We have discovered recent star formation in the outermost portion ( 1–4 \times 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 \sim 2.2 \times 10 ^ { -5 } M _ { \odot } yr ^ { -1 } kpc ^ { -2 } .
Of the total FUV flux , 70 % comes from the HI ring which is forming stars at a rate of 2.5 \times 10 ^ { -3 } M _ { \odot } yr ^ { -1 } .
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 \sim 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 .