The role of stellar feedback and dark matter in the evolution of dwarf galaxies

Abstract

Supernova and multiple supernova events regulate several structural properties of dwarf galaxies. In particular, they govern the metal enrichment and the energy budget of the ISM; they might induce partial (blowout) or total (blowaway) gas removal from the galaxy; they also regulate the pressure of the ISM, and consequently the morphology of the galactic gaseous body. Significant amounts of dark matter may play an equally important role: the dark matter gravitational potential tends to concentrate baryons towards the centre, thus enhancing both the star formation rate and the metal production. Also, the dynamical properties of the ISM, and the occurrence of a blowout or blowaway, are shown to be determined by the dark matter content. We present detailed analytical/numerical models describing the evolution of dwarf irregular galaxies (dIs), including the above and other effects. The main results are: (i) dwarfs with total masses M ≲ 5 × 106 M⊙ are blown away; those with gas masses up to ≃109 M⊙ lose mass in an outflow; (ii) metallicities are found to correlate tightly with dark matter content, and are consistent with a range of dark-to-visible mass ratios θ ≈ 0-30, with about 65 per cent of the dwarfs in the sample having θ ≈ 0-10; (iii) we predict a lower limit to the oxygen abundance in dIs of 12 + log(O/H) ≈ 7.2; (iv) outflows are not particularly important for the metallicity evolution of dwarf galaxies, and certainly less important than star formation for gas consumption; however, dwarfs with gas masses of a few × 108 M⊙ are shown to be the major pollutants of the IGM; (v) the ISM H I velocity dispersion correlates with metallicity and, independently of dark matter, scales as Z3.5. Specific comparisons with well-studied dIs, such as for example Leo A, yield excellent agreement with the data. Based on our results, we discuss a scenario in which late-type and early-type dwarfs had common progenitors in the past, but differences in their total mass forced these objects to follow different evolutionary paths. We therefore consider dI → dE transitions occurring at present cosmic times as very unlikely.