Cosmological simulations of low-mass galaxies: some potential issues

Cosmological Adaptive Mesh Refinement simulations are used to study the specific star formation rate (sSFR=SSF/M_s) history and the stellar mass fraction, f_s=M_s/M_T, of small galaxies, total masses M_T between few × 10¹⁰ M_solar to few ×10¹¹ M_solar. Our results are compared with recent observational inferences that show the so-called ``downsizing in sSFR'' phenomenon: the less massive the galaxy, the higher on average is its sSFR, a trend seen at least since z ~ 1. The simulations are not able to reproduce this phenomenon, in particular the high inferred values of sSFR, as well as the low values of f_s constrained from observations. The effects of resolution and sub-grid physics on the SFR and f_s of galaxies are discussed.