Abundance gradients in spiral discs: is the gradient inversion at high redshift real?

Abstract

We compute the abundance gradients along the disc of the Milky Way by means of the two-infall model: in particular, the gradients of oxygen and iron and their temporal evolution. First, we explore the effects of several physical processes which influence the formation and evolution of abundance gradients. They are (i) the inside-out formation of the disc, (ii) a threshold in the gas density for star formation, (iii) a variable star formation efficiency along the disc, (iv) radial flows and their speed and (v) different total surface mass density (gas plus stars) distributions for the halo. We are able to reproduce at best the present day gradients of oxygen and iron if we assume an inside-out formation, no threshold gas density, a constant efficiency of star formation along the disc and radial gas flows. It is particularly important the choice of the velocity pattern for radial flows and the combination of this velocity pattern with the surface mass density distribution in the halo. Having selected the best model, we then explore the evolution of abundance gradients in time and find that the gradients in general steepen in time and that at redshift z ∼ 3 there is a gradient inversion in the inner regions of the disc, in the sense that at early epochs the oxygen abundance decreases towards the Galactic Centre. This effect, which has been observed, is naturally produced by our models if an inside-out formation of the disc and a constant star formation efficiency are assumed. The inversion is due to the fact that in the inside-out formation a strong infall of primordial gas, contrasting chemical enrichment, is present in the innermost disc regions at early times. The gradient inversion remains also in the presence of radial flows, either with constant or variable speed in time, and this is a new result.