2008AJ....135..408B

The spiral galaxy NGC4622 was recognized by Byrd et al. (1989, Celest. Mech., 45, 31) to have a single inner arm winding outward counterclockwise (CCW) and an outer pair of arms winding outward clockwise (CW). Using HST and ground-based observations, Buta et al. (2003, AJ, 125, 634) used a dust silhouette method to determine that the disk of NGC4622 rotates CW on the sky. The outer pair of arms winding outward CW on the sky should thus lead (wind outward in the direction of disk orbital motion), contrary to most expectations. We have discovered in Fourier component images an additional pair of weak CCW arms: this pair is inside the already-known strong outer CW pair. We also find an additional weak single CW arm component: this arm is outside the strong, already-known, CCW single inner arm. Thus, regardless of disk rotation sense, NGC4622 has a pair of leading arms as well as a single leading arm. NGC4622's moderate (19°) tilt is a plus for the methods in the present paper. We use a well-demonstrated IVB color/age method to locate co-rotation (CR) radii. At CR, the Fourier position angle peaks switch position angle sequence (e.g., IVB to BVI or the reverse). We find two possible CR radii (21.4'' and 36''). The boundary between the outer CW arms and the inner CCW arms is between these two radii. From the IVB switching senses crossing the CR radii and the observed orbital angular rate decline with increasing radius across the first CR, the orbit sense is CW and the outer pair thus is the leading pair. We thus place on a firm footing for future research our earlier conclusions about the unusual nature of the NGC4622 spiral pattern.