Galaxy morphology and star formation in the Illustris Simulation at<i>z</i> = 0

Abstract

We study how optical galaxy morphology depends on mass and star formation\nrate (SFR) in the Illustris Simulation. To do so, we measure automated galaxy\nstructures in 10808 simulated galaxies at z=0 with stellar masses 10^9.7 &lt;\nM_*/M_sun &lt; 10^12.3. We add observational realism to idealized synthetic images\nand measure non-parametric statistics in rest-frame optical and near-IR images\nfrom four directions. We find that Illustris creates a morphologically diverse\ngalaxy population, occupying the observed bulge strength locus and reproducing\nmedian morphology trends versus stellar mass, SFR, and compactness. Morphology\ncorrelates realistically with rotation, following classification schemes put\nforth by kinematic surveys. Type fractions as a function of environment agree\nroughly with data. These results imply that connections among mass, star\nformation, and galaxy structure arise naturally from models matching global\nstar formation and halo occupation functions when simulated with accurate\nmethods. This raises a question of how to construct experiments on galaxy\nsurveys to better distinguish between models. We predict that at fixed halo\nmass near 10^12 M_sun, disc-dominated galaxies have higher stellar mass than\nbulge-dominated ones, a possible consequence of the Illustris feedback model.\nWhile Illustris galaxies at M_* ~ 10^11 M_sun have a reasonable size\ndistribution, those at M_* ~ 10^10 M_sun have half-light radii larger than\nobserved by a factor of two. Furthermore, at M_* ~ 10^10.5-10^11 M_sun, a\nrelevant fraction of Illustris galaxies have distinct "ring-like" features,\nsuch that the bright pixels have an unusually wide spatial extent.\n