The Disc Origin of the Milky Way Bulge

Abstract

The Galactic bulge, that is the prominent out-of-plane over-density present in the inner few kiloparsecs of the Galaxy, is a complex structure, as the morphology, kinematics, chemistry, and ages of its stars indicate. To understand the nature of its main components-those at [Fe/H] −1 dex-it is necessary to make an inventory of the stellar populations of the Galactic disc(s), and of their borders: the chemistry of the disc at the solar vicinity, well known from detailed studies of stars over many years, is not representative of the whole disc. This finding, together with the recent revisions of the mass and sizes of the thin and thick discs, constitutes a major step in understanding the bulge complexity. N-body models of a boxy-/peanut-shaped bulge formed from a thin disc through the intermediary of a bar have been successful in interpreting a number of global properties of the Galactic bulge, but they fail in reproducing the detailed chemo-kinematic relations satisfied by its components and their morphology. It is only by adding the thick disc to the picture that we can understand the nature of the Galactic bulge.

Quantifying the (X/peanut)-shaped structure of the Milky Way – new constraints on the bar geometry

“…This scenario is supported by numerical simulations as well as observational evidence that most of the stars in the bulge originate from the disc (Shen et al 2010;Ness et al 2013Ness et al , 2014Di Matteo et al 2014;Di Matteo 2016), implying that it formed predominantly from the buckling and secular evolution of the disc and bar. In support of this picture, WGP15 have argued that the angle of the long bar is smaller than previously thought, and is consistent with that of the elongated 'bulge'.…”

“…Multiple studies, based on stellar populations and numerical simulations, have shown evidence that the Milky Way's central 'bulge' is not (primarily) the remnant of past merger events, i.e., a 'classical' bulge, but rather it was built predominantly from disc stars through the buckling and secular evolution of the Galactic bar, the latter itself originating from the disc (Shen et al 2010, Ness et al 2012Di Matteo et al 2014;Di Matteo 2016;Abbott et al 2017; see also Fragkoudi et al 2017). This result is consistent with the X/P morphology and indicates that the X/P 'bulge' and bar are aligned, since one has formed from, and is still the thick central part of, the other (see also Martinez-Valpuesta & Gerhard 2011, Romero-Gómez et al 2011and Wegg, Gerhard & Portail 2015.…”

What the Milky Way bulge reveals about the initial metallicity gradients in the disc

“…Our findings suggest that the stellar populations that make up the inner MW arose from an ISM that was well mixed and turbulent and whose radial metallicity gradients were mostly flat (Haywood et al 2013;Nidever et al 2014;Feng & Krumholz 2014;Di Matteo et al 2015;Haywood et al 2015;Wuyts et al 2016;Di Matteo 2016), with stars first forming in a geometrically thick layer and then in thinner layers in an upsidedown fashion (e.g. Bird et al 2013).…”

“…Another recently proposed scenario for reconciling the disc origin of the MW bulge with its vertical metallicity gradient is one in which the metal-poor, α-enhanced stars in the bulge are part of the same population as the thick disc stars at the solar vicinity (Bekki & Tsujimoto 2011;Haywood et al 2013;Di Matteo et al 2015;Di Matteo 2016); thus the vertical metallicity gradient is present in the disc initially, and is enhanced through vertical heating by the bar. This scenario also reproduces the morphological and kinematic properties of the MW bulge (Athanassoula et al 2017;Debattista et al 2017;Fragkoudi et al 2017).…”

Building blocks of the Milky Way's accreted spheroid

“…Although it is still unclear to which extent accretion plays a role besides instabilities of the disc in the formation of the Galactic bulge (e.g., Combes 2000;Gerhard 2015;Di Matteo 2016), there is growing consensus on the formation of the Galactic halo. Since the proposed scenario of Searle & Zinn (1978) in which the stellar halo formed via the merging of several protogalactic clouds, there have been many pieces of evidence suggesting indeed a hierarchical build-up of the Milky Way's stellar halo (e.g., Ibata et al 1994;Helmi et al 1999;Belokurov et al 2006;Bell et al 2008;Starkenburg et al 2009;Janesh et al 2015).…”