Effects of the radial flows on the chemical evolution of the Milky Way disk

Abstract

Context. The majority of chemical evolution models assume that the Galactic disk forms by means of infall of gas and divide the disk into several independent rings without exchange of matter between them. However, if gas infall is important, radial gas flows should be taken into account as a dynamical consequence of infall. Aims. We test the effects of radial gas flows on detailed chemical evolution models (one-infall and two-infall) of the Milky Way disk with different prescriptions for the infall law and star formation rate. Methods. We modified the equation of chemical evolution to include radial gas flows according to the method described in Portinari & Chiosi (2000, A&A, 355, 929). Results. We found that with a gas radial inflow of constant speed the metallicity gradient tends to steepen. Taking into account a constant timescale for the infall rate along the Galaxy disk and radial flows with a constant speed, we obtained too flat a gradient, at variance with data, implying that an inside-out formation and/or a variable gas flow speed are required. To explain the observed gradients, the gas flow should increase in modulus with the galactocentric distance, in both the one-infall and two-infall models. However, the inside-out disk formation coupled with a threshold in the gas density (only in the two-infall model) for star formation and/or a variable efficiency of star formation with galactocentric distance can also reproduce the observed gradients without radial flows. Conclusions. We show that the radial flows can be the most important process in reproducing abundance gradients but only with a variable gas speed. Finally, one should consider that uncertainties in the data concerning gradients prevent us from drawing firm conclusions. Future more detailed data will help us to ascertain whether the radial flows are a necessary ingredient in the formation and evolution of the Galactic disk and disks in general.

The thick disk rotation-metallicity correlation as a fossil of an “inverse chemical gradient” in the early Galaxy

“…In the light of our results, the thick disk rotation-metallicity correlation (Spagna et al 2010;Lee et al 2011) seems to represent an important signature of the disk evolution. This feature can indeed be explained by the role of the dynamical migration in our N-body simulations, once we assume an initial radial chemical gradient as that suggested by the chemical evolution models of Spitoni & Matteucci (2011), which prescribes a positive slope for the inner early disk, R < ∼ 10 kpc, combined with the usual decreasing slope in the outer disk. The crucial role of a positive inner slope for the primordial chemical distribution to produce a positive rotation-metallicity correlation was presented in Curir et al (2012), where a distribution consisting of two simple linear functions with positive slope for inner radii up to 6 kpc and a negative one outwards was plugged in the early (N-body) disk 3 .…”

“…Radial flows were not considered 1 . The model adopted in this paper is Model S2IT of Spitoni & Matteucci (2011), their Table 1: it is a two-infall model similar to that presented in Chiappini et al (2001) and Cescutti et al (2007). This particular model was chosen because it accurately reproduces the most recent estimate of the present time abundance gradients by Luck & Lambert (2011) in a galactocentric range 4-14 kpc.…”

Are ancient dwarf satellites the building blocks of the Galactic halo?

“…In this work, we did not explore different values of the threshold in the halo, our aim being to test the effects of the enriched infall on chemical evolution models of the MW, which are able to reproduce the majority of the observations in the solar neighbourhood and also the abundance gradients along the disc. As shown in Mott, Spitoni & Matteucci (2013), if we do not take into account radial gas flows (Portinari & Chiosi 2000;Spitoni & Matteucci 2011;Cavichia et al 2014), a threshold in the gas density is required to explain the abundance gradients along the Galactic disc, and in particular the values of 4 M pc −2 in the halo phase and 7 M pc −2 in the thin disc phase, provide a very good agreement with the data.…”

Understanding inverse metallicity gradients in galactic discs as a consequence of inside-out formation

“…Curir et al (2012) linked this to inverse metallicity gradients (i.e. larger metallicities in the outer regions of a disc) at early times, identified in the chemical evolution models of Spitoni & Matteucci (2011) and Chiappini et al (2001), as well as occasionally observed in high-redshift galaxies, such as those studied by Cresci et al (2010).…”