Galaxy Cold Gas Contents in Modern Cosmological Hydrodynamic Simulations.

We present a comparison of galaxy atomic and molecular gas properties in three recent cosmological hydrodynamic simulations, Simba, EAGLE, and IllustrisTNG, versus observations from $z\sim 0-2$. All simulations qualitatively reproduce the observed z=0 HI and H$_2$ mass functions (HIMF, H2MF), CO(1-0) luminosity functions (COLF), and gas scaling relations versus stellar mass, specific star formation rate, and stellar surface density $\mu_*$. Quantitatively, Simba and TNG show significantly more cold gas than EAGLE, particularly in massive systems. To compare to the COLF, we employ a prescription for the H$_2$-to-CO conversion factor $\alpha_{CO}$ from Narayanan et al (2012), yielding substantial variations in $\alpha_{CO}$ and significant differences between models. Predicted $z=0$ COLFs agree better with data than predicted H2MFs. Out to $z\sim 2$, EAGLE's and Simba's HIMF and COLF strongly increase, while TNG's HIMF declines and COLF evolves slowly. EAGLE and Simba reproduce high $L_{\rm CO1-0}$ galaxies at $z\sim 1-2$ as observed, owing partly to a median $\alpha_{CO}(z=2)\sim 1$ versus $\alpha_{CO}(z=0)\sim 3$. Examining \HI, H$_2$, and CO scaling relations, their trends with $M_*$ are broadly reproduced in all models, but EAGLE tends to produce somewhat too little HI, TNG overproduces cold gas in massive galaxies, and Simba overproduces molecular gas. Specifically in Simba, we test how individual black hole feedback modules impact cold gas contents. We find that Simba's AGN jet feedback is primarily responsible by lowering cold gas contents from $z\sim 1\to0$ by suppressing cold gas in $M_*>\sim 10^{10}M_\odot$ galaxies, while X-ray feedback suppresses the formation of high-$\mu_*$ systems.