The environment of "E+A" galaxies

The spectrum of an "E+A" galaxy (Dressier & Gunn) - which is dominated by a young stellar component but lacks the emission lines characteristic of any significant, on-going star formation-suggests that the galaxy experienced a brief, powerful starburst within the last gigayear (Dressler & Gunn; Couch & Sharples). In past work, this violent star formation history and the detection of these galaxies almost exclusively in distant clusters linked them to the Butcher-Oemler (B-O) effect (Butcher & Oemler) and argued for the influence of cluster environment in the evolution of galaxies. However, no statistical survey of the environments of "E+A"s had ever been made. From 11,113 galaxy spectra in the Las Campanas Redshift Survey (Shectman and coworkers), we have obtained a unique and well-defined sample of 21 nearby "E+A" galaxies with the same spectral characteristics as "E+A"s in distant clusters. These "E+A"s are selected to have the strongest Balmer absorption lines (the average of the equivalent widths of Hβ, γ, δ is > 5.5 Å) and weakest [O II] emission-line equivalent widths (<2.5 Å, which corresponds to a detection of [O II] of less than 2 σ significance) of any of the galaxies in the survey. In contrast to inferences drawn from previous studies, we find that a large fraction (∼75%) of nearby " E + A "s lie in the field, well outside of clusters and rich groups of galaxies. We conclude that interactions with the cluster environment, in the form of the intracluster medium or cluster potential, are not essential for "E+A" formation and therefore that the presence of these galaxies in distant clusters does not provide strong evidence for the effects of cluster environment on galaxy evolution. If one mechanism is responsible for "E+A" formation, then the observations that "E+A"s exist in the field and that at least five of the 21 in our sample have clear tidal features argue that galaxy-galaxy interactions and mergers are that mechanism. The most likely environments for such mergers are poor groups of galaxies, which have lower velocity dispersions than clusters and higher galaxy densities than the field. Groups are correlated with rich clusters and, in hierarchical models, fall into clusters in greater numbers at intermediate redshifts than they do today (cf. Bower; Lacey & Cole; Kauffmann). When combined with the strong evolution observed in the field population (cf. Broadhurst and coworkers; Lilly and coworkers), our work suggests that the B-O effect may reflect the typical evolution of galaxies in groups and in the field, rather than the influence of clusters on the star formation history of galaxies.