Chemo-spectrophotometric evolution of spiral galaxies – IV. Star formation efficiency and effective ages of spirals |
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Authors: | S Boissier A Boselli N Prantzos G Gavazzi |
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Institution: | Institut d'Astrophysique de Paris, 98bis Bd. Arago, 75104 Paris, France;Laboratoire d'Astronomie Spatiale, Traverse du Siphon, F-13376 Marseille Cedex 12, France;Universitàdegli Studi di Milano –Bicocca, Pizza dell'Ateneo Nuovo 1, 20126 Milano, Italy |
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Abstract: | We study the star formation history of normal spirals by using a large and homogeneous data sample of local galaxies. For our analysis we utilize detailed models of chemical and spectrophotometric galactic evolution, calibrated on the Milky Way disc. We find that star formation efficiency is independent of galactic mass, while massive discs have, on average, lower gas fractions and are redder than their low-mass counterparts; put together, these findings convincingly suggest that massive spirals are older than low-mass ones. We evaluate the effective ages of the galaxies of our sample and we find that massive spirals must be several Gyr older than low-mass ones. We also show that these galaxies (having rotational velocities in the 80–400 km s?1 range) cannot have suffered extensive mass losses, i.e. they cannot have lost during their lifetime an amount of mass much larger than their current content of gas+stars. |
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Keywords: | galaxies: evolution galaxies: general galaxies: photometry galaxies: spiral galaxies: stellar content |
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