4. Spiral structure
The presence of the spiral implies the presence of a disk. If what we see is a nearly face-on disk then the small spiral amplitude is quite unusual. We can estimate the mean displacements and velocities that are needed to produce the observed density contrast.
A density wave is created by coherent oscillations of stars around their equilibrium orbits. Because the spiral is tightly wrapped, the largest contribution comes from the radial displacements, , whose phases are rapidly varying functions of the equilibrium position r. If we write the phase as , then a little bit of algebra shows that the change in surface density, satisfies the relation
when . For our spiral , which means that when , , a very small number indeed!
In a similar spirit we can estimate the velocities, , associated with such displacements. The radial displacements will oscillate with a frequency typically less than the natural radial frequency , which in turn is roughly times the mean angular rotation rate . Hence , or 0.3 km s-1 .
These small numbers can be increased by reducing the disk light contribution and attributing it to a spheroid. Even a ten-fold reduction would produce fractional displacements of , and velocities of only 3 km s-1 . Such low velocities are unlikely to lead to shocks in any neutral gas that may be present in the disk, and therefore to any star formation that would signal the presence of gas.
By reducing the disk light contribution we also reduce the importance of the spiral's self-gravity and increase the likelihood of a tidal origin. Fig. 6 shows several possible perturbers. When the self-gravity becomes negligible, one can rule out even a distant passage of a big perturber like NGC4380, since the tidal distortions will not propagate to the center. That leaves close passages by the fainter objects, such as the two VCC dwarf galaxies, as possible cause of the spiral seen in IC3328.
The other plausible scenario is that most of the light does come from the disk and we are seeing swing amplified noise (Toomre & Kalnajs 1991). The gain of the swing amplifier is not large enough to amplify the stellar density fluctuations to the observed level, but a small amount lumpy gas could provide the necessary leading perturbations which then are amplified by a factor of 10-30 as the shear transforms them into a trailing spirals (Toomre 1981). Huchtmeier & Richter's (1986) upper limit of for the HI content of IC3328 does not rule out this possibility.
© European Southern Observatory (ESO) 2000
Online publication: June 20, 2000