Reproducing the Stellar Mass/Halo Mass Relation in Simulated LCDM
  Galaxies: Theory vs Observational Estimates
  We examine the present-day total stellar-to-halo mass (SHM) ratio as a
function of halo mass for a new sample of simulated field galaxies using fully
cosmological, LCDM, high resolution SPH + N-Body simulations.These simulations
include an explicit treatment of metal line cooling, dust and self-shielding,
H2 based star formation and supernova driven gas outflows. The 18 simulated
halos have masses ranging from a few times 10^8 to nearly 10^12 solar masses.
At z=0 our simulated galaxies have a baryon content and morphology typical of
field galaxies. Over a stellar mass range of 2.2 x 10^3 to 4.5 x 10^10 solar
masses, we find extremely good agreement between the SHM ratio in simulations
and the present-day predictions from the statistical Abundance Matching
Technique presented in Moster et al. (2012). This improvement over past
simulations is due to a number systematic factors, each decreasing the SHM
ratios: 1) gas outflows that reduce the overall SF efficiency but allow for the
formation of a cold gas component 2) estimating the stellar masses of simulated
galaxies using artificial observations and photometric techniques similar to
those used in observations and 3) accounting for a systematic, up to 30 percent
overestimate in total halo masses in DM-only simulations, due to the neglect of
baryon loss over cosmic times. Our analysis suggests that stellar mass
estimates based on photometric magnitudes can underestimate the contribution of
old stellar populations to the total stellar mass, leading to stellar mass
errors of up to 50 percent for individual galaxies. These results highlight the
importance of using proper techniques to compare simulations with observations
and reduce the perceived tension between the star formation efficiency in
galaxy formation models and in real galaxies.
