Phylogeny of the Milky Way’s inner disk and bulge populations: Implications for gas accretion, (the lack of) inside-out thick disk formation, and quenching

¹ GEPI, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Paris Cité, 5 place Jules Janssen, 92190 Meudon, France
e-mail: misha.haywood@obspm.fr
² Sorbonne Université, CNRS, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
³ School of Physics, Korea Institute for Advanced Study, 85 Hoegiro, Dongdaemun-gu, Seoul, 02455 Republic of Korea
⁴ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany

Received: 13 June 2017
Accepted: 23 February 2018

Abstract

We show that the bulge and the disk of the Milky Way (MW) at R ≲ 7 kpc are well described by a unique chemical evolution and a two-phase star formation history (SFH). We argue that the populations within this inner disk, not the entire disk, are the same, and that the outer Lindblad resonance (OLR) of the bar plays a key role in explaining this uniformity. In our model of a two-phase SFH, the metallicity, [α/Fe] and [α/H] distributions, and age-metallicity relation are all compatible with the observations of both the inner disk and bulge. The dip at [Fe/H] ∼ 0 dex seen in the metallicity distributions of the bulge and inner disk reflects the quenching episode in the SFH of the inner MW at age ∼8 Gyr, and the common evolution of the bulge and inner disk stars. Our results for the inner region of the MW, R ≲ 7 kpc, are consistent with a rapid build-up of a large fraction of its total baryonic mass within a few billion years. We show that at z ≤ 1.5, when the MW was starting to quench, transitioning between the end of the α-enhanced thick disk formation to the start of the thin disk, and yet was still gas rich, the gas accretion rate could not have been significant. The [α/Fe] abundance ratio before and after this quenching phase would be different, which is not observed. The decrease in the accretion rate and gas fraction at z ≤ 2 was necessary to stabilize the disk allowing the transition from thick to thin disks, and for beginning the secular phase of the MW’s evolution. This possibly permitted a stellar bar to develop which we hypothesize is responsible for quenching the star formation. The present analysis suggests that MW history, and in particular at the transition from the thick to the thin disk – the epoch of the quenching – must have been driven by a decrease of the star formation efficiency. We argue that the decline in the intensity of gas accretion, the formation of the bar, and the quenching of the star formation rate (SFR) at the same epoch may be causally connected thus explaining their temporal coincidence. Assuming that about 20% of the gas reservoir in which metals are diluted is molecular, we show that our model is well positioned on the Schmidt-Kennicutt relation at all times.