The colours of Virgo dEs as seen by SDSS

Abstract

A colour analysis of a carefully selected sample of over 200 Virgo cluster dEs and 8 giant ellipticals (Es) in the SDSS yields the following results: a) the nucleated dEs (dENs) follow a tight colour-magnitude relation (CMR) which does not broaden significantly at fainter (Mr > -15) magnitudes, b) the CMR of dENs smoothly transitions to the CMR of Es, but the latter changes its slope at about Mr = -20, c) the brighter dENs are consistent with ages of t > 5Gyr and a decrease of metallicity towards fainter magnitudes, but a possible additional luminosity-age relation is not ruled out, d) it is crucial to treat dS0s separately from dEs since their different colour properties would otherwise bias the comparison of nucleated and non-nucleated dEs (dEnoNs), e) the dEnoNs show a weak trend towards bluer colours than dENs.