Figure: Kinematics of M31


UPPER PANEL: The top figure shows the velocity dispersion as function of distance from the center of the galaxy in arcseconds, along the major axis (in the disk plane). The velocity dispersion is the width of the velocity distribution of stars. It is a measure of the amount of random stellar motion.

As the density of stars increases, so does their gravitational pull on the other stars. In this way, the velocity dispersion increases towards the galaxy center, where the density of stars if much higher than in the outer parts of the galaxy.

If this increase is too rapid to be explained only by the gravitational forces of the stars, one has to invoke the presence of a massive black hole in the center, as was the case for M31.



LOWER PANEL: The bottom figure shows the (line-of-sight) stellar velocity as function of distance along the major axis (as in the upper panel). The velocity along the line-of-sight is the mean of the velocity distribution. It is a measure of the ordinate quasi-circular motion of stars in the disk.

As the density of stars increases, like for the velocity dispersion, the rotation velocity is affected. The central behavior is due to instrumental resolution and seeing convolution, summing up light from stars rotating in opposite directions results in a (central) zero mean velocity.

The steepness of the rotation curve at the galaxy center depends on the mass concentration. Again, if the visible stellar mass can not explain the velocity data, one has to invoke some dark mass (a black hole) that could be at the origin of the rapid stellar motion.

Figure credit: John Kormendy (University of Texas, Austin) and Luis C. Ho (Carnegie Observatories, Pasadena).

Click here for a web review that discusses stellar dynamical evidence for BHs in inactive and weakly active galaxies by J. Kormendy and L. C. Ho. The dispersion velocity and rotation velocity curves were taken from that review.