Galactic-Field IMFs of Massive Stars

Over the past years observations of young and populous star clusters have shown that the stellar IMF appears to be an invariant featureless Salpeter power-law with an exponent alpha=2.35 for stars more massive than a few Msun. A consensus has also emerged that most, if not all, stars form in stellar groups and star clusters, and that the mass function of young star clusters in the solar-neighborhood and in interacting galaxies can be described, over the mass range of a few 10 Msun to 10^7 Msun, as a power-law with an exponent near beta=2. These two results imply that galactic-field IMFs for early-type stars cannot, under any circumstances, be a Salpeter power-law, but that they must have a steeper exponent alpha_field>2.8. This has important consequences for the distribution of stellar remnants and for the chemo-dynamical and photometric evolution of galaxies.