The Dependence of Galaxy Morphology and Structure on Environment and Stellar Mass

From the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5), we extract a sample of 4594 galaxies at redshifts 0.02<z<0.03, complete down to a stellar mass of M=10^10 Msol. We quantify their structure (Sersic index), morphology (Sersic index + ``Bumpiness''), and local environment. We show that morphology and structure are intrinsically different galaxy properties, and we demonstrate that this is a physically relevant distinction by showing that these properties depend differently on galaxy mass and environment. Structure mainly depends on galaxy mass whereas morphology mainly depends on environment. This is driven by variations in star formation activity, as traced by color, which only weakly affects the structure of a galaxy but strongly affects its morphological appearance. The implication of our results is that the existence of the morphology-density relation is intrinsic and not just due to a combination of more fundamental, underlying relations. Our findings have consequences for high-redshift studies, which often use some measure of structure as a proxy for morphology. A direct comparison with local samples selected through visually classified morphologies may lead to biases in the inferred evolution of the morphological mix of the galaxy population, and misinterpretations in terms of how galaxy evolution depends on mass and environment.