Evidence for a Non-uniform Initial Mass Function in the Local Universe

Many of the results in modern astrophysics rest on the notion that the Initial Mass Function (IMF) is universal. Our observations of a sample of H I selected galaxies in the light of Hα and the far-ultraviolet (FUV) challenge this result. The extinction corrected flux ratio FHα/ fFUV from these two tracers of star formation shows strong correlations with the surface-brightness in Hα and the R band: Low Surface Brightness (LSB) galaxies have lower FHα/ fFUV ratios compared to High Surface Brightness (HSB) galaxies as well as compared to expectations from equilibrium models of constant star formation rate (SFR) using commonly favored IMF parameters. Weaker but significant correlations of FHα/ fFUV with luminosity, rotational velocity and dynamical mass are found as well as a systematic trend with morphology. The correlated variations of FHα/ fFUV with other global parameters are thus part of the larger family of galaxy scaling relations. The FHα/ fFUV correlations can not be due to residual extinction correction errors, while systematic variations in the star formation history can not explain the trends with both Hα and R surface brightness nor with other global properties. The possibility that LSB galaxies have a higher escape fraction of ionizing photons seems inconsistent with their high gas fraction, and observations of color-magnitude diagrams of a few systems which indicate a real deficit of O stars. The most plausible explanation for the correlations is the systematic variations of the upper mass limit Mu and/or the slope γ which define the upper end of the IMF. We outline a scenario of pressure driving the correlations by setting the efficiency of the formation of the dense star clusters where the highest mass stars preferentially form. Our results imply that the star formation rate measured in a galaxy is highly sensitive to the tracer used in the measurement. A non-universal IMF would also call into question interpretation of metal abundance patterns in dwarf galaxies as well star formation histories derived from color magnitude diagrams. Subject headings: galaxies: stellar content – stars: luminosity function, mass function – stars: formation – galaxies: ISM * Based on observations made with the NASA Galaxy Evolution Explorer. GALEX is operated for NASA by the California Institute of Technology under NASA contract NAS5-98034. 1 Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218; [email protected] 2 Department of Astronomy, Yale University, New Haven, CT 06520 3 School of Physics, University of Melbourne, VIC 3010, Australia 4 University of Michigan, Department of Astronomy, 830 Denison Building, Ann Arbor, MI 48109-1042 5 Institute of Astronomy, School of Physics, University of Sydney, Australia 6 Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia 7 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany 8 Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, NSW 1710, Australia 9 Observatories of the Carnegie Institute of Washington, Pasadena, CA, 91101 10 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 11 Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027 12 WIYN Consortium, Inc., 950 North Cherry Avenue, Tucson, AZ 85726 13 Department of Physics, University of Queensland, Brisbane, QLD 4072, Australia 14 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Mail 31, PO Box 218, Hawthorn, VIC 3122, Australia 15 School of Physics, University of Western Australia, Crawley, WA 6009, Australia 16 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, United Kingdom 17 Cerro Tololo Inter-American Observatory (CTIO), Casilla 603, La Serena, Chile