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/Ms ) history and the stellar mass fraction, fs = Ms/MT , of small galaxies, total masses MT between few ×1010 M_ to few ×1011 M_. 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 fs constrained from observations. The effects of resolution and sub-grid physics on the SFR and fs of galaxies are discussed.