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 func tion (IMF) in the mass range m > 2 M.. For spirals (Sa-Sd), Magellanic irre gulars (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-ban d luminosities) and, only secondarily, on morphological type. The present s tar formation activity of massive spirals is up to 100 times smaller than t hat averaged over their lifetime, while in low-mass galaxies it is comparab le 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 t o 10 Gyr) are independent of the luminosity and morphological type. This ev idence is consistent with the idea that galaxies are coeval systems, that t hey evolved as closed boxes, forming stars following a simple, universal st ar 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 si milar conclusion was drawn by Gavazzi & Scodeggio in 1996 to explain the co lor-magnitude relation of late-type galaxies. The consequences of this inte rpretation on the evolution of the star formation rate and the gas density per comoving volume of the universe with look-back time are discussed.