The S4G view of stellar mass, mid-IR dust, and evolved, intermediate-age stars in nearby galaxies

Abstract

AbstractWith deep imaging at 3.6 and 4.5 μm where the light in nearby galaxies is dominated by old stars, the Spitzer Survey of Stellar Structure in Nearby Galaxies (S4G) promises to be the ultimate inventory of stellar mass and structure in the local universe. We present results from a novel technique that makes it possible to fully exploit the information contained in these images, pertaining not only to the stellar light (and, ultimately, mass distribution), but also the nature and distribution of the mid-IR dust and the properties of evolved, intermediate age stars (e.g. in AGB-dominated star clusters). We apply Independent Component Analysis (ICA) to the 3.6 and 4.5 μm bands to separate the light from the old stars from the secondary non-stellar (i.e. PAH and hot dust) sources of emission, which are identified via comparison to the non-stellar emission imaged at 8 μm. Then, within the context of age and mass estimation at high z, we extract optical-to-mid-IR SEDs for a sample of ICA-identified AGB-dominated clusters to constrain the typically uncertain fractional contribution of AGB light to the total stellar emission in (rest-frame) NIR bands.