1996A&A...312..797C

This paper aims to see whether the observed distribution of the Roberts time for different samples of late-type galaxies of a given class (normal, active) may be fitted by a single theoretical distribution, and what additional constraints are put on models obeying a generalized Schmidt star formation law. We find that an extended Poisson distribution with expected value k^*=2.5Gyr and median value k^{ddaggfer}=2.83Gyr is consistent with five different samples of normal galaxies, provided systems with Roberts time T_R>7Gyr are excluded, and an extended Poisson distribution with expected value k^*=0.05Gyr and median value k^{ddaggfer}=0.52Gyr is consistent with both a sample of active galaxies and a sample of compact HII regions. In this view, active galaxies and ``normal'' galaxies with T<sub>R</sub>>7Gyr should be regarded as merger products in different stages of evolution. In dealing with a generalized Schmidt star formation law, we take into consideration closed, comoving models of chemical evolution. The history of a galaxy is described by two main phases: contraction (which produces the extended component) and equilibrium (which gives the disk). Negative values of the ratio of contraction time to age of a galaxy, T<sub>c</sub>/T, relate with present-day, fractional star formation rates C<sub>D</sub>≲0.025G/yr and Roberts times T<sub>R</sub>>5Gyr but, in general, the reverse is not true. Again, an extended Poisson distribution with expected value k<sup>*</sup>=2.5Gyr and median value k<sup>{ddaggfer}</sup>=2.83Gyr is consistent with two different samples (for which total masses also are reported and then ratios T<sub>c</sub>/T can be calculated) of normal galaxies, provided systems for which T<sub>c</sub>/T ≤0 are excluded. In this view, active galaxies and ``normal'' galaxies for which T_c/T≤0 should be regarded as mergers products in different stages of evolution. Further analysis provides additional support to two conclusions already established in a previous paper, namely: (i) models where star formation obeys a pure Schmidt law cannot fit the whole set of observations; and (ii) for assumed T_R=k^{ddaggfer}; T_c/T>0 for normal galaxies; and T_c/T<0.5 for at least a fraction of large-mass (M>10¹¹M_☉) normal galaxies; a generalized Schmidt star formation law with an exponent n≃1 is favoured over one with n≃2. More precisely, 1≲n≲1.8 with a preferred value n≃1.2, which makes a generalized Schmidt star formation law with n≃1 consistent with the observations. Finally, a stochastic process of star formation, related to a Poisson distribution, is briefly outlined.