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

We obtain constraints on the slope of a universal stellar initial massfunction (IMF) over a range of cosmic star-formation histories (SFH) usingz=0.1 luminosity densities in the range from 0.2 to 2.2 microns. The age-IMFdegeneracy of integrated spectra of stellar populations can be broken for theUniverse as a whole by using direct measurements of (relative) cosmic SFH fromhigh-redshift observations. These have only marginal dependence onuncertainties in the IMF, whereas, fitting to local luminosity densitiesdepends strongly on both cosmic SFH and the IMF. We fit to these measurementsusing population synthesis and find the best-fit IMF power-law slope to beGamma=1.15+-0.2 (0.5 < M/M_solar < 120). This slope is in good agreement withthe Salpeter IMF slope (Gamma=1.35). A strong upper limit of Gamma<1.7 isobtained which effectively rules out the Scalo IMF due to its too low fractionof high-mass stars. This upper limit is at the 99.7% confidence level if weassume a closed-box chemical evolution scenario and 95% if we assume constantsolar metallicity. Fitting to the H-alpha line luminosity density, we obtain abest-fit IMF slope in good agreement with that derived from broadbandmeasurements. Marginalizing over cosmic SFH and IMF slope, we obtain (95% conf.ranges, h=1): omega_stars = 1.1-2.0 E-3 for the stellar mass density; rho_sfr =0.7-4.1 E-2 M_solar/yr/Mpc^3 for the star-formation rate density, and; rho_L =1.2-1.7 E+35 W/Mpc^3 for the bolometric, attenuated, stellar, luminositydensity (0.09-5 microns). Comparing this total stellar emission with anestimate of the total dust emission implies a relatively modest averageattenuation in the UV (<=1 magnitude at 0.2 microns).Comment: 16 pages, accepted by Ap