The He I 2.06 microns/Br-gamma ratio in starburst galaxies - an objective constraint on the upper mass limit to the initial mass function

Abstract

The use of the He I λ2.06 micron/Brγ ratio as a constraint on the massive stellar population in star-forming galaxies is developed. A theoretical relationship between the He I λ2.06 micron/Brγ ratio and the effective temperature of the exciting star in H II regions is derived. The effects of collisional excitation and dust within the nebula on the ratio are also considered. It is shown that the He I λ2.06 micron/Brγ ratio is a steep function of the effective temperature, a property which can be used to determine the upper mass limit of the initial mass function (IMF) in galaxies. This technique is reliable for upper mass limits less than ~40 M_sun_. New near-infrared spectra of starburst galaxies are presented. The He I λ2.06 micron/Brγ ratios observed imply a range of upper mass limits from 27 to >~ 40 M_sun_. There is also evidence that the upper mass limit is spatially dependent within a given galaxy. These results suggest that the upper mass limit is not a uniquely defined parameter of the IMF and probably varies with local physical conditions.