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G. Byrd, R. Buta, (Univ. of Alabama, Tuscaloosa), T. Freeman (Bevill State College)
NGC 4622 is a nearly face-on spiral with an intriguing morphology. Byrd et al (1989) pointed out that besides a pair of outer arms winding outward clockwise (CW), NGC4622 has a weaker, single inner arm winding outward counter clockwise (CCW). Byrd et al noted that one set of arms must be leading, a very rare configuration. Buta, Crocker, and Byrd (1992) showed using BVI photometry that the single inner arm is a stellar dynamical feature, not an unusual dust distribution. NGC 4622 was thus the most convincing case of a galaxy having leading spiral structure. However, which set of arms is leading? Based on theory and numerical simulations, Byrd, Freeman, and Howard (1993) suggested that the inner arm leads. We have obtained new HST WFPC2 images of NGC4622 in B, V, and I (http://bama.ua.edu/~rbuta/ngc4622/). These images, with a ground based Fabry-Perot H-alpha velocity field, have given the surprising result that THE TWO OUTER ARMS IN NGC4622 MUST BE LEADING! de Vaucouleurs (1958) used silhouetted dust lanes to identify the near side of disk galaxies. Although the inclination is low (23 deg.), clear dust lanes are seen silhouetted on the east side of NGC4622's kinematic line of nodes (p.a. 20 deg.). We observe that the north half of the galaxy is receding, which implies the disk rotates CW. The two outer CW arms thus lead, not the single inner CCW arm. This unexpected result requires a completely new theoretical interpretation. Alternatively, the old model could be valid with the dust silhouette method being wrong for NGC 4622, given its low inclination. However, the dust features are much more clearly silhouetted east of the line of nodes than west. Our HST images also indicate that NGC4622's unusual arm pattern could be due to a merger with a small companion. A short, linear dust lane splits the nucleus nearly in half suggesting that the perturber was destroyed in a passage close to the nucleus. Supported by NASA/STScI Grant GO 8707 and by NSF Grant RUI 9802918.
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The author(s) of this abstract have provided an email address for comments about the abstract: byrd@possum.astr.ua.edu