AAS 200th meeting, Albuquerque, NM, June 2002
Session 43. Galaxies
Display, Tuesday, June 4, 2002, 10:00am-6:30pm, SW Exhibit Hall

## [43.12] A Beautiful Galaxy :" Further HST Evidence and Mechanisms for Two Leading Arms in NGC 4622

G. Byrd, R. Buta (U. of AL), T. Freeman (Bevill State Coll.)

Shu (1982) noted the galaxy NGC4622's beautiful spiral pattern composed of two trailing spiral arms." which E. M. Burbidge (1982) described as extraordinarily symmetric." Besides the two arms which wind outward clockwise (CW), Byrd et al. (1989) pointed out a weaker, single, inner arm which winds outward counterclockwise (CCW). Byrd et al. suspected the single arm must lead, a very rare configuration. Buta, Crocker, and Byrd (1992)'s BVI photometry showed the inner arm is a mostly stellar disk feature. Byrd, Freeman, and Howard (1993) simulated creation of a single leading and outer trailing pair via a plunging low-mass perturber. At the Jan. 02 AAS meeting, we discussed new HST WFPC2 BVI images of NGC4622 (http://bama.ua.edu/~ rbuta/ngc4622/). Despite the low inclination i=26o±4o, sharp dust lanes are silhouetted on the east side of NGC4622's kinematic line of nodes (p.a. 22o) but not the west, indicating the east is nearer. In a ground-based H\alpha velocity field, the north half of the galaxy is receding. Therefore, the disk rotates CW. The two CW-opening arms thus lead, NOT the single inner CCW-opening arm. At the June 02 AAS meeting, we will discuss further the silhouetted dust lanes and show they are also found west of the line of nodes, but these are less sharp and less red because they are on the far side, viewed through the bright bulge. We will show via a model galaxy with an r1/4 bulge and an exponential disk, that a measurable reddening and dust asymmetry across the line of nodes results even with i=20o-30o, if the bulge is nearly spherical and is a significant fraction of the total luminosity. The HST images reveal a globular cluster system in NGC4622 which we will discuss. Despite previous descriptions, this beautiful galaxy apparently has a pattern far from a classical trailing density wave. We will discuss how a pair of leading arms in NGC4622 may originate via tidal perturbation and/or a merger. Supported by Grants NASA/STScI GO 8707 and by NSF RUI 9802918.