Combining Ha and IRAC images of the nearby spiral galaxy NGC 628, we find that between 30-43% of its 8um dust emission is not related to recent star formation. Contributions from dust heated by young stars are separated by identifying HII regions in
the Ha map and using these areas as a mask to determine the 8um dust emission that must be due to heating by older stars. Corrections are made for sub-detection-threshold HII regions, photons escaping from HII regions and for young stars not directly associated to HII regions (i.e. 10-100 Myr old stars). A simple model confirms this amount of 8um emission can be expected given dust and PAH absorption cross-sections, a realistic star-formation history, and the observed optical extinction values. A Fourier power spectrum analysis indicates that the 8um dust emission is more diffuse than the Ha emission (and similar to observed HI), supporting our analysis that much of the 8um-emitting dust is heated by older stars. The 8um dust-to-Ha emission ratio declines with galactocentric radius both within and outside of HII regions, probably due to a radial increase in disk transparency. In the course of this work, we have also found that intrinsic diffuse Ha fractions may be lower than previously thought in galaxies, if the differential extinction between HII regions and diffuse regions is taken into account.
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 galaxys HI ring (formed by a merger event accor
ding 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.
We have initiated a search for extended ultraviolet disk (XUV-disk) galaxies in the local universe. Herein, we compare GALEX UV and visible--NIR images of 189 nearby (D$<$40 Mpc) S0--Sm galaxies included in the GALEX Atlas of Nearby Galaxies and pres
ent the first catalogue of XUV-disk galaxies. We find that XUV-disk galaxies are surprisingly common but have varied relative (UV/optical) extent and morphology. Type~1 objects ($ga$20% incidence) have structured, UV-bright/optically-faint emission features in the outer disk, beyond the traditional star formation threshold. Type~2 XUV-disk galaxies ($sim$10% incidence) exhibit an exceptionally large, UV-bright/optically-low-surface-brightness (LSB) zone having blue $UV-K_s$ outside the effective extent of the inner, older stellar population, but not reaching extreme galactocentric distance. If the activity occuring in XUV-disks is episodic, a higher fraction of present-day spirals could be influenced by such outer disk star formation. Type~1 disks are associated with spirals of all types, whereas Type~2 XUV-disks are predominantly found in late-type spirals. Type~2 XUV-disks are forming stars quickly enough to double their [presently low] stellar mass in the next Gyr (assuming a constant SF rate). XUV-disk galaxies of both types are systematically more gas-rich than the general galaxy population. Minor external perturbation may stimulate XUV-disk incidence, at least for Type~1 objects. XUV-disks are the most actively evolving galaxies growing via inside-out disk formation in the current epoch, and may constitute a segment of the galaxy population experiencing significant, continued gas accretion from the intergalactic medium or neighboring objects.
