[Paper Review] Bars & boxy/peanut bulges in thin & thick discs: I. Morphology and line-of-sight velocities of a fiducial model

Francesca Fragkoudi, P. Di Matteo+5 more|arXiv (Cornell University)|Apr 3, 2017

Stellar, planetary, and galactic studies84 references28 citations

TL;DR

This study uses N-body simulations to investigate how thin (kinematically cold) and thick (kinematically hot) discs form distinct bars and boxy/peanut (b/p) bulges in disc galaxies. It finds that thin disc stars form a stronger, X-shaped b/p bulge due to efficient trapping in the bar, while thick disc stars, with higher angular momentum and radial excursions, produce a weaker, boxier b/p and exhibit line-of-sight velocities up to 40% higher than thin disc stars in outer bar regions—highlighting strong dependence on stellar kinematics in bar-bulge morphology and kinematics.

ABSTRACT

We explore trends in the morphology and line-of-sight (los) velocity of stellar populations in the inner regions of disc galaxies, using N-body simulations with both a thin (kinematically cold) and a thick (kinematically hot) disc which form a bar and boxy/peanut (b/p) bulge. The bar in the thin disc component is $\sim$50\% stronger than the thick disc bar and is more elongated, with an axis ratio almost half that of the thick disc bar. The thin disc b/p bulge has a pronounced X-shape, while the thick disc b/p is weaker with a rather boxy shape. This leads to the signature of the b/p bulge in the thick disc to be weaker and further away from the plane than in the thin disc. Regarding the kinematics, we find that the los velocity of thick disc stars in the outer parts of the b/p bulge can be \emph{larger} than that of thin disc stars, by up to 40\% and 20\% for side-on and Milky Way-like orientations of the bar respectively. This is due to the different orbits followed by thin and thick disc stars in the bar-b/p region, which are affected by the fact that: i) thin disc stars are trapped more efficiently in the bar - b/p instability and thus lose more angular momentum than their thick disc counterparts and ii) thick disc stars have large radial excursions and therefore stars from large radii with high angular momenta can be found in the bar region. We also find that the difference between the los velocities of the thin and thick disc in the b/p bulge ($\Delta v_{los}$) correlates with the initial difference between the radial velocity dispersions of the two discs ($\Delta \sigma$) . We therefore conclude that stars in the bar - b/p bulge will have considerably different morphologies and kinematics depending on the kinematic properties of the disc population they originate from.

Motivation & Objective

• Investigate how kinematically distinct thin and thick discs map into bars and boxy/peanut (b/p) bulges in disc galaxies.
• Address the lack of systematic studies on how the dynamical properties of thick discs (mass, scalelength, velocity dispersion) affect bar and b/p morphology and kinematics.
• Understand the role of angular momentum transfer and orbital structure in shaping the morphology and line-of-sight velocities of stars in bar-b/p systems.
• Provide physical interpretations for observed kinematic and morphological differences between stellar populations in the inner regions of disc galaxies, especially in the context of the Milky Way.

Proposed method

• Conduct N-body simulations of isolated disc galaxies with both a thin disc (scaleheight 0.3 kpc, kinematically cold) and a thick disc (scaleheight 0.9 kpc, kinematically hot).
• Track stellar particles from each disc component separately to analyze their distinct contributions to the bar and b/p bulge formation.
• Use cylindrical rotation and angular momentum transfer analysis to study kinematic behavior and orbital structure in the bar region.
• Measure line-of-sight (los) velocities and density profiles along the bar major axis to quantify morphological differences (e.g., X-shape vs. boxy shape).
• Correlate initial radial velocity dispersion differences (∆σ) with final los velocity differences (∆vlos) to identify scaling trends.
• Compare simulation results with observational data from ARGOS, GES, and APOGEE surveys to validate kinematic trends.

Experimental results

Research questions

• RQ1 How do the morphological properties of bars and b/p bulges differ between thin and thick disc components in a dual-disc galaxy model?
• RQ2 What causes the line-of-sight velocity of thick disc stars to exceed that of thin disc stars in the outer regions of the b/p bulge?
• RQ3 To what extent does the initial radial velocity dispersion difference between discs correlate with the final los velocity difference in the bar-b/p region?
• RQ4 How do angular momentum transfer and orbital structure explain the distinct morphological and kinematic signatures of thin and thick disc stars in the bar-b/p system?
• RQ5 What role do radial excursions and high angular momentum stars from the outer thick disc play in populating the bar region?

Key findings

• The thin disc bar is ~50% stronger and has an axial ratio nearly half that of the thick disc bar, indicating a more elongated structure.
• The thin disc forms a pronounced X-shaped b/p bulge, while the thick disc b/p is weaker and more boxy, with a less distinct signature.
• The b/p density dip along the bar major axis is weaker and located at larger heights above the plane for thick disc stars compared to thin disc stars.
• Line-of-sight velocities of thick disc stars in the outer b/p region exceed those of thin disc stars by up to 40% in side-on views and 20% in Milky Way-like orientations.
• This velocity difference arises because thin disc stars lose more angular momentum and are trapped on more elongated orbits, while thick disc stars with high angular momentum from large radii can reach the bar region via radial excursions.
• A significant correlation exists between the initial radial velocity dispersion difference (∆σ) and the final line-of-sight velocity difference (∆vlos), confirming that kinematic properties of stellar populations govern bar-b/p characteristics.