THE GALEX/S4G UV–IR COLOR–COLOR DIAGRAM: CATCHING SPIRAL GALAXIES AWAY FROM THE BLUE SEQUENCE

We obtained GALEX FUV, NUV, and Spitzer/ IRAC 3.6 μm photometry for > 2000 galaxies, available for 90% of the S^(4)G sample. We find a very tight. GALEX blue sequence (GBS) in the (FUV- NUV) versus (NUV-[3.6]) color- color diagram, which is populated by irregular and spiral galaxies, and is mainly driven by changes in the formation timescale (tau) and a degeneracy between (tau) and dust reddening. The tightness of the GBS provides an unprecedented way of identifying star- forming galaxies and objects that are just evolving to (or from) what we call the. GALEX green valley (GGV). At the red end of the GBS, at (NUV-[3.6]) > 5, we find a wider. GALEX red sequence (GRS) mostly populated by E/ S0 galaxies that has a perpendicular slope to that of the GBS and of the optical red sequence. We find no such dichotomy in terms of stellar mass (measured by M_([3.6])) since both massive (M. > 10^(11)M.) blue- and red- sequence galaxies are identified. The type that is proportionally more often found in the GGV is the S0- Sa's, and most of these are located in high- density environments. We discuss evolutionary models of galaxies that show a rapid transition from the blue to the red sequence on a timescale of 10^(8) yr.