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 α = 2.35 for stars more massive than a few M⊙. 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 10M⊙ to 10 M⊙, as a power-law with an exponent β ≈ 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 αfield>∼ 2.8. This has important consequences for the distribution of stellar remnants and for the chemo-dynamical and photometric evolution of galaxies. Subject headings: stars: formation – stars: luminosity function, mass function – galaxies: star clusters – galaxies: evolution – galaxies: stellar content – Galaxy: stellar content