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 Λ cold dark matter (ΛCDM) scenario we explore the specific star formation rates (SSFR = SFR/Ms , Ms is the stellar mass) and stellar mass fractions (Fs ≡ Ms /Mh , Mh is the halo mass) for sub-M* field galaxies at different redshifts (0 ∼< z ∼< 1.5). Distinct low-mass halos (2.5 ∼< Mh /1010 Msun ∼< 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 ∼ 1-1.5, although it is less drastic. The Fs of simulated galaxies increases with Mh as semi-empirical inferences show. However, the values of Fs at z ∼ 0 are ∼5-10 times larger in the simulations than in the inferences; these differences increases probably to larger factors at z ∼ 1-1.5. The inconsistencies reported here imply that simulated low-mass galaxies (0.2 ∼< Ms /109 Msun ∼< 30 at z = 0) assembled their stellar masses much earlier than observations suggest. Our results confirm the predictions found by means of ΛCDM-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.