We discuss the observational constraints to the chemical evolution of the interstellar medium, ISM, provided by the abundances of HII regions. We present a review of the results derived from these constraints for the solar vicinity and for metal-poor galaxies. It is found that, contrary to previous results, black holes do not play an important role in the chemical evolution of galaxies. Chemical evolution considerations indicate that substellar objects (M ≤ 0.08 M⊙) have a mass density smaller than 0.02 M⊙ pc−3 (2σ) in the solar neighborhood. One or more of the following ingredients are needed to explain the Z versus µ diagram of metal-poor galaxies: a) a heavy-element yield increasing with Z due to a variation of the low-mass end of the IMF, b) outflow of Z-rich material, c) outflow of well-mixed material, and d) dark matter that does not participate in the chemical evolution process.

Introduction

HII regions are excellent probes of the chemical composition of the ISM of the Galaxy and of other galaxies. From the study of bright HII regions it is possible to derive accurate abundances of H, He, C, N, O, Ne, S, and Ar for galaxies that are located at many megaparsecs from us. Two important sources of error in the abundance determinations of bright HII regions are the temperature structure and the fraction of heavy elements that is embedded in dust.