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Astron. Astrophys. 358, 845-849 (2000)

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3. Dynamics

In 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 [FORMULA] corresponds to 77.5 pc.

The only kinematic data available for IC3328 to date is a measurement of the central velocity dispersion of [FORMULA] km s-1 (Peterson & Caldwell 1993). In the absence of a proper velocity field determination we have to resort to the light distribution and estimates of the mass-to-light ratio, [FORMULA].

The square of the rotational velocity, [FORMULA], can be determined from [FORMULA], where [FORMULA] 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 [FORMULA]. The two [FORMULA] 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 [FORMULA] around the peak [FORMULA] is what really matters.

[FIGURE] Fig. 5. A plot of [FORMULA] and the two rotation curves, [FORMULA] and [FORMULA], produced by it. The surface density [FORMULA] was obtained from [FORMULA] by assuming the [FORMULA] (in solar units). The [FORMULA] curve is a Nuker profile fit to [FORMULA], shown by the dots.

Assuming [FORMULA] gives a maximum disk rotation velocity, [FORMULA] km s-1 . A better estimate of 55 km s-1 comes from the average mass-to-light ratio for globular clusters [FORMULA], based on the quantities [FORMULA] (Pryor & Meylan 1993) and [FORMULA] (Peterson 1993). Such a value does not clash with [FORMULA] km s-1 .

The best option would be to measure the rotation curve. Then one could use the above arguments to obtain the actual [FORMULA] ratio of IC3328.

The estimated peak rotation velocity of 55 km s-1 occurs around 1.4 kpc, which means that the angular rotation rate there is 39 km s-1 kpc-1, a value comparable to the 25 km s-1 kpc-1 measured near the Sun. Thus IC3328 has had ample time to settle into an equilibrium.

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000