Galactic fountains and outflows in star-forming dwarf galaxies: interstellar medium expulsion and chemical enrichment

We investigate the impact of supernova (SN) feedback in gas-rich dwarf galaxies experiencing a low-to-moderate star formation rate, typical of relatively quiescent phases between starbursts. We calculate the long-term evolution of the interstellar medium (ISM) and the metal-rich SN ejecta using three-dimensional hydrodynamic simulations, in which the feedback energy is deposited by Type II SNe exploding in distinct OB associations. We find that a circulation flow similar to galactic fountains is generally established, with some ISM lifted at heights of one to a few kpc above the galactic plane. This gas forms an extraplanar layer, which falls back to the plane in ≈108 yr, once the star formation stops. Very little or no ISM is expelled outside the galaxy system for the star formation rates considered, even though in the most powerful model the SN energy is comparable to the gas binding energy. The metal-rich SN ejecta is instead more vulnerable to the feedback and we find that a significant fraction (25–80 per cent) is vented in the intergalactic medium, even for low SN rates (7 × 10−5 to 7 × 10−4 yr−1). About half of the metals retained by the galaxy are located far (z > 500 pc) from the galactic plane. Moreover, our models indicate that the circulation of the metal-rich gas out from and back to the galactic disc is not able to erase the chemical gradients imprinted by the (centrally concentrated) SN explosions.