The fate of heavy elements in dwarf galaxies – the role of mass and geometry

Abstract

Context. Energetic feedback from supernovae and stellar winds can drive galactic winds. Dwarf galaxies, due to their shallower potential wells, are assumed to be more vulnerable to this phenomenon. Metal loss through galactic winds is also commonly invoked to explain the low metal content of dwarf galaxies. Aims. Our main aim in this paper is to show that galactic mass cannot be the only parameter determining the fraction of metals lost by a galaxy. In particular, the distribution of gas must play an equally important role. Methods. We performed 2D chemo-dynamical simulations of galaxies characterized by different gas distributions, masses, and gas fractions.Results. The gas distribution can change the fraction of lost metals through galactic winds by up to one order of magnitude. In particular, disk-like galaxies tend to lose metals more easily than roundish ones. Consequently, the final metallicities attained by models with the same mass but with different gas distributions can also vary by up to one dex. Confirming previous studies, we also show that the fate of gas and freshly produced metals strongly depends on the mass of the galaxy. Smaller galaxies (with shallower potential wells) more easily develop large-scale outflows, so that the fraction of lost metals tends to be higher.

The chemical evolution of galaxies with a variable integrated galactic initial mass function

“…that the metallicity of the outflowing gas is larger than the average gas-phase metallicity in the galaxy. Both observations (Martin et al 2002;Ott et al 2005) and numerical models (Mac-Low & Ferrara 1999;Recchi & Hensler 2013) support this assumption. Notice that probably also the efficiency with which the various chemical species are ejected vary from element to element (selective winds; see Recchi et al 2001;) but for simplicity we will not consider further this hypothesis.…”

“…Although we are convinced that galactic winds can play an important role in the chemical evolution of dwarf galaxies (Recchi & Hensler 2013; see also Sect. 5), we want to explore here the possibility that the IMF is not constant but that it depends on time though its dependence on the SFR and on Z, as per the IGIMF theory described in Sect.…”

Molecular gas in low-metallicity starburst galaxies:

“…This is largely due to our poor knowledge of the physical mechanisms behind starburst activity, as well as of the feedback induced by the massive star formation in the interstellar medium (ISM). The strong UV radiation and the mechanical energy from stellar winds and SNe are likely agents of feedback in starbursts like those present in BCDs, leading to the ejection of metalenriched gas into the galactic halo, which limits the star formation and shapes the large-scale structure and kinematics of the surrounding ISM (e.g., Mac Low & Ferrara 1999;Silich & Tenorio-Tagle 2001;Tenorio-Tagle et al 2006;Recchi & Hensler 2013).…”

The Supernova – ISM/Star-formation interplay

“…From fig.3 it is discernible that only flat gas disks of preferably low-mass galaxies allow a hot wind to escape from the galaxy. Consequences for the chemical evolution are in the focus of those models (Recchi & Hensler 2013). And finally, observed superbubbles also reveal a mismatch of their spatial extent and…”

“…(from Recchi & Hensler 2013) A superbubble expanding from a stellar association embedded in a Hi disk has, at first, to act against the surrounding medium, by this, is cooling due to its pressure work and radiation, but compresses the swept-up shell material and implies turbulent energy to the ISM. How much the superbubble expansion is efficiently hampered depends on the surrounding gas density and pressure, the Hi disk shape (Recchi & Hensler 2013), and the energy loss by radiative cooling. From fig.3 it is discernible that only flat gas disks of preferably low-mass galaxies allow a hot wind to escape from the galaxy.…”