Constraints on a Universal Stellar Initial Mass Function from Ultraviolet to Near‐Infrared Galaxy Luminosity Densities

Abstract

We obtain constraints on the slope of a universal stellar initial mass function (IMF) over a range of model cosmic star formation histories (SFHs) using z ? 0.1 luminosity densities in the range from 0.2 to 2.2 ?m. The age-IMF degeneracy of the integrated spectra of stellar populations can be broken for the universe as a whole by using direct measurements of (relative) cosmic SFH from high-redshift observations. These have only marginal dependence on uncertainties in the IMF, whereas fitting to local luminosity densities depends strongly on both cosmic SFH and the IMF. We fit to these measurements using population synthesis and find the best-fit IMF power-law slope to be ? = 1.15 ? 0.2 (assuming dN/d log m ? m-? for 0.5-120 M? and m-0.5 for 0.1-0.5 M ?). This M > 0.5 M? slope is in good agreement with the Salpeter IMF slope (? = 1.35). A strong upper limit of ? < 1.7 is obtained, which effectively rules out the Scalo IMF because its fraction of high-mass stars is too low. This upper limit is at the 99.7% confidence level if we assume a closed-box chemical evolution scenario and 95% if we assume constant solar metallicity. Fitting to the Ha line luminosity density, we obtain a best-fit IMF slope in good agreement with that derived from broadband measurements. Marginalizing over cosmic SFH and IMF slope, we obtain (95% confidence ranges) ?stars = (1.1-2.0) ? 10-3 h-1 for the stellar mass density, ? SFR = (0.7-4.1) ? 10-2 h M? yr -1 Mpc-3 for the star formation rate density, and ?L = (1.2-1.7) ? 1035 h W Mpc-3 for the bolometric, attenuated, stellar luminosity density (0.09-5 ?m). Comparing this total stellar emission with an estimate of the total dust emission implies a relatively modest average attenuation in the UV ( ? 1 mag at 0.2 ?m).