Tracing Galaxy Evolution in the Field: the UV Emission in Shell Galaxies Mapped by GALEX

Abstract

Nearby early-type galaxies (ETGs) showing shells inhabit low density regions. Whatever their origin is, either a weak interaction [1] or a merging/accretion between galaxies [2, 3], the event should have triggered a secondary burst of star formation. We imaged with Galaxy Evolution Explorer (GALEX) in far (FUV) and near (NUV) ultraviolet three nearby shell galaxies, namely NGC 2865, NGC 5018 and NGC 7135, whose optical line-strength indices [4] suggest that they host recent (≤2 Gyr) star formation. Their FUV emission is always confined in the innermost regions while the NUV emission shows fine structures very similar to those detected in the optical band. Remarkable in NGC 7135 is the coincidence between the optical and NUV fine structures which give to the galaxy a “medusa-like” morphology in both bands. NUV and FUV emissions likely have different origins [5]. The NUV emission in a passively evolving galaxy is dominated by the turnoff and sub-giant stars. The FUV emission traces one (or more) hot, plausibly high-metallicity, stellar components (e.g. hot-HB and/or post-AGB and AGB manque stars) giving origin to their wellknown UV-upturn phenomenon. At the same time, the UV light is also a sensitive tracer of the residual star formation in ETGs, a necessary ingredient to constrain the galaxy rejuvenation suggested by the optical line-strength indices which suffer from age-metallicity degeneracy effects.