[Paper Review] The disc origin of the Milky Way bulge: Dissecting the chemo-morphological relations using N-body simulations and APOGEE

Francesca Fragkoudi, P. Di Matteo+5 more|arXiv (Cornell University)|Feb 1, 2018

Stellar, planetary, and galactic studies83 references3 citations

TL;DR

This paper proposes that the Milky Way bulge originates from the secular evolution of a composite thin and thick disc, with the boxy/peanut (b/p) bulge forming via bar-driven vertical heating. Using N-body simulations and APOGEE DR13 data, it shows that the differential mapping of cold (thin disc) and hot (thick disc) populations reproduces observed chemo-morphological patterns—including metallicity gradients, MDF trends, and azimuthal variations—supporting a disc origin over a classical spheroid for the metal-poor bulge population.

ABSTRACT

There is a long-standing debate on the origin of the metal-poor stellar populations of the Milky Way (MW) bulge, with the two leading scenarios being that these populations are either i) part of a classical metal-poor spheroid or ii) the same population as the chemically defined thick disc seen at the Solar neighbourhood. Here we test whether the latter scenario can reproduce the observed chemical properties of the MW bulge. To do so we compare an N-body simulation of a composite (thin+thick) stellar disc -- which evolves secularly to form a bar and a boxy/peanut (b/p) bulge -- to data from APOGEE DR13. This model, in which the thick disc is massive and centrally concentrated, can reproduce the morphology of the metal-rich and metal-poor stellar populations in the bulge, as well as the mean metallicity and [$α$/Fe] maps as obtained from the APOGEE data. It also reproduces the trends, in both longitude and latitude, of the bulge metallicity distribution function (MDF). Additionally, we show that the model predicts small but measurable azimuthal metallicity variations in the inner disc due to the differential mapping of the thin and thick disc in the bar. We therefore see that the chemo-morphological relations of stellar populations in the MW bulge are naturally reproduced by mapping the thin and thick discs of the inner MW into a b/p.

Motivation & Objective

• To test whether the metal-poor stellar populations in the Milky Way bulge can originate from the chemically defined thick disc rather than a classical spheroid.
• To investigate whether secular evolution of a composite thin+thick disc can reproduce the observed morphology and chemical properties of the bulge.
• To examine the chemo-morphological relations in the bulge, including metallicity gradients and distribution functions, using both simulations and observational data.
• To determine if azimuthal metallicity variations in the inner disc arise from differential mapping of disc components in the bar.
• To assess whether a massive, centrally concentrated thick disc can explain the kinematic and chemical trends observed in the bulge and inner disc

Proposed method

• Conducting N-body simulations of a composite stellar disc (thin + thick) that secularly evolves into a bar and forms a boxy/peanut (b/p) bulge.
• Using APOGEE DR13 data to compare simulated metallicity and [α/Fe] maps with observed distributions in the bulge and inner disc.
• Analyzing the morphology of metal-rich and metal-poor populations in the simulated bulge to match observed X-shaped structure and red clump splits.
• Measuring trends in the metallicity distribution function (MDF) as a function of longitude and latitude in both simulation and data.
• Quantifying azimuthal metallicity variations in the inner disc due to differential mapping of thin and thick disc components in the bar.
• Applying spatial and kinematic selection functions to align simulated stellar populations with APOGEE survey geometry and completeness

Experimental results

Research questions

• RQ1 Can a secularly evolving composite disc model reproduce the observed morphology and chemo-dynamical structure of the Milky Way bulge?
• RQ2 Do the observed trends in the metallicity distribution function (MDF) of the bulge—especially in longitude and latitude—match predictions from a disc-origin model?
• RQ3 Is the vertical metallicity gradient in the bulge, with increasing metal-poor fraction at higher latitudes, naturally explained by the mapping of thin and thick disc populations?
• RQ4 Can small but measurable azimuthal metallicity variations in the inner disc (≈0.1 dex) be reproduced by differential mapping of cold and hot populations in the bar?
• RQ5 Does the kinematic and chemical structure of the bulge support a disc origin over a classical spheroid for the metal-poor population?

Key findings

• The N-body simulation with a massive, centrally concentrated thick disc successfully reproduces the observed X-shaped morphology of the Milky Way bulge.
• The simulated mean metallicity and [α/Fe] maps match the APOGEE DR13 data in both the bulge and inner disc regions.
• The model reproduces the observed trends in the metallicity distribution function (MDF) as a function of longitude and latitude, including the positive longitudinal metallicity gradient.
• The simulation predicts small but measurable azimuthal metallicity variations of approximately 0.1 dex in the inner disc due to differential mapping of thin and thick disc populations in the bar.
• The model explains the vertical metallicity gradient in the bulge, with increasing metal-poor fraction at higher latitudes, as a consequence of the kinematic and spatial segregation of disc components.
• The results support a disc origin for the Milky Way bulge, with the metal-poor population being part of the chemically defined thick disc rather than a classical spheroid