The Massive Stellar Content in Starburst Galaxies and its Impact on Galaxy Evolution

Starburst galaxies are powered by massive stars. These stars dominate the heating and enrichment with heavy elements of the interstellar medium, gas out of which new stars form. Thus, high-mass stars, and in consequence starburst galaxies, are an important (in some cases the dominant) energy source for the evolution of galaxies and the universe. In this contribution I review techniques to derive the massive stellar content in starburst galaxies and their evolutionary state. They are based on the analysis of their integrated light with evolutionary synthesis models. The massive stellar population is derived in a self-consistent way using the continuum and stellar wind resonance lines in the ultraviolet, the photospheric H Balmer and HeI lines and the nebular emission lines at optical wavelengths. Comparison with observations provides constraints on the most recent star formation history in starburst galaxies and their evolutionary state. 1. Starburst Galaxies and its Cosmological relevance Massive stars are defined as having masses higher than 10 M⊙ when they are at the zero age main sequence (ZAMS). They are very luminous and dominate the heating of the interstellar medium (ISM), gas out of which new stars form. They emit photons that can ionize the surrounding interstellar gas and deposit mechanical energy both via stellar winds and supernovae. Most of the heavy elements are formed in massive stars, and these metals are dispersed in the ISM when these stars finally explode as supernova. Therefore, massive stars are an important source (sometimes the dominant one) for the evolution of a galaxy. Massive stars are formed in the arms of normal spiral galaxies. However, the star formation rate is low (≃ 1 M⊙ yr) and the gas available in these galaxies is enough to sustain the rate for many Gyr. On the contrary, much higher rate of star formation occurs in the galaxies know as starburst galaxies. A starburst is defined as a brief episode of intense star formation that is taking place in small regions (100 to 1000 pc) of the galaxy and dominates the overall luminosity of the galaxy (Weedman 1983; Heckman 1998). The star formation rate is so high (10-100 M⊙ yr and in some cases up to 1000 M⊙ yr) that galaxies can be in this phase only for a small fraction of the age of the universe (≤10 yr) because the gas available in these galaxies cannot sustain