THE GALAXY NGC 4622 - LONG-LASTING LEADING ARMS AND RINGS VIA LOW MASS PERTURBERS

There are few candidates for leading arm galaxies. Only NGC 4622 is a confirmed case because it has both leading and trailing arms. In simulations, we successfully reproduce NGC 4622's leading arm and outer pair of trailing arms. A small mass companion (0.01 of the galaxy's mass) passing close (within one-tenth of the disk radius) seems to produce the best match. This very close passage produces a leading arm in our simulations for both direct and retrograde orbit senses. The close retrograde passage seems to match the entire pattern of NGC 4622 a bit better. Moreover, the leading arm from the direct close passage does not last as long. A heavy halo mass (eight times the disk) of the primary galaxy is also necessary to reproduce the present-day features. The massive, distant, and retrograde companions considered in previous simulations produced leading arms but not the sort of outer trailing arms seen in NGC 4622. If tidal encounters only produce leading arms when the inert halo is more important than the disk, leading arms ought to be common if halos are massive. There are only a few other one-armed spirals known with no information yet as to their arm sense. This rarity either indicates that halos tend to be comparable in mass to the disk, leading arms are short lived, or close encounters with small companions are rare. The leading arm is still present in our NGC 4622 close encounter simulations (retrograde) after a billion years, so these leading arms are long lived. Small galaxies are very numerous compared to larger ones so small-large encounters should be frequent. Thus our results may indicate that halos are comparable to disks in most galaxies. Finally, NGC 4622 also provides a different way to make rings in disk galaxies besides bar excitation: the low mass companion itself initiates a leading arm and ring as it passes near the nucleus.