Dark Matter and the Chemical Evolution of Irregular Galaxies

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Abstract
We discuss the chemical evolution of the irregular galaxies II Zw 33 and NGC 1560, and a ``typical'' irregular galaxy. Closed box models are better than O-rich outflow models in explaining the C/O, Delta Y/Delta O, and Z/O observed values. The fraction of low-mass stars needed by our models is larger than that derived for the solar vicinity, but similar to that found in globular clusters. For our typical irregular galaxy we need a mass fraction of about 40% in the form of substellar objects plus non baryonic dark matter inside the Holmberg radius, in good agreement with the results derived for II Zw 33 and NGC 1560 where we do have an independent estimate of the mass fraction in non baryonic dark matter.

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