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 e \sim 0.8 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 .