NGC 404, A Rejuvenated Lenticular Galaxy on a Merger-Induced, Blueward
  Excursion into the Green Valley
  We have discovered recent star formation in the outermost portion (1-4x R_25)
of the nearby lenticular (S0) galaxy NGC 404 using GALEX UV imaging. FUV-bright
sources are strongly concentrated within the galaxy's HI ring (formed by a
merger event according to del Rio et al.), even though the average gas density
is dynamically subcritical. Archival HST imaging reveals resolved upper main
sequence stars and conclusively demonstrates that the UV light originates from
recent star formation activity. We present FUV, NUV radial surface brightness
profiles and integrated magnitudes for NGC 404. Within the ring, the average
star formation rate surface density (Sigma_{SFR}) is 2.2x10^-5 Msun/yr/kpc^2.
Of the total FUV flux, 70% comes from the HI ring which is forming stars at a
rate of 2.5x10^-3 Msun/yr. The gas consumption timescale, assuming a constant
SFR and no gas recycling, is several times the age of the Universe. In the
context of the UV-optical galaxy CMD, the presence of the SF HI ring places NGC
404 in the green valley separating the red and blue sequences. The rejuvenated
lenticular galaxy has experienced a merger-induced, disk-building excursion
away from the red sequence toward bluer colors, where it may evolve quiescently
or (if appropriately triggered) experience a burst capable of placing it on the
blue/star-forming sequence for up to ~1 Gyr. The green valley galaxy population
is heterogeneous, with most systems transitioning from blue to red but others
evolving in the opposite sense due to acquisition of fresh gas through various
channels.
