NGC 404: A REJUVENATED LENTICULAR GALAXY ON A MERGER-INDUCED, BLUEWARD EXCURSION INTO THE GREEN VALLEY

Abstract

We have discovered recent star formation in the outermost portion (1-4x R_25)\nof the nearby lenticular (S0) galaxy NGC 404 using GALEX UV imaging. FUV-bright\nsources are strongly concentrated within the galaxy's HI ring (formed by a\nmerger event according to del Rio et al.), even though the average gas density\nis dynamically subcritical. Archival HST imaging reveals resolved upper main\nsequence stars and conclusively demonstrates that the UV light originates from\nrecent star formation activity. We present FUV, NUV radial surface brightness\nprofiles and integrated magnitudes for NGC 404. Within the ring, the average\nstar formation rate surface density (Sigma_{SFR}) is 2.2x10^-5 Msun/yr/kpc^2.\nOf the total FUV flux, 70% comes from the HI ring which is forming stars at a\nrate of 2.5x10^-3 Msun/yr. The gas consumption timescale, assuming a constant\nSFR and no gas recycling, is several times the age of the Universe. In the\ncontext of the UV-optical galaxy CMD, the presence of the SF HI ring places NGC\n404 in the green valley separating the red and blue sequences. The rejuvenated\nlenticular galaxy has experienced a merger-induced, disk-building excursion\naway from the red sequence toward bluer colors, where it may evolve quiescently\nor (if appropriately triggered) experience a burst capable of placing it on the\nblue/star-forming sequence for up to ~1 Gyr. The green valley galaxy population\nis heterogeneous, with most systems transitioning from blue to red but others\nevolving in the opposite sense due to acquisition of fresh gas through various\nchannels.\n