The Effects of Boxy/Peanut Bulges on Galaxy Models

TL;DR

This paper demonstrates that neglecting Boxy/Peanut bulges in galaxy models leads to significant errors in gravitational force estimates, affecting orbital dynamics and bar strength assessments.

Contribution

It introduces a variable height function method for more accurate galaxy potential modeling that accounts for B/P bulge geometry, improving upon traditional flat models.

Findings

Neglecting B/P bulges causes up to 40% errors in force calculations.

Including B/P bulges reduces estimated bar strength.

Modeling uncertainties are less impactful than ignoring B/P structures.

Abstract

We examine the effects that the modelling of a Boxy/Peanut (B/P) bulge will have on the estimates of the stellar gravitational potential, forces, orbital structure and bar strength of barred galaxies. We present a method for obtaining the potential of disc galaxies from surface density images, assuming a vertical density distribution (height function), which is let to vary with position, thus enabling it to represent the geometry of a B/P. We construct a B/P height function after the results from a high-resolution, N-body+SPH simulation of an isolated galaxy and compare the resulting dynamical model to those obtained with the commonly used, position-independent "flat" height functions. We show that methods that do not allow for a B/P can induce errors in the forces in the bar region of up to 40% and demonstrate that this has a significant impact on the orbital structure of the model, which in turn determines its kinematics and morphology. Furthermore, we show that the bar strength is reduced in the presence of a B/P. We conclude that neglecting the vertical extent of a B/P can introduce considerable errors in the dynamical modelling. We also examine the errors introduced in the model due to uncertainties in the parameters of the B/P and show that even for generous but realistic values of the uncertainties, the error will be noticeably less than that of not modelling a B/P bulge at all.