Galaxy Zoo: A sample of blue early-type galaxies at low redshift
  We report the discovery of a population of nearby, blue early-type galaxies
with high star formation rates (0.5 < SFR < 50 Msun/yr). They are identified by
their visual morphology as provided by Galaxy Zoo for SDSS DR6 and their u-r
colour. We select a volume-limited sample in the redshift range 0.02 < z <
0.05, corresponding to luminosities of approximately L* and above, and with u-r
colours significantly bluer than the red sequence. We confirm the early-type
morphology of the objects in this sample and investigate their environmental
dependence and star formation properties. Blue early-type galaxies tend to live
in lower-density environments than `normal' red sequence early-types and make
up 5.7 +/-0.4% of the low-redshift early-type galaxy population. We find that
such blue early-type galaxies are virtually absent at high velocity dispersions
above 200 km/s. Our analysis uses emission line diganostic diagrams and we find
that ~25% of them are actively starforming, while another ~25% host both star
formation and an AGN. Another ~12% are AGN. The remaining 38% show no strong
emission lines. When present and uncontaminated by an AGN contribution, the
star formation is generally intense. We consider star formation rates derived
from Halpha, u-band and infrared luminosities, and radial colour profiles, and
conclude that the star formation is spatially extended. Of those objects that
are not currently undergoing star formation must have ceased doing so recently
in order to account for their blue optical colours. The gas phase metallicity
of the actively starforming blue early-types galaxies is supersolar in all
cases. We discuss the place of these objects in the context of galaxy
formation. A catalogue of all 204 blue early-type galaxies in our sample,
including star formation rates and emission line classification, is provided.
