A pathway toward optimally exploiting spectrally-synthesized, panchromatic survey imaging of nearby galaxies

By David Thilker

Due to recent wide-area surveys conducted by GALEX, Pan-STARRS1, and WISE, plus targeted observations by HST, our generation is afforded the remarkable opportunity to accomplish panchromatic dissection of unprecedented numbers of galaxies in the local universe, even at the scale of individual stars in the nearest objects. Such results may be compared against the emerging view of predecessor objects at high redshift. The broad wavelength coverage of space-based surveys allows the substructure of nearby galaxies to be interpreted meaningfully in terms of realistically complex stellar populations while allowing robust correction for dust obscuration. We will demonstrate our innovative approach to generating a more comprehensive, physically-interpreted view of z ~ 0 galaxy structure via pixel-SED fitting of resolved galaxies. To accomplish such data processing, we have created a successful distributed computing network, enabled by volunteer citizen scientists. Our early work has been focused on visible-light observations from Pan-STARRS1 and SDSS, but we are now in the process of broadening our spectral scope over the UV-IR domain. For thousands of galaxies, we will determine maps of key physical parameters based on UV-IR pixel-SED fitting. We will compute the entire suite of CAS+Gini+M_20 non-parametric morphology indicators from the resulting images of stellar mass (M*) and extinction-free star formation rate (SFR). Further, parametric models of galaxy structure (bulge, disk, bar) will be derived directly using the stellar mass maps. Our overarching science goal is understanding the evolution of galaxies with respect to their position and trajectory in the global (M*, SFR) plane, using maximally orthogonal, quantitative measures of stellar structure and SF modes obtained from our physical parameter maps. We now quantify the z~0 view, but envision that our pixel-SED fitting resource can later be applied to galaxy populations at moderate (LSST+WFIRST) and high z (CANDELS, Frontier fields), even if rest-frame IR bands are unavailable in that case. If time permits, we will provide a brief overview of LEGUS, the Legacy ExtraGalactic UV Survey, which is complementary to our pixel-SED fitting in the sense that it links the ~kpc scale results of our investigation to the constituent stars and clusters at the start of the SF hierarchy.