MOLECULAR GAS, CO, AND STAR FORMATION IN GALAXIES: EMERGENT EMPIRICAL RELATIONS, FEEDBACK, AND THE EVOLUTION OF VERY GAS-RICH SYSTEMS

We use time-varying models of the coupled evolution of the Hi, H-2 gas phases and stars in galaxy-sized numerical simulations to (1) test for the emergence of the Kennicutt-Schmidt (K-S) and the H-2-pressure relation, (2) explore a realistic H-2-regulated star formation recipe which brings forth a neglected and potentially significant SF-regulating factor, and (3) go beyond typical galactic environments (for which these galactic empirical relations are deduced) to explore the early evolution of very gas-rich galaxies. In this work, we model low-mass galaxies (M-baryon <= 10(9) M-circle dot), while incorporating an independent treatment of CO formation and destruction, the most important tracer molecule of H-2 in galaxies, along with that for the H-2 gas itself. We find that both the K-S and the H-2-pressure empirical relations can robustly emerge in galaxies after a dynamic equilibrium sets in between the various interstellar medium (ISM) states, the stellar component and its feedback (T greater than or similar to 1 Gyr). The only significant dependence of these relations seems to be for the CO-derived (and thus directly observable) ones, which show a strong dependence on the ISM metallicity. The H-2-regulated star formation recipe successfully reproduces the morphological and quantitative aspects of previous numerical models while doing away with the star formation efficiency parameter. Most of the HI -> H-2 mass exchange is found taking place under highly non-equilibrium conditions necessitating a time-dependent treatment even in typical ISM environments. Our dynamic models indicate that the CO molecule can be a poor, nonlinear, H-2 gas tracer. Finally, for early evolutionary stages (T less than or similar to 0.4 Gyr), we find significant and systematic deviations of the true star formation from that expected from the K-S relation, which are especially pronounced and prolonged for metal-poor systems. The largest such deviations occur for the very gas-rich galaxies, where deviations of a factor similar to 3-4 in global star formation rate (SFR) can take place with respect to those expected from the CO-derived K-S relation. This is particularly important since gas-rich systems at high redshifts could appear as having unusually high SFRs with respect to their CO-bright H-2 gas reservoirs. This points to a possibly serious deficiency of K-S relations as elements of the sub-grid physics of star formation in simulations of structure formation in the early universe.