The different star-formation histories of blue and red spiral and
  elliptical galaxies
  [Abridged] We study the spectral properties of intermediate mass galaxies as
a function of colour and morphology. We use Galaxy Zoo to define three
morphological classes of galaxies, namely early-types (ellipticals), late-type
(disk-dominated) face-on spirals and early-type (bulge-dominated) face-on
spirals. We classify these galaxies as blue or red according to their SDSS g-r
colour and use the spectral fitting code VESPA to calculate time-resolved
star-formation histories, metallicity and total starlight dust extinction from
their SDSS fibre spectra. We find that red late-type spirals show less
star-formation in the last 500 Myr than blue late-type spirals by up to a
factor of three, but share similar star-formation histories at earlier times.
This decline in recent star-formation explains their redder colour: their
chemical and dust content are the same. We postulate that red late-type spirals
are recent descendants of blue late-type spirals, with their star-formation
curtailed in the last 500 Myrs. The red late-type spirals are however still
forming stars approximately 17 times faster than red ellipticals over the same
period. Red early-type spirals lie between red late-type spirals and red
ellipticals in terms of recent-to-intermediate star-formation and dust content.
Therefore, it is plausible that these galaxies represent an evolutionary link
between these two populations. They are more likely to evolve directly into red
ellipticals than red late-type spirals. Blue ellipticals show similar
star-formation histories as blue spirals (regardless of type), except they have
formed less stars in the last 100 Myrs. However, blue ellipticals have
different dust content, which peaks at lower extinction values than all spiral
galaxies.
