Early‐type galaxies in low‐density environments

We describe the construction and study of an objectively defined sample of early‐type galaxies in low‐density environments. The sample galaxies are selected from a recently completed redshift survey using uniform and readily quantified isolation criteria, and are drawn from a sky area of ∼700 deg 2 , to a depth of 7000 km s −1 and an apparent magnitude limit of b J ≤ 16.1 . Their early‐type (E/S0) morphologies are confirmed by subsequent CCD imaging. Five out of the nine sample galaxies show signs of morphological peculiarity such as tidal debris or blue circumnuclear rings. We confirm that E/S0 galaxies are rare in low‐density regions, accounting for only ≈8 per cent of the total galaxy population in such environments. We present spectroscopic observations of nine galaxies in the sample, which are used, in conjunction with updated stellar population models, to investigate star formation histories. Our line‐strength analysis is conducted at the relatively high spectral resolution of 4.1 Å. Environmental effects on early‐type galaxy evolution are investigated by comparison with a sample of Fornax cluster E/S0s (identically analysed). Results from both samples are compared with predictions from semi‐analytic galaxy formation models. From the strength of [O  ii ]λ3727 emission we infer only a low level of ongoing star formation (<0.15 M ⊙ yr −1 ) . Relative to the Fornax sample, a larger fraction of the galaxies exhibit [O  iii ]λ5007 nebular emission and, where present, these lines are slightly stronger than typical for cluster E/S0s. The Mg–σ relation of E/S0s in low‐density regions is shown to be indistinguishable from that of the Fornax sample. Luminosity‐weighted stellar ages and metallicities are determined by considering various combinations of line‐indices; in particular the Hγ F versus Fe5015 diagram cleanly resolves the age–metallicity degeneracy at the spectral resolution of our analysis. At a given luminosity, the E/S0 galaxies in low‐density regions are younger than the E/S0s in clusters (by ∼2–3 Gyr), and also more metal‐rich (by ≈0.2 dex). We infer that an anti‐correlation of age and metallicity effects is responsible for maintaining the zero‐point of the Mg–σ relation. The youngest galaxies in our sample show clear morphological signs of interaction. The lower mean age of our sample, relative to cluster samples, confirms, at least qualitatively, a robust prediction of hierarchical galaxy formation models. By contrast, the enhanced metallicity in the field is contrary to the predictions and highlights shortcomings in the detailed treatment of star formation processes in current models. The [Mg/Fe] abundance ratio appears to span a similar, mostly super‐solar, range both in low‐density regions and in Fornax cluster galaxies. This result is quite unexpected in simple hierarchical models.