  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.
