Bar Imprints on the Inner Gas Kinematics of M33

Abstract

We present measurements of the stellar and gaseous velocities in thecentral 5' of the Local Group spiral M33. The data were obtained withthe ARC 3.5 m telescope. Blue and red spectra with resolutions from 2 to4 {Å} covering the principal gaseous emission and stellar absorptionlines were obtained along the major and minor axes and six otherposition angles. The observed radial velocities of the ionized gas alongthe photometric major axis of M33 remain flat at 22 km s^{-1}all the way into the center, while the stellar velocities show a gradualrise from 0 to 22 km s^{ -1} over that same region. The centralstar cluster is at or very close to the dynamical center, with avelocity that is in accordance with M33's systemic velocity to withinour uncertainties. Velocities on the minor axis are nonzero out to about1' from the center in both the stars and gas. Together with themajor-axis velocities, they point at significant deviations fromcircular rotation. The most likely explanation for the bulk of thevelocity patterns are streaming motions along a weak inner bar with aposition angle close to that of the minor axis, as suggested bypreviously published IR photometric images. The presence of bar imprintsin M33 implies that all major Local Group galaxies are barred. Thenoncircular motions over the inner 200 pc make it difficult to constrainthe shape of M33's inner dark matter halo profile. If the noncircularmotions we find in this nearby Sc galaxy are present in other moredistant late-type galaxies, they might be difficult to recognize.