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 H delta absorption equivalent widths, W-o(H delta) > 4 angstrom. 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 H alpha or [OII]lambda 3727 emission (known as k + a galaxies) are predominantly bulge-dominated ( with a mode of B/T similar to 0.6), while galaxies with nebular emission [known as e( a) galaxies] are mostly disc-dominated (B/T similar to 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 H delta 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 (tau(V) similar to 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.