On the typical timescale for the chemical enrichment from Type Ia supernovae in galaxies

We calculate the Type Ia supernova (SN) rate for different star formation histories in galaxies by adopting the most popular and recent progenitor models. We show that the timescale for the maximum in the SN Ia rate, which corresponds also to time of the maximum enrichment, is not unique but is a strong function of the adopted stellar lifetimes, initial mass function, and star formation rate. This timescale varies from similar to 40-50 Myr for an instantaneous starburst to similar to0.3 Gyr for a typical elliptical galaxy to similar to 4.0-5.0 Gyr for a disk of a spiral galaxy like the Milky Way. We also show that the typical timescale of 1 Gyr, often quoted as the typical timescale for the SNe la, is just the time at which, in the solar neighborhood, the Fe production from SNe Ia starts to become important and not the time at which SNe Ia start to explode. As a consequence of this, a change in slope in the [O/Fe] ratio is expected in correspondence of this timescale. We conclude that the suggested lack of SNe Ia at low metallicities produces results at variance with the observed [O/Fe] versus [Fe/H] relation in the solar region. We also compute the SN Ia rates for different galaxies as a function of redshift and predict an extended maximum between redshift z similar to 3.6 and z similar to 1.6 for elliptical galaxies, and two maxima, one at z similar to 3 and the other at z similar to 1, for spiral galaxies, under the assumption that galaxies start forming stars at z(f) similar to 5 and Omega (M) = 0.3, Q(Lambda) = 0.7.