Disk origin of the Milky Way bulge: the necessity of the thick disk

Abstract

In the Milky Way bulge, metal-rich stars form a strong bar and are more peanut-shaped than metal-poor stars. It has recently been claimed that this behavior is driven by the initial (i.e., before bar formation) in-plane radial velocity dispersion of these populations, rather than by their initial vertical random motions. This has led to the suggestion that a thick disk is not necessary to explain the characteristics of the Milky Way bulge. We discuss this issue again by analyzing two dissipationless N-body simulations of boxy or peanut-shaped bulges formed from composite stellar disks that consist of kinematically cold and hot stellar populations. These two models represent two extreme cases: one where all three components of the disk have a fixed vertical velocity dispersion and different in-plane radial dispersion, and another where they all have a fixed radial dispersion and different vertical random motions (thickness). This is intended to quantify the drivers of the main features that are observed in composite boxy or peanut-shaped bulges and their origin. We quantify the mapping into a boxy or peanut-shaped bulge of disk populations in these two cases, and we conclude that initial vertical random motions are as important as in-plane random motions in determining the relative contribution of cold-and hotdisk populations with height above the plane, the metallicity and age trends. Previous statements emphasizing the dominant role of in-plane motions in determining these trends are not confirmed. However, significant differences exist in the morphology and strength of the resulting boxy or peanut-shaped bulges. In particular, the model where disk populations initially have only different in-plane random motions, but similar thickness, results in a boxy or peanut-shaped bulge where all populations have a similar peanut shape, independent of their initial kinematics or metallicity. This is at odds with the trends observed in the Milky Way bulge. We discuss the reasons behind these differences, and also predict the signatures that these two extreme initial conditions would leave on the vertical age and metallicity gradients of disk stars outside the bulge region. As a consequence of this analysis, we conclude that given our current knowledge of the Milky Way bulge and of the properties of its main stellar components, a metal-poor, kinematically (radial and vertical) hot component, that is, a thick disk, is necessary in the Milky Way before bar formation. This supports the scenario that has been traced in previous works. Boxy or peanut-shaped bulges and their surrounding regions are fossil records of the conditions present at early times in disk galaxies, and by dissecting their stellar components by chemical compositions and/or age, it may be possible to reconstruct their early state.

Escapees from the bar resonances

“…1, top right panel). At t = 5 Gyr, the bar strength starts to decline because of the vertical buckling of the bar and the subsequent formation of a boxy/peanut bulge, as has been reported in a number N-body simulations (Combes & Sanders 1981;Combes et al 1990;Martinez-Valpuesta et al 2006;Athanassoula 2008;Di Matteo et al 2019;Parul et al 2020;Ciambur et al 2020). As a result of the bar slowdown (see Debattista & Sellwood 2000;Athanassoula 2003), its length increases and thereby its resonances migrate outward (see, e.g., Halle et al 2018) because the whole resonance region can be very wide in radius, as shown when an analysis of individual orbits of stars is performed (Ceverino & Klypin 2007;Halle et al 2018;Khoperskov et al 2020).…”

A2A: 21 000 bulge stars from the ARGOS survey with stellar parameters on the APOGEE scale

“…N-body models of b/p bulges built from single thin disks with inside-out population gradients have reproduced several characteristics observed in the Milky Way's b/p bulge, such as cylindrical rotation (Shen et al 2010) or the vertical metallicity gradient (Martinez-Valpuesta & Gerhard 2011). However, these models also miss many of the finer chemo-kinematic properties seen in the bulge (Di Matteo et al 2015Matteo et al , 2019Fragkoudi et al 2017).…”

Box/peanut-shaped bulges in action space

“…A monotonically increasing vertical profile of JR,0 develops in most models as a result of B/P formation, while the vertical gradient in Jz,0 is largely erased. Both these facts argue that the radial motions are much more important for determining the final vertical thickening of a population, as proposed by Debattista et al (2017), and at odds with the suggestion of Di Matteo et al (2019). Nevertheless, a thick, vertically hot, disc is also likely to be radially hot; as such the combination of a thin and a thick disc is still likely to retain some of the vertical gradient inherent in the different metallicities of the two discs once a bar forms.…”

“…For this reason, they proposed that the radial random motion of stars determined the vertical height stars can reach during buckling, with radially hotter stars reaching larger heights. Recently, Di Matteo et al (2019) argued that the vertical dispersion before the bar forms is just as important as the radial dispersion. We have shown here that the vertical thickening of populations is a strongly monotonic function of JR,0, but depends less strongly on Jz,0.…”