The GALEX UV emission in shell galaxies: tracing galaxy "rejuvenation" episodes

We present the Galaxy Evolution Explorer (GALEX) far-ultraviolet (FUV) and near-ultraviolet (NUV) imaging of three nearby shell galaxies, namely NGC 2865, NGC 5018 and NGC 7135 located in low-density environments. The system of shells and fine structures visible in the optical is detected in the NUV image of NGC 2865 and in both NUV and FUV images of NGC 7135. The NUV image of NGC 5018 does not present shell structures. We detect absorption features in the nuclear region of all three galaxies. NGC 2865 has a nearly flat colour profile with (FUV-NUV) ~ 2 throughout the whole galaxy. NGC 7135 is blue in the centre (FUV-NUV) ~ 0 and as red as (FUV-NUV) ~ 1.5 in the outskirts, including the faint shell-like feature. The three shell galaxies are members of poor groups of galaxies. We compare GALEX NUV observations with available HI large-scale measurements, and determine the UV magnitudes of likely companions. Most of the known (and possible) companions are gas-rich late-type galaxies, suggesting that our shell galaxies inhabit the ideal environment for hosting rejuvenating episodes driven by accretion events. We investigate the ability of the nuclear GALEX (FUV-NUV) colour to provide information about rejuvenation phenomena in the stellar populations of the shell galaxies. To this aim, we derive from theory the relationship between the Mg2, Hβ, HγA, HδA Lick line-strength indices and the (FUV-NUV) colour. We extend the study to a sample of early-type galaxies in low-density environments which includes shell galaxies and/or galaxies with emission lines in their optical spectra. In the index versus (FUV-NUV) colour diagrams, most of the galaxies are well explained by passively evolving single stellar populations. On the average, ages and metallicities of the galaxies in our sample estimated from optical line-strength indices are consistent with those inferred from the (FUV-NUV) colour. We note that the GALEX (FUV-NUV) and (UV-V) colours have different response to age and metallicity. In general, all the colours but for (FUV-NUV) and (FUV-V), become nearly age-insensitive when 1-2 Gyr have elapsed from the last star-forming event. Finally, considering composite stellar population models with a recent burst of star formation, we suggest that the positions of the NGC 7135 and NGC 2865 nuclei in the (FUV-NUV)-Hβ plane could be explained in terms of a recent rejuvenation episode.