AAS 201st Meeting, January, 2003
Session 13. Galaxy - Structure
Poster, Monday, January 6, 2003, 9:20am-6:30pm, Exhibit Hall AB

## [13.14] Leading Pairs and Trailing Single Spiral Arms: Observations and Simulations of NGC4622

G. Byrd (Univ. of Alabama), T. Freeman (Bevill State Coll., Alabama), R. Buta (Univ. of Alabama)

NGC4622 has a most unusual spiral arm pattern. Besides a pair of outer arms which wind outward clockwise (CW), Byrd et al. (1989) pointed out a weaker, single, inner arm winding outward counterclockwise. Byrd et al. suspected the single arm must lead, a rare configuration. Buta, Crocker, and Byrd's (1992) BVI photometry showed the inner arm is a real stellar disk feature. Assuming a flat rotation curve, Byrd, Freeman, and Howard (1993) simulated a single leading and outer trailing pair of arms for NGC4622 via a plunging low-mass perturber along the plane of a disk in a massive halo. However, new HST data (Byrd, Buta, and Freeman 2001) indicate via dust cloud silhouettes that the east edge of the galaxy is nearer. Additional H\alpha spectra indicate that the north edge along the disk line of nodes is receding relative to the center. The disk material thus orbits CW and the arm pair leads, winding out in the direction of disk orbital motion counter" to common theoretical expectations. Conversely, the inner single arm trails. The spectra indicate that the rotation curve is rising and getting steeper with increasing radius in the outer disk where the pair of arms is found. There is some indication that it is flat in the single arm region (although there is little ionized gas there). This unusual curve appears to be a key to understanding the NGC4622 arm pattern. An encounter much like our previous work but with the new rotation curve successfully produces an inner single trailing, outer pair leading spiral arm pattern. See http://bama.ua.edu/~rbuta/ngc4622. We find support for the encounter in the lack of a systematic BVI azimuthal color displacement in the outer pair, indicating a young impulsive perturbation which is winding up. We find evidence in the HST images that the inner pattern contains a one-sided periodic orbit ring whose pattern speed results in a clear angular displacement between the B, V, and I maximum radial distances. Grant support: NASA/STScI GO 8707 and NSF AST-0206177.