Assessment of SDSS-Derived Galaxy Morphologies Using HST Imaging

Abstract: The Sloan Digital Sky Survey (SDSS) was foundational to the study of galaxy evolution, having revealed the bimodality of galaxies and the relationship between their structure and star-forming activity. However, ground-based optical surveys like SDSS are limited in resolution and depth which may lead to biases or poor quality in the derived morphological properties, potentially impacting our understanding of how and why galaxies cease their star formation (quench). We use archival HST imaging of ~2,000 SDSS objects to assess the reliability of SDSS-derived morphologies, taking advantage of both SDSS statistical samples and of HST's superior resolution and sensitivity. Single Sersic fitting and bulge-disk decomposition is performed on HST images for direct comparison with SDSS results. Of the three catalogs of SDSS-derived morphologies considered, none are significantly more accurate than the others. For disk-dominated galaxies (n<2.5), global Sersic indices (n) from Meert et al. 2015 (M15) are preferred. For bulge-dominated galaxies (n>2.5), Simard et al. 2011 (S11) and M15 overestimate n by ~20%, and NYU-derived global n are preferred. Global R_eff from S11 are preferred, but overestimate R_eff for the largest galaxies by 0.1 dex. SDSS-derived single-component parameters are generally significantly more robust than SDSS-derived two-component parameters. The bulge Sersic index (n_bulge) cannot be reliably constrained from SDSS imaging. The bulge-to-total (B/T) ratio can be reliably inferred from SDSS for galaxies with SDSS B/T<0.6 provided that n_bulge=4 is enforced. The difference in global n between HST and SDSS depends strongly on B/T; an empirical correction based only on it accounts for most of the systematics in global n.