Do strong bars exhibit strong non-circular motions?

TL;DR

This study demonstrates a clear correlation between bar strength and non-circular stellar motions in barred galaxies, highlighting that stronger bars induce more significant non-circular motions, especially in galaxies with B/P bulges.

Contribution

First observational confirmation that stronger galactic bars produce stronger non-circular motions, using MUSE data and a bisymmetric kinematic model.

Findings

Stronger bars correlate with increased non-circular motions.

Galaxies with B/P bulges show higher non-circular motions and radial velocities.

Non-circular motions beyond the bar are less than 10% of circular motions.

Abstract

Galactic bars induce characteristic motions deviating from pure circular rotation, known as non-circular motions. As bars are non-axisymmetric structures, stronger bars are expected to show stronger non-circular motions. However, this has not yet been confirmed by observations. We use a bisymmetric model to account for the stellar kinematics of 14 barred galaxies obtained with the Multi-Unit Spectroscopic Explorer (MUSE) and characterize the degree of bar-driven non-circular motions. For the first time, we find tight relations between the bar strength (bar ellipticity and torque parameter) and the degree of stellar non-circular motions. We also find that bar strength is strongly associated with the stellar radial velocity driven by bars. Our results imply that stronger bars exhibit stronger non-circular motions. Non-circular motions beyond the bar are found to be weak, comprising less than 10% of the strength of the circular motions. We find that galaxies with a boxy/peanut (B/P) bulge exhibit a higher degree of non-circular motions and higher stellar radial velocity compared to galaxies without a B/P bulge, by 30-50%. However, this effect could be attributed to the presence of strong bars in galaxies with a B/P feature in our sample, which would naturally result in higher radial motions, rather than to B/P bulges themselves inducing stronger radial motions. More observational studies, utilizing both stellar and gaseous kinematics on statistically complete samples, along with numerical studies, are necessary to draw a comprehensive view of the impact that B/P bulges have on bar-driven non-circular motions.