Constraints on a Universal Imf from Uv to Near-ir Galaxy Luminosity Densities

We obtain constraints on the slope of a universal stellar initial mass function (IMF) over a range of cosmic star-formation histories (SFH) using z ≈ 0.1 luminosity densities in the range from 0.2μm to 2.2μm. The age-IMF degeneracy of 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 Γ = 1.15 ± 0.2 (assuming dN/d logm ∝ m for 0.5–120M⊙ and m −0.5 for 0.1–0.5M⊙). This M > 0.5M⊙ slope is in good agreement with the Salpeter IMF slope (Γ = 1.35). A strong upper limit of Γ < 1.7 is obtained which effectively rules out the Scalo IMF due to its too low fraction of high-mass stars. 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 Hα 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% conf. ranges): Ωstars = 1.1–2.0 ×10 −3 h for the stellar mass density; ρSFR = 0.7–4.1 ×10 −2 hM⊙ yr −1 Mpc for the star-formation rate density, and; ρL = 1.2–1.7 ×10 35 hWMpc for the bolometric, attenuated, stellar, luminosity density (0.09–5μm). Comparing this total stellar emission with an estimate of the total dust emission implies a relatively modest average attenuation in the UV (. 1 magnitude at 0.2μm). Subject headings: stars: formation — stars: luminosity function, mass function — dust, extinction — galaxies: luminosity function, mass function — galaxies: stellar content — cosmology: miscellaneous