2011MNRAS.415.1155B

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 Ia SNe is also accounted for in our scenario. Chemical enhancement is assessed in terms of the so-called `yield metallicity' ({img}), that is the metal abundance of processed mass inside stars, as constrained by the galaxy photometric history. For a Salpeter initial mass function (IMF), {img} being nearly insensitive to the galaxy star formation history. The interstellar medium (ISM) metallicity can be set in terms of {img}, 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 ≲ 0.3 at any age.

The comparison with the observed age-metallicity relation allows us to set a firm upper limit to the Galaxy birthrate, such as b ≲ 0.5, and to the chemical enrichment ratio ΔY/ΔZ ≲ 5. About four out of five stars in the solar vicinity are found to match the expected {img} figure within a factor of 2, 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 discussed 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_galdependence, 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 {img} versus galaxy type, the increasingly poorer gas metallicity, as traced by the [O/H] abundance of H II regions along the Sa -> 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 time-scale, then star formation might continue only at a maximum birthrate b_max ≪ f/(1 -f) ≲ 0.45, for a Salpeter IMF. As a result, only massive (M_gal≳ 10¹¹ M_☉) Sa/Sb spirals may have some chance to survive ∼30 per cent 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_maxthreshold.