Peculiar early-type galaxies in the Sloan Digital Sky Survey Stripe82

Abstract

We explore the properties of 'peculiar' early-type galaxies (ETGs) in the local Universe that show (faint) morphological signatures of recent interactions such as tidal tails, shells and dust lanes. Standard-depth (∼51-s exposure) multicolour galaxy images from the Sloan Digital Sky Survey (SDSS) are combined with the significantly (∼2 mag) deeper monochromatic images from the public SDSS Stripe82 to extract, through careful visual inspection, a robust sample of nearby (z <0.05), luminous (M <-20.5) ETGs, including a subset of ∼70 peculiar systems. ∼18 per cent of ETGs exhibit signs of disturbed morphologies (e.g. shells), while ∼7 per cent show evidence of dust lanes and patches. An analysis of optical emission-line ratios indicates that the fraction of peculiar ETGs that are Seyferts or LINERs (19.4 per cent) is twice the corresponding values in their relaxed counterparts (10.1 per cent). LINER-like emission is the dominant type of nebular activity in all ETG classes, plausibly driven by stellar photoionization associated with recent star formation. An analysis of ultraviolet-optical colours indicates that, regardless of the luminosity range being considered, the fraction of peculiar ETGs that have experienced star formation in the last Gyr is a factor of ∼1.5 higher than that in their relaxed counterparts. The spectrophotometric results strongly suggest that the interactions that produce the morphological peculiarities also induce low-level recent star formation which, based on the recent literature, are likely to contribute a few per cent of the stellar mass over the last ∼1 Gyr. Peculiar ETGs preferentially inhabit low-density environments (outskirts of clusters, groups or the field), either due to high peculiar velocities in clusters making merging unlikely or because shell systems are disrupted through frequent interactions within a cluster crossing time. The catalogue of galaxies that forms the basis of this paper can be obtained at http://www.mssl.ucl.ac.uk/~ska/stripe82/skavirajstripe82.dat or on request from the author.