The problem of chemo-photometric evolution of late-type galaxies is dealt with relying on prime physical arguments of energetic self-consistency between chemical enhancement of galaxy mass , through nuclear processing inside stars , and luminosity evolution of the system .
Our analysis makes use of the Buzzoni template galaxy models along the Hubble morphological sequence .
The contribution of Type ii and i a SNe is also accounted for in our scenario .
Chemical enhancement is assessed in terms of the so-called “ yield metallicity ” ( \cal Z ) , that is the metal abundance of processed mass inside stars , as constrained by the galaxy photometric history .
For a Salpeter IMF , { \cal Z } \propto t ^ { 0.23 } being nearly insensitive to the galaxy star formation history .
The ISM metallicity can be set in terms of { \cal Z } , and just modulated by the gas fraction and the net fraction of returned stellar mass ( f ) .
For the latter , a safe upper limit can be placed , such as f \lesssim 0.3 at any age .

The comparison with the observed age-metallicity relation allows us to to set a firm upper limit to the Galaxy birthrate , such as b \lesssim 0.5 , and to the chemical enrichment ratio \Delta Y / \Delta Z \lesssim 5 .
About four out of five stars in the solar vicinity are found to match the expected \cal Z figure within a factor of two , a feature that leads us to conclude that star formation in the Galaxy must have proceeded , all the time , in a highly contaminated environment where returned stellar mass is in fact the prevailing component to gas density .

The possible implication of the Milky Way scenario for the more general picture of late-type galaxy evolution is dicussed moving from three relevant relationships , as suggested by the observations .
Namely , i ) the down-sizing mechanism appears to govern star formation in the local Universe ; ii ) the “ delayed ” star formation among low-mass galaxies , as implied by the inverse b - M _ { gal } dependence , naturally leads to a more copious gas fraction when moving from giant to dwarf galaxies ; iii ) although lower-mass galaxies tend more likely to take the look of later-type spirals , it is mass , not morphology , that drives galaxy chemical properties .
Facing the relatively flat trend of \cal Z vs. galaxy type , the increasingly poorer gas metallicity , as traced by the [ O / H ] abundance of H ii regions along the Sa \to Im Hubble sequence , seems to be mainly the result of the softening process , that dilute enriched stellar mass within a larger fraction of residual gas .

The problem of the residual lifetime for spiral galaxies as active star-forming systems has been investigated .
If returned mass is left as the main ( or unique ) gas supplier to the ISM , as implied by the Roberts timescale , then star formation might continue only at a maximum birthrate b _ { max } \ll f / ( 1 - f ) \lesssim 0.45 , for a Salpeter IMF .
As a result , only massive ( M _ { gal } \gtrsim 10 ^ { 11 } ~ { } M _ { \odot } ) Sa/Sb spirals may have some chance to survive \sim 30 % or more beyond a Hubble time .
Things may be worse , on the contrary , for dwarf systems , that seem currently on the verge of ceasing their star formation activity unless to drastically reduce their apparent birthrate below the b _ { max } threshold .