Are passive red spirals truly passive? - The current star formation
  activity of optically-red disc galaxies
  We use GALEX ultraviolet and WISE 22 micron observations to investigate the
current star formation activity of the optically-red spirals recently
identified as part of the Galaxy Zoo project. These galaxies were accurately
selected from the Sloan Digital Sky Survey in order to be pure discs with low
or no current star formation activity, representing one of the best
optically-selected samples of candidate passive spirals. However, we show that
these galaxies are not only still forming stars at a significant rate >= 1
M_sun/yr but, more importantly, their star formation activity is not different
from that of normal star-forming discs of the same stellar mass (M* >= 10^10.2
M_sun). Indeed, these systems lie on the UV-optical blue sequence, even without
any corrections for internal dust attenuation, and they follow the same
specific star formation rate vs. stellar mass relation of star-forming
galaxies. Our findings clearly show that, at high stellar masses, optical
colours do not allow to discriminate between actively star-forming and truly
quiescent systems.
