Red star-forming and blue passive galaxies in clusters
  We explore the relation between colour and specific star formation rate
(derived from optical spectra obtained by SDSS DR4) of over 6,000 galaxies
(M_r<=-20.5) in and around low redshift (z<0.12) clusters. Even though most red
galaxies have little or no ongoing star formation, and most blue galaxies are
currently forming stars, there are significant populations of red star-forming
(SF) and blue passive galaxies. This paper examines various properties of
galaxies belonging to the latter two categories. These properties include
morphological parameters, internal extinction, spectral features such as
EW(H_delta) and the 4000 ang break, and metallicity. Our analysis shows that
the blue passive galaxies have properties very similar to their SF
counterparts, except that their large range in EW(H_delta) indicates recent
truncation of star formation. The red SF galaxies fall into two broad
categories, one of them being massive galaxies in cluster cores dominated by an
old stellar population, but with evidence of current star formation in the
core. For the remaining red SF galaxies it is evident from various metallicity
measures and mean stellar ages, that their colours result from the predominance
of a metal-rich stellar population. The implication of the properties of these
SF galaxies on environmental studies, like that of the Butcher-Oemler effect,
is discussed.
