A generalized Schmidt star formation law: Observational constraints

This paper aims to see whether a generalized Schmidt star formation law is consistent with observations in the light of a closed, comoving model of chemical evolution, where two main phases occurred: contraction (extended component) and equilibrium (flat component). It is shown that the empirical relation between present-day gas mass which is being turned into stars and gas mass, M-dotga proportional to Mga, i.e. a linear correlation between the model present-day star formation rate and gas mass fraction, CD proportional to mua, yields a connection between the ratio of contraction time to age, Tc/T, and mua. The comparison between model predictions and observations leads to the following main conclusions: (1) models where star formation obeys a pure Schmidt law cannot fit the whole set of observations; (2) if we demand (in dealing with an averaged CD/mua) Tc/T greater than 0 for all the galaxies and Tc/T less than 0.5 for at least a fraction of large-mass (M greater than 1011 solar mass) galaxies of the sample considered, then a generalized Schmidt star formation law with an exponent n approximately equal to 1 is preferred in respect to n approximately equal to 2; (3) if contraction and equilibrium phase were characterized by different star formation rates and mass spectra, then a correlation exists between present-day metal content and gas mass fraction. Thus a generalized Schmidt star formation law with exponent n approximately equal to 1 appears to be consitent with observations.