The time-scales of chemical enrichment are fundamental to understand the evolution of abundances and abundance ratios in galaxies .
In particular , the time-scales for the enrichment by SNe II and SNe Ia are crucial in interpreting the evolution of abundance ratios such as [ \alpha /Fe ] .
In fact , the \alpha -elements are produced mainly by SNe II on time-scales of the order of 3 to 30 Myr , whereas the Fe is mainly produced by SNe Ia on a larger range of time-scales , going from 30 Myr to a Hubble time .
This produces differences in the [ \alpha /Fe ] ratios at high and low redshift and it is known as ” time-delay ” model .
In this talk we review the most common progenitor models for SNe Ia and the derived rates together with the effect of the star formation history on the [ \alpha /Fe ] versus [ Fe/H ] diagram in the Galaxy .
From these diagrams we can derive the timescale for the formation of the inner halo ( roughly 2 Gyr ) , the timescale for the formation of the local disk ( roughly 7-8 Gyr ) as well the time-scales for the formation of the whole disk .
These are functions of the galactocentric distance and vary from 2-3 Gyr in the inner disk up to a Hubble time in the outer disk ( inside-out formation ) .
Finally , the timescale for the formation of the bulge is found to be no longer than 0.3 Gyr , similar to the timescale for the formation of larger spheroids such as elliptical galaxies .
We show the time-delay model applied to galaxies of different morphological type , identified by different star formation histories , and how it constrains differing galaxy formation models .