The apparent “reversed” motion of gas and stars in M 82

Abstract

We summarize earlier and unpublished long-slit spectroscopic measurements of radial velocities of ionized gas and stars along and parallel to M 82’s major axis to a radial distance of ∼±2.5 kpc (∼±140 �� ) from the center. In the position-velocity diagram, these measurements indicate a velocity reversal of ∼100 km s −1 of gas and/or stars at ∼±1.0 kpc (∼±50−70 �� ) on either side of the center, outside the bar. Although seen in earlier observations, and perhaps neglected because they were assumed to be only an effect of heavy local extinction, the positional symmetry of the velocity reversal with respect to the center of M 82, as well as the absence of the reversal in the motion of stars seen in the near-IR Ca ii absorption lines, points to another origin. M 82’s two-armed spiral, as outlined by Mayya et al. (2005, ApJ, 628, L33), may explain a part of the velocity reversal, although this interpretation leaves inconsistencies so that other explanations must also be investigated. A simple, conclusive explanation of the velocity reversal has not yet been found. While restricted observationally in radial distance to ∼120 �� (2 kpc), the near-IR stellar Ca ii absorption lines, which do not show the velocity reversal, indicate a flat radial velocity curve of the stellar disk that remained after the encounter with M 81.