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Dust origin in late-type dwarf galaxies: ISM growth vs. type II supernovae

We re-evaluate the roles of different dust sources in dust production as a function of metallicity in late-type dwarf galaxies, with the goal of understanding the relation between dust content and metallicity. The dust content of late-type dwarf galaxies with episodic star formation is studied with a multicomponent model of dust evolution, which includes dust input from AGB stars, type II SNe and dust mass growth by accretion of gas species in the ISM. Dust growth in the ISM becomes an important dust source in dwarf galaxies, on the timescale of 0.1 - few Gyrs. It increases the dust-to-gas ratio (DGR) during post-burst evolution, unlike type II SNe, which eject grains into the ISM only during starbursts. Before the dust growth in the ISM overtakes the dust production, AGB stars can be major sources of dust in metal-poor dwarf galaxies. Our models reproduce the relation between the DGR and oxygen abundance, derived from observations of a large sample of dwarf galaxies. The steep decrease in the DGR at low O values is explained by the relatively low efficiency of dust condensation in stars. The scatter observed at higher O values is determined mainly by different critical metallicities for the transition from stardust- to ISM-growth dominated dust production, depending on the star formation history. In galaxies with episodic star formation, additional dispersion in the DGR is introduced by grain destruction during starbursts, followed by an increase of the dust mass due to dust growth in the ISM during post-burst evolution. We find that the carbon-to-silicate ratio changes dramatically, when the ISM growth becomes the dominant dust source, therefore this ratio can be used as an indicator of the transition. The observed DGR-O relation in dwarf galaxies favours low condensation efficiencies in type II SNe, together with an increase in the total dust mass by means of dust growth in the ISM.