## 3. DynamicsIn order to estimate the dynamical time scales associated with the spiral pattern we need to know the distance to and the velocities within IC3328. There are two distance estimates for IC3328. The first comes from the radial velocity which coincides with the mean velocity of the Virgo cluster. It agrees well with the preliminary SBF distance of 15.5 Mpc (Jerjen et al. in preparation). At this distance corresponds to 77.5 pc. The only kinematic data available for IC3328 to date is a
measurement of the central velocity dispersion of
km s The square of the rotational velocity, , can be determined from , where is the projected surface mass density. On a logarithmic scale these two quantities are related by a convolution (Kalnajs 1999). Fig. 5 shows the relation between the two quantities in the case when . The two curves correspond to the limiting cases where the projected surface density comes from a flat or a spherical mass distribution. Fig. 5 also makes it clear that the value of around the peak is what really matters.
Assuming gives a maximum disk
rotation velocity, km s The best option would be to measure the rotation curve. Then one could use the above arguments to obtain the actual ratio of IC3328. The estimated peak rotation velocity of 55 km s © European Southern Observatory (ESO) 2000 Online publication: June 20, 2000 |