New clues to the evolution of dwarf early‐type galaxies

Surface photometry of 18 Virgo cluster dwarf elliptical (dE) and dwarf lenticular (dS0) galaxies, made by Gavazzi et al. in the H band (1.65 μm) and in the B band (0.44 μm), shows that the ratio of the effective radii of these stellar systems in the B and H bands, r e B / r e H , ranges between 0.7 and 2.2. In particular, dwarf ellipticals and lenticulars with a red total colour index B ‐ H (i.e. with 3.2< B ‐ H <4) have equal effective radii in these two passbands. By contrast, blue (i.e. with 2.5< B ‐ H <3.1) dEs and dS0s have B ‐band effective radii about 50 per cent larger than the H ‐band ones, on average. Consistently, strong negative gradients in B ‐ H along the galactocentric radius are found to be associated with blue total colours. This trend is not found in a sample of 29 giant E and S0 galaxies of the Coma cluster with analogous data available in the literature. These early‐type giants span a broad range in  r e B / r e H (0.2–2.2) , with a mean r e B / r e H ∼1.1 , but a narrow range in (red) colour (3.3< B ‐ H <4.2) . In these stellar systems, colour gradients are usually interpreted as arising either from age/metallicity gradients along the radial coordinate or from dust attenuation, whatever the total colour of the system is. Assuming each of these three distinct interpretations of the origin of colour gradients, we discuss the origin of the association of strong negative colour gradients with blue colours found in the early‐type dwarfs under study, in relation with current scenarios of formation and evolution of dE and dS0 galaxies.