THE SPECIFIC STAR FORMATION RATE AND STELLAR MASS FRACTION OF LOW-MASS CENTRAL GALAXIES IN COSMOLOGICAL SIMULATIONS

By means of cosmological N-body + hydrodynamics simulations of galaxies in the context of the. cold dark matter (Lambda CDM) scenario we explore the specific star formation rates (SSFR = SFR/M-s, M-s is the stellar mass) and stellar mass fractions (F-s = M-s/M-h, M-h is the halo mass) for sub-M* field galaxies at different redshifts (0 less than or similar to z less than or similar to 1.5). Distinct low-mass halos (2.5 less than or similar to M-h/10(10) M-circle dot less than or similar to 50 at z = 0) were selected for the high-resolution re-simulations. The Hydrodynamics Adaptive Refinement Tree (ART) code was used and some variations of the sub-grid parameters were explored. Most simulated galaxies, specially those with the highest resolutions, have significant disk components and their structural and dynamical properties are in reasonable agreement with observations of sub-M* field galaxies. However, the SSFRs are 5-10 times smaller than the averages of several (compiled and homogenized here) observational determinations for field blue/star-forming galaxies at z < 0.3 (at low masses, most observed field galaxies are actually blue/star forming). This inconsistency seems to remain even at z similar to 1-1.5, although it is less drastic. The F-s of simulated galaxies increases with M-h as semi-empirical inferences show. However, the values of F-s at z approximate to 0 are similar to 5-10 times larger in the simulations than in the inferences; these differences increases probably to larger factors at z similar to 1-1.5. The inconsistencies reported here imply that simulated low-mass galaxies (0.2 less than or similar to M-s/10(9) M-circle dot less than or similar to 30 at z = 0) assembled their stellar masses much earlier than observations suggest. Our results confirm the predictions found by means of Lambda CDM-based models of disk galaxy formation and evolution for isolated low-mass galaxies, and highlight that our understanding and implementation of astrophysics into simulations and models are still lacking vital ingredients.