Near-infrared imaging of 222 nearby Hδ-strong galaxies from the Sloan Digital Sky Survey

We present UFTI K-band imaging observations of 222 galaxies that are selected from the Sloan Digital Sky Survey to have unusually strong Hδ absorption equivalent widths, W_∘(Hδ) > 4 Å. Using GIM2D, the images are fitted with two-dimensional surface-brightness models consisting of a simple disc and bulge component to derive the fraction of luminosity in the bulge B/T. We find that the galaxies with weak or absent Hα or [OII]λ3727 emission (known as k+a galaxies) are predominantly bulge-dominated (with a mode of B/T~ 0.6), while galaxies with nebular emission [known as e(a) galaxies] are mostly disc-dominated (B/T~ 0.1). The morphologies and (r-k) colours of most k+a galaxies are inconsistent with the hypothesis that they result from the truncation of star formation in normal, spiral galaxies. However, their (u-g) and (r-k) colours, as well as their Hδ line strengths, form a sequence that is well matched by a model in which >5 per cent of the stellar mass has been produced in a recent starburst. The lack of scatter in the dust-sensitive (r-k) colours suggests that the unusual spectra of k+a galaxies are not due to the effects of dust. The e(a) galaxies, on the other hand, have a colour distribution that is distinct from the k+a population, and typical of normal or dusty (τ_V~ 2) spiral galaxies. We conclude that many e(a) galaxies are not progenitors of k+a galaxies, but are a separate phenomenon. Both k+a and e(a) galaxies reside in environments (characterized by the local density of galaxies brighter than M_r=-20) that are typical of normal galaxies and that are inconsistent with overdense regions like rich galaxy clusters.