The ALMA Spectroscopic Survey in the HUDF: the molecular gas content of
  galaxies and tensions with IllustrisTNG and the Santa Cruz SAM
  The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS)
provides new constraints for galaxy formation models on the molecular gas
properties of galaxies. We compare results from ASPECS to predictions from two
cosmological galaxy formation models: the IllustrisTNG hydrodynamical
simulations and the Santa Cruz semi-analytic model (SC SAM). We explore several
recipes to model the H$_2$ content of galaxies, finding them to be consistent
with one another, and take into account the sensitivity limits and survey area
of ASPECS. For a canonical CO-to-H$_2$ conversion factor of $\alpha_{\rm CO} =
3.6\,\rm{M}_\odot/(\rm{K}\,\rm{km/s}\,\rm{pc}^{2})$ the results of our work
include: (1) the H$_2$ mass of $z>1$ galaxies predicted by the models as a
function of their stellar mass is a factor of 2-3 lower than observed; (2) the
models do not reproduce the number of H$_2$-rich ($M_{\rm H2} > 3\times
10^{10}\,\rm{M}_\odot$) galaxies observed by ASPECS; (3) the H$_2$ cosmic
density evolution predicted by IllustrisTNG (the SC SAM) is in tension (only
just agrees) with the observed cosmic density, even after accounting for the
ASPECS selection function and field-to-field variance effects. The tension
between models and observations at $z>1$ can be alleviated by adopting a
CO-to-H$_2$ conversion factor in the range $\alpha_{\rm CO} = 2.0 -
0.8\,\rm{M}_\odot/(\rm{K}\,\rm{km/s}\,\rm{pc}^{2})$. Additional work on
constraining the CO-to-H$_2$ conversion factor and CO excitation conditions of
galaxies through observations and theory will be necessary to more robustly
test the success of galaxy formation models.
