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

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 star formation history, 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, R 7 kpc, of the MW, are consistent with a rapid build-up of a large fraction of its total baryonic mass within a few Gyr. 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 Milky Way 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 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.

Chemodynamics of barred galaxies in cosmological simulations: On the Milky Way’s quiescent merger history and in-situ bulge

“…The most massive mergers these galaxies experience since z ∼ 3.5 are <1:20 mergers, which is broadly consistent, although on the low mass end, with respect to recent estimates of a Gaia Sausage/Enceladus-type merger for the Milky Way 11 (e.g. Haywood et al 2018a;Helmi et al 2018;Belokurov et al 2019;Deason et al 2019; see also Bignone et al 2019). These 10 We note that the bars in our sample tend to form soon after a merger, since mergers increase the disc mass, and can also remove angular momentum from the disc thus kick-starting the formation of the bar (we will discuss in more detail the mechanisms responsible for bar formation in Auriga galaxies in Fragkoudi et al in prep.).…”

“…This suggests a stellar mass ratio of ∼ 1 : 20 for the recently proposed Gaia Sausage/Enceladus merger (see e.g. Haywood et al 2018a;Helmi et al 2018;Di Matteo et al 2018;Belokurov et al 2019 and see Sections 5.2 and 6).…”

“…On the theoretical side, recent studies using tailored, isolated simulations of Milky Way-type galaxies, have shown that the metal-poor populations in the bulge of the Milky Way are in fact consistent with being composed of the thick disc seen at the Solar neighbourhood, with no need for an additional 'classical' bulge component (Di Matteo 2016;Fragkoudi et al 2017a;Debattista et al 2017;Portail et al 2017;Haywood et al 2018a;Fragkoudi et al 2018). These models are able to explain the chemo-morphological and chemo-kinematic relations of stellar populations in the bulge (Fragkoudi et al 2018;Gomez et al 2018), as well as its vertical and radial metallicity gradients (Fragkoudi et al 2017b).…”

Age dissection of the Milky Way discs: Red giants in theKeplerfield

“…In particular, as it became clear after Gaia DR2, the Milky Way suffered a large collision with another dwarf Galaxy, the socalled Gaia-Enceladus (Helmi et al 2018) or Gaia-Sausage (Belokurov et al 2018), roughly estimated to have happened around 10 Gyrs ago, contributing to the halo and/or thick disc population observed today (Haywood et al 2018;Sahlholdt et al 2019;Di Matteo et al 2019;Mackereth et al 2019b;Deason et al 2019;Mackereth & Bovy 2020). However, many questions remain open, namely: What was the state of the Milky Way when these mergers occurred?…”

“…From the colour-magnitude diagram (CMD) analysis of stars within 2 kpc from the Sun, Gallart et al (2019) suggested that the components of the double sequence observed in the CMD of a sample of kinematically-defined halo stars are coeval but have different metallicity, with the bluer sequence being associated with the accretion event (e.g. Helmi et al 2018;Belokurov et al 2018;Haywood et al 2018). The ages inferred by comparing stellar-model tracks to observed quantities in the CMD suggest a sharper halo age cut around 10 Gyr upon the major accretion event to the MW, while these authors also suggested the thick disc component to be younger.…”