Top-Heavy IMFs in Starburst Galaxies

Abstract

Evidence for abnormal IMFs in some starburst galaxies is reviewed. Recent determinations of the near-infrared extinction in M82 weaken, or at least complicate, the suggestion that a large lower mass limit m _l is required to account for the 2µm luminosity. Weak Balmer absorption lines in several interacting and blue compact starburst galaxies may indicate a large lower mass limit, but a definite conclusion must await comparison with galaxy evolution models which include the pre-main sequence phase of stellar evolution. Three additional approaches to the problem are considered: 1. Comparison of stellar mass produced in starbursts with dynamical masses; 2. Comparison of models with observed ratios of ultraviolet absorption line equivalent widths; 3. Comparison of models with observed nebular emission line ratios. These three methods all point to an excess of massive stars in a significant fraction (~10–50 percent) of starburst galaxies if the upper mass limit is not significantly larger than about 80–100 M_⊙. Difficulties involved in testing for very large upper mass limits are outlined. It is also emphasized that the majority of starburst galaxies do not seem to require a top-heavy IMF, so models postulating a feedback between the star formation rate and the IMF must be taken with reservation. The dependence of the yield of metals on ml suggests that ml cannot be much larger than about 1M_⊙ in blue compact dwarfs if the chemical mixing is confined to HII regions.