Galaxy Zoo: Morphological classification of galaxy images from the
  Illustris simulation
  Modern cosmological simulations model the universe with increasing
sophistication and at higher spatial and temporal resolutions. These
enhancements permit detailed comparisons between the simulation outputs and
real observational data. Recent projects such as Illustris are capable of
producing simulated images that are comparable to those obtained from local
surveys. This paper tests how well Illustris achieves this goal across a
diverse population of galaxies using visual morphologies derived from Galaxy
Zoo citizen scientists. Morphological classifications provided by volunteers
for simulated galaxies are compared with similar data for a compatible sample
of images drawn from the SDSS Legacy Survey. This paper investigates how simple
morphological characterization by human volunteers asked to distinguish smooth
from featured systems differs between simulated and real galaxy images.
Differences are identified, which are likely due to the limited resolution of
the simulation, but which could be revealing real differences in the dynamical
evolution of populations of galaxies in the real and model universes.
Specifically, for stellar masses $M_{\star}\lesssim10^{11}M_{\odot}$, a larger
proportion of Illustris galaxies that exhibit disk-like morphology or visible
substructure, relative to their SDSS counterparts. Toward higher masses,
simulated and observed galaxies converge and exhibit similar morphology
distributions. The stellar mass threshold indicated by this divergent behavior
confirms recent works using parametric measures of morphology from Illustris
simulated images. When $M_{\star}\gtrsim10^{11}M_{\odot}$, the Illustris
dataset contains fewer galaxies that classifiers regard as unambiguously
featured. These results suggest that comparison between the detailed properties
of observed and simulated galaxies, even when limited to reasonably massive
systems, may be misleading.
