NGC 404, A Rejuvenated Lenticular Galaxy on a Merger-Induced, Blueward Excursion into the Green Valley

TL;DR

NGC 404, a nearby lenticular galaxy, shows recent star formation in its HI ring caused by a merger, placing it in the green valley and indicating a rejuvenation process that may lead to future starburst activity.

Contribution

This study provides direct evidence of recent star formation in the outer regions of NGC 404, linking merger events to galaxy rejuvenation and green valley placement.

Findings

Star formation occurs predominantly in the HI ring with a rate of 2.5x10^-3 Msun/yr.

70% of FUV flux originates from the HI ring, indicating active star formation.

Gas consumption timescale exceeds the age of the Universe, suggesting sustained star formation potential.

Abstract

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.