parameters of the galaxies, such as ellipticity, luminosity and kinematic anisotropy. We demonstrate the existence of a galaxy age–metallicity relation similar to that seen for local galactic disc stars, whereby young galaxies have high metallicity, while old galaxies span a large range in metallicities. We also investigate the influence of environment and morphology on the galaxy age and metallicity, especially the predictions made by semi‐analytic hierarchical clustering models (HCM). We confirm that non‐cluster ellipticals are indeed younger on average than cluster ellipticals as predicted by the HCM models. However we also find a trend for the more luminous galaxies to have a higher [Mg/Fe] ratio than the lower luminosity galaxies, which is opposite to the expectation from HCM models.

The colours and magnitudes of early‐type galaxies in galaxy clusters are strongly correlated. The existence of such a correlation has been used to infer that early‐type galaxies must be old passively evolving systems. Given the dominance of early‐type galaxies in the cores of rich clusters, this view sits uncomfortably with the increasing fraction of blue galaxies found in clusters at intermediate redshifts, and with the late formation of galaxies favoured by cold dark matter type cosmologies. In this paper, we make a detailed investigation of these issues and examine the role that the colour–magnitude relation can play in constraining the formation history of galaxies currently found in the cores of rich clusters. We start by considering the colour evolution of galaxies after star formation ceases. We show that the scatter of the colour–magnitude relation places a strong constraint on the spread in age that is allowed for the bulk of the stellar population. In the extreme case that the stars are formed in a single event, the spread in age cannot be more than 4 Gyr. Although the bulk of stars must be formed in a short period, continuing formation of stars in a fraction of the galaxies is not so strongly constrained. We examine a model in which star formation occurs over an extended period of time in most galaxies with star formation being truncated randomly. This model is consistent with the formation of stars in a few systems until look‐back times of ∼ 5 Gyr. An extension of this type of star formation history allows us to reconcile the small present‐day scatter of the colour–magnitude relation with the observed blue galaxy fractions of intermediate redshift galaxy clusters. In addition to setting a limit on the variations in luminosity‐weighted age between the stellar populations of cluster galaxies, the colour–magnitude relation can also be used to constrain the degree of merging between pre‐existing stellar systems. This test relies on the slope of the colour–magnitude relation: mergers between galaxies of unequal mass tend to reduce the slope of the relation and to increase its scatter. We show that random mergers between galaxies very rapidly remove any well‐defined colour–magnitude correlation. This model is not physically motivated, however, and we prefer to examine the merger process using a self‐consistent merger tree. In such a model there are two effects. First, massive galaxies preferentially merge with systems of similar mass. Secondly, the rate of mass growth is considerably smaller than for the random merger case. As a result of both of these effects, the colour–magnitude correlation persists through a larger number of merger steps. The passive evolution of galaxy colours and their averaging in dissipationless mergers provide opposing constraints on the formation of cluster galaxies in a hierarchical model. At the level of current constraints, a compromise solution appears possible. The bulk of the stellar population must have formed before z = 1, but cannot have formed in mass units much less tha...

Hickson Compact Groups (HCGs) constitute an interesting extreme in the range of environments in which galaxies are located, as the space density of galaxies in these small groups are otherwise only found in the centres of much larger clusters. The work presented here uses Lick indices to make a comparison of ages and chemical compositions of galaxies in HCGs with those in other environments (clusters, loose groups and the field). The metallicity and relative abundance of 'α-elements' show strong correlations with galaxy age and central velocity dispersion, with similar trends found in all environments. However, we show that the previously reported correlation between α-element abundance ratios and velocity dispersion disappears when a full account is taken of the the abundance ratio pattern in the calibration stars. This correlation is thus found to be an artifact of incomplete calibration to the Lick system.Variations are seen in the ranges and average values of age, metallicity and α-element abundance ratios for galaxies in different environments. Age distributions support the hierarchical formation prediction that field galaxies are on average younger than their cluster counterparts. However, the ages of HCG galaxies are shown to be more similar to those of cluster galaxies than those in the field, contrary to the expectations of current hierarchical models. A trend for lower velocity dispersion galaxies to be younger was also seen. This is again inconsistent with hierarchical collapse models, but is qualitatively consistent with the latest N-body-SPH models based on monolithic collapse in which star formation continues for many Gyr in low mass halos.

We present a new photometric catalogue of the Coma galaxy cluster in the Johnson U and V bands. We cover an area of 3360 arcmin2 of sky, to a depth of in a 13‐arcsec diameter aperture, and produce magnitudes for ∼1400 extended objects in metric apertures from 8.8‐ to 26‐arcsec diameters. The mean internal rms scatter in the photometry is 0.014 mag in V, and 0.026 mag in U, for . We place new limits on the levels of scatter in the colour–magnitude relation (CMR) in the Coma cluster, and investigate how the slope and scatter of the CMR depend on galaxy morphology, luminosity and position within the cluster. As expected, the lowest levels of scatter are found in the elliptical galaxies, while the late‐type galaxies have the highest numbers of galaxies bluewards of the CMR. We investigate whether the slope of the CMR is an artefact of colour gradients within galaxies, and show that it persists when the colours are measured within a diameter that scales with galaxy size. Looking at the environmental dependence of the CMR, we find a trend of systematically bluer galaxy colours with increasing projected radius from the centre of the cluster. Surprisingly, this is accompanied by a decreased scatter of the CMR. We investigate whether this gradient could be caused by dust in the cluster potential, however the reddening required would produce too large a scatter in the colours of the central galaxies. The gradient appears to be better reproduced by a gradient in the mean galactic ages with projected radius.

We have derived ages, metallicities and enhanced‐element ratios [α/Fe] for a sample of 83 early‐type galaxies essentially in groups, the field or isolated objects. The stellar‐population properties derived for each galaxy correspond to the nuclear re/8 aperture extraction. The median age found for Es is 5.8±0.6 Gyr and the average metallicity is +0.37±0.03 dex. For S0s, the median age is 3.0±0.6 Gyr and [Z/H]= 0.53±0.04 dex. We compare the distribution of our galaxies in the Hβ‐[MgFe] diagram with Fornax galaxies. Our elliptical galaxies are 3–4 Gyr younger than Es in the Fornax cluster. We find that the galaxies lie in a plane defined by [Z/H]= 0.99 log σ0− 0.46 log(age) − 1.60, or in linear terms Z ∝σ0× (age) −0.5. More massive (larger σ0) and older galaxies present, on average, large [α/Fe] values, and therefore must have undergone shorter star‐formation time‐scales. Comparing group against field/isolated galaxies, it is not clear that environment plays an important role in determining their stellar‐population history. In particular, our isolated galaxies show ages differing by more than 8 Gyr. Finally we explore our large spectral coverage to derive log (O/H) metallicity from the Hα and N iiλ6584 and compare it with model‐dependent [Z/H]. We find that the O/H abundances are similar for all galaxies, and we can interpret it as if most chemical evolution has already finished in these galaxies.