Intrinsic errors of the central galactic mass derived from rotation curves under the influence of a weak non-axisymmetric potential

¹ Institute of Astronomy, University of Tokyo 2-21-1, Osawa, Mitaka, Tokyo 181-0015, Japan
² National Astronomical Observatory, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan

Corresponding author: J. Koda, jin.koda@nao.ac.jp

Received: 12 July 2002
Accepted: 1 October 2002

Abstract

Rotation curves are often used to estimate the mass distribution of spiral galaxies, assuming that the circular velocities of the interstellar medium balance with the galactic centrifugal force. However, non-circular motions caused by a non-axisymmetric gravitational potential, such as a stellar bar, may disturb the velocity field, resulting in errors in mass estimation, especially in the central regions of galaxies. This is because the line-of-sight velocity depends on the viewing angles in a non-axisymmetric flow. Observing rotation curves of edge-on galaxies in time-dependent numerical simulations from different viewing angles, we obtain errors in the estimation of galactic mass from the rotation curves. In the most extreme case, the ellipticity of gas orbits is as high as in the central regions, even if the bar potential is weak. When rotation curves are defined as the highest velocity envelope of position-velocity diagrams, the mass estimated from the rotation curves is larger than the true mass by a factor of five for 15% of the viewing angles, and the ratio between the apparent mass and true mass is less than six for any viewing angle. The overestimation in mass occurs more frequently than the underestimation.