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 \gt 2000 galaxies, available for 90% of the S⁴G 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 (τ) and a degeneracy between τ 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]) \gt 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}_\star }\gt {{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 10⁸ yr.