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تسجيل مستخدم جديدThe GALEX/S4G UV-IR color-color diagram: Catching spiral galaxies away from the Blue Sequence
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We obtained GALEX FUV, NUV, and Spitzer/IRAC 3.6$mu$m photometry for > 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 ($tau$) 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 $rm{M}_{[3.6]}$), since both massive ($M_{star} > 10^{11} M_{odot}$) blue and red sequence galaxies are identified. The type that is proportionally more often found in the GGV are the S0-Sas 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 timescale of $10^{8}$years.
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