1.65 Micron (H band) surface photometry of galaxies.: VI.: The history of star formation in normal late-type galaxies

We have collected a large body of near-IR (H band), UV (2000 Angstrom) and H alpha measurements of late-type galaxies galaxies. These are used jointly with spectral evolutionary synthesis models to study the initial mass function (IMF) in the mass range m > 2 M.. For spirals (Sa-Sd), Magellanic irregulars (Im), and blue compact dwarfs, our determination is consistent with a Salpeter IMF with an upper mass cutoff M-up similar to 80 M.. The history of star formation and the amount of total gas (per unit mass) of galaxies are found to depend primarily on their total masses (as traced by the H-band luminosities) and, only secondarily, on morphological type. The present star formation activity of massive spirals is up to 100 times smaller than that averaged over their lifetime, while in low-mass galaxies it is comparable to or higher than that at earlier epochs. Dwarf galaxies presently have larger gas reservoirs per unit mass than massive spirals. The efficiency in transforming gas into stars and the timescale for gas depletion (similar to 10 Gyr) are independent of the luminosity and morphological type. This evidence is consistent with the idea that galaxies are coeval systems, that they evolved as closed boxes, forming stars following a simple, universal star formation law whose characteristic timescale is small (tau similar to 1 Gyr) in massive spirals and large (tau > 10 Gyr) in low-mass galaxies. A similar conclusion was drawn by Gavazzi & Scodeggio in 1996 to explain the color-magnitude relation of late-type galaxies. The consequences of this interpretation on the evolution of the star formation rate and the gas density per comoving volume of the universe with look-back time are discussed.