HIGH-MASS STAR FORMATION IN NORMAL LATE-TYPE GALAXIES: OBSERVATIONAL CONSTRAINTS TO THE INITIAL MASS FUNCTION

We use H alpha and far-ultraviolet (FUV, 1539 angstrom) Galaxy Evolution Explorer (GALEX) data for a large sample of nearby objects to study the high-mass (m >= 2 M-circle dot) star formation activity of normal late-type galaxies. The data are corrected for dust attenuation using the most accurate techniques available at present, namely the Balmer decrement for H alpha data and the total far-infrared to FUV flux ratio for GALEX data. The sample shows a highly dispersed distribution in the H alpha to FUV flux ratio (log f (H alpha)/ f (FUV) = 1.10 +/- 0.34 angstrom) indicating that two of the most commonly used star formation tracers give star formation rates (SFRs) with uncertainties up to a factor of 2 -3. The high dispersion is partly due to the presence of active galactic nuclei, where the UV and the H alpha emission can be contaminated by nuclear activity, highly inclined galaxies, for which the applied extinction corrections are probably inaccurate, or starburst galaxies, where the stationarity in the star formation history required for transforming H alpha and UV luminosities into SFRs is not satisfied. Excluding these objects, normal late-type galaxies have log f (H alpha)/ f (FUV) = 0.94 +/- 0.16 angstrom, which corresponds to an uncertainty of similar to 50% on the SFR. The H alpha to FUV flux ratio of the observed galaxies increases with their total stellar mass. If limited to normal star-forming galaxies, however, this relationship reduces to a weak trend that might be totally removed using different extinction correction recipes. In these objects, the H alpha to FUV flux ratio seems also barely related to the FUV-H color, the H-band effective surface brightness, the total star formation activity, and the gas fraction. The data are consistent with a Kroupa and Salpeter initial mass function (IMF) in the high-mass stellar range (m > 2M(circle dot)) and imply, for a Salpeter IMF, that the variations of the slope gamma cannot exceed 0.25, from gamma = 2.35 for massive galaxies to gamma = 2.60 in low luminosity systems. We show however that these observed trends, if real, can be due to the different micro-history of star formation in massive galaxies with respect to dwarf systems.