The ALMA Spectroscopic Survey in the HUDF: the molecular gas content of galaxies and tensions with IllustrisTNG and the Santa Cruz SAM

Understand

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.