Bars disrupt normal galaxy rotation

Typical disk galaxies have well-ordered rotation and are axially symmetric since most of their stellar motion is in the tangential direction. Their rotation can be easily modeled with a parametric model using any number of common rotation curve models, or with a nonparametric model that describes the specific shape of the galaxy's rotation curve.

Bars disrupt this orderly rotation. Bars are linear features at the center of a galaxy with bilateral symmetry. According to dynamical theory and models, the stars in galactic bars move radially in and out near the galactic center. The galaxy velocity field on the right has a significant distortion in its normal rotation due to the motions in its bar.

Because of this irregular motion, traditional axisymmetric velocity field models often do not describe the velocity fields of barred galaxies, so different models should be applied to them.

Nirvana fits bisymmetric distortions

I developed a velocity field fitting code called Nirvana (Nonaxisymmetric Irregular Rotational Velocity Analysis) that can more accurately describe bisymmetric motions in galaxies. The Nirvana model of the above barred galaxy is shown on the left.

Based on a second-order Fourier model from Spekkens & Sellwood (2007), Nirvana uses the nested sampling package dynesty to fit a nonparametric Bayesian velocity model to galactic rotational data. The code is optimized to be galaxy agnostic despite the highly degenerate and multimodal likelihood space. It also accounts for observation quality by taking into account velocity dispersion, PSF smearing, and observational errors.

Picking apart velocity modes

Nirvana models galaxy rotation by splitting the galaxy's rotation into three components: the first-order tangential component that represents the dominant rotational component and the second-order radial and tangential components that come from the bar. These three components are shown separately here.

I also don't assume any functional form for the galaxy's rotation, allowing it to describe the data in a more flexible model agnostic manner. Nirvana reproduces typical rotation curves on its own and generates consistent bar rotation curves as well.

A catalog of >1000 barred galaxies

I ran Nirvana on all of the barred galaxies in the MaNGA survey, producing models and rotation curves for 1263 unique barred galaxies along with a matched control sample of similar unbarred galaxies. This is the largest sample of barred galaxy kinematic maps assembled to date. Nirvana finds systematically higher second-order rotational modes in the barred galaxies, and the elevated bisymmetric motions are preferentially aligned with visually identified bars, supporting theoretical models of the origins of bars.

The catalog and the modeling code are described in DiGiorgio Zanger et al. (2024), ApJ 973, 116.

Ongoing and Future Work

The Nirvana catalog is large enough that it opens up a number of follow-up questions, several of which make good undergraduate projects. Two papers are currently in preparation.

  • What do the stellar populations of bars look like? Previous studies find that bar stellar populations are older than the surrounding disk, but that the stellar population gradients are shallower because of increased radial mixing within the bar. Is radial motion correlated with stellar population gradients? I found early evidence of a correlation in my dissertation, and Colby undergraduate Zora Lewis continued this work with me in 2023-24. A paper on this is in preparation.
  • What is a good model for bar rotation curves? There is at present no accepted kinematic model for the rotation curves of radial flows within bars. With Nirvana we have enough nonparametric bar rotation curves to test candidate models statistically, which should let future fits use far simpler models. A paper on this is in preparation.
  • Is Tully-Fisher scatter dependent on second-order motions? Past studies have found some correlation between galaxy asymmetry and scatter about the Tully-Fisher relation. Is that scatter caused by rotational models poorly describing asymptotic velocity, or do nonaxisymmetric galaxies follow a genuinely different velocity-size relation? Colby undergraduate Nico Flota-Sanchez investigated this with me in 2023-24, and Swarthmore student Josephine Tuft is picking it up in 2026.
  • How do bars change as galaxies evolve? JWST is obtaining resolved spectroscopy of galaxies at high redshift, giving us snapshots of galaxy evolution across cosmic time. Prior studies suggest the bar fraction has increased steadily, so how did early bars differ from modern ones?
  • Can we detect outflows with Nirvana? Nirvana currently assumes no inflows or outflows, but the model could be modified to look for AGN or other galaxies that violate the continuity equation. Can Nirvana detect outflows that are otherwise obscured?