Formation and evolution of late-type dwarf galaxies - I. NGC 1705 and NGC 1569

We present one-zone chemical evolution models for two dwarf starburst galaxies, NGC 1705 and NGC 1569. Though especially designed for the inner similar to 1 kpc region, where numerous H II regions and most of the stars are observed, the models also account for the presence of extended gaseous and dark matter haloes, and properly compute the binding energy of the gas heated by supernova explosions. Using information about the past star formation history and initial mass function of the systems previously obtained from Hubble Space Telescope optical and near-infrared colour-magnitude diagrams, we identify possible scenarios of chemical enrichment and development of galactic winds. We assume that the galactic winds are proportional to the Type II and Type Ia supernova rates. As a consequence, they do not necessarily go to zero when the star formation stops. In order not to overestimate the current metallicity of the interstellar gas inferred from H II region spectroscopy, we suggest that the winds efficiently remove from the galaxies the metal-rich ejecta of dying stars. Conversely, requiring the final mass of neutral gas to match the value inferred from 21-cm observations implies a relatively low efficiency of interstellar medium entrainment in the outflow, thus confirming previous findings that the winds driving the evolution of typical starbursts are differential. These conclusions could be different only if the galaxies accrete huge fractions of unprocessed gas at late times. By assuming standard stellar yields we obtain a good fit to the observed nitrogen-to-oxygen (N/O) ratio of NGC 1569, while the mean N/O ratio in NGC 1705 is overestimated by the models. Reducing the extent of hot bottom burning in low-metallicity intermediate-mass stars does not suffice to solve the problem. Localized self-pollution from stars more massive than 60 M-. in NGC 1705 and/or funnelling of larger fractions of nitrogen through its winds are then left to explain the discrepancy between model predictions and observations. Inspection of the log(N/O) versus log(O/H)+12 diagram for a large sample of dwarf irregular and blue compact dwarf galaxies in the literature favours the latter hypothesis, but the physical mechanisms responsible for such a selective loss of metals remain unclear.