1.65 Micron ([ITAL]H[/ITAL] Band) Surface Photometry of Galaxies. VI. The History of Star Formation in Normal Late-Type Galaxies

We have collected a large body of NIR (H band), UV (2000 A) and Halphameasurements of late-type galaxies. These are used, jointly with spectralevolutionary synthesis models, to study the initial mass function (IMF) in themass range m > 2 Mo. For spirals (Sa-Sd), Magellanic irregulars (Im) and bluecompact dwarfs (BCD), our determination is consistent with a Salpeter IMF withan upper mass cutoff M_up = 80 Mo. The history of star formation and the amountof total gas (per unit mass) of galaxies are found to depend primarily on theirtotal masses (as traced by the H band luminosities) and only secondarily onmorphological type. The present star formation activity of massive spirals isup to 100 times smaller than that average over their lifetime, while in lowmass galaxies it is comparable to or higher than that at earlier epochs. Dwarfgalaxies have presently larger gas reservoirs per unit mass than massivespirals. The efficiency in transforming gas into stars and the time scale forgas depletion (10 Gyrs) are independent of the luminosity and/or of themorphological type. These evidences are consistent with the idea that galaxiesare coeval systems,that they evolved as closed-boxes forming stars following asimple, universal star formation law whose characteristic time scale is small(1 Gyr) in massive spirals and large (10 Gyr) in low mass galaxies. A similarconclusion was drawn by Gavazzi and Scodeggio (1996) to explain thecolour-magnitude relation of late-type galaxies. The consequences of thisinterpretation on the evolution of the star formation rate and of the gasdensity per comoving volume of the Universe with look-back time are discussed.Comment: LaTex, 24 pages, 12 figures, accepted for publication on Astronomical Journa