Galaxy Formation in the Santa Cruz semi-analytic model compared with
  IllustrisTNG -- I. Galaxy scaling relations, dispersions, and residuals at
  z=0
  We present the first results from applying the Santa Cruz semi-analytic model
(SAM) for galaxy formation on merger trees extracted from a dark matter only
version of the IllustrisTNG (TNG) simulations. We carry out a statistical
comparison between the predictions of the Santa Cruz SAM and TNG for a subset
of central galaxy properties at z=0, with a focus on stellar mass, cold and hot
gas mass, star formation rate (SFR), and black hole (BH) mass. We find fairly
good agreement between the mean predictions of the two methods for stellar mass
functions and the stellar mass vs. halo mass (SMHM) relation, and qualitatively
good agreement between the SFR or cold gas mass vs. stellar mass relation and
quenched fraction as a function of stellar mass. There are greater differences
between the predictions for hot (circumgalactic) gas mass and BH mass as a
function of halo mass. Going beyond the mean relations, we also compare the
dispersion in the predicted scaling relations, and the correlation in residuals
on a halo-by-halo basis between halo mass and galaxy property scaling
relations. Intriguingly, we find similar correlations between residuals in SMHM
in the SAM and in TNG, suggesting that these relations may be shaped by similar
physical processes. Other scaling relations do not show significant
correlations in the residuals, indicating that the physics implementations in
the SAM and TNG are significantly different.
