Do Strong Bars Exhibit Strong Noncircular Motions?

Abstract

Galactic bars induce characteristic motions deviating from pure circular rotation, known as noncircular motions. As bars are nonaxisymmetric structures, stronger bars are expected to show stronger noncircular 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 and characterize the degree of bar-driven noncircular motions. For the first time, we find tight relations between the bar strength (bar ellipticity and torque parameter) and the degree of stellar noncircular motions. We also find that the bar strength is strongly associated with the stellar radial velocity driven by bars. Our results imply that stronger bars exhibit stronger noncircular motions. Noncircular 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 noncircular 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 the 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 noncircular motions.