An Independent Determination that NGC4622's Two Outer Arms Lead

Details An Independent Determination that NGC4622's Two Outer Arms Lead An Independent Determination that NGC4622's Two Outer Arms Lead

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...211.1308b&link_type=abstract

American Astronomical Society, AAS Meeting #211, #13.08; Bulletin of the American Astronomical Society, Vol. 39, p.756

Astronomy and Astrophysics

Astronomy

Scientific paper

Byrd et al(1989) first noted the unusual two-way spiral arms of NGC4622, a single inner arm winding outward counter clockwise (CCW) on the sky and an outer pair winding outward clockwise (CW). Buta, Byrd, and Freeman (2003) used dust silhouettes, color asymmetry across the disk line of nodes plus Doppler shifts to determine that the disk rotates CW. The outer CW pair of arms should thus lead (wind outward in the direction of disk orbital motion)! To double check this unusual finding, we use an independent Fourier IVB image method which also works best for NGC4622's moderate tilt from face-on. We discover two new arm components: (1) an inner CCW pair hidden by the stronger inner CCW single arm component. (2) an outer CW single arm hidden by the stronger outer CW pair. Thus, regardless of disk rotation sense, NGC4622 has a pair of leading arms as well as a single leading arm. Stellar associations "light up” in B down stream in position angle from the old (I,V) stellar arms in images and Fourier components. The IVB sequence reverses crossing co-rotation (CR) radii (e.g. IVB to BVI or the reverse). We find CR reversals at 21.4″ and 36''. From the IVB reversal and the observed orbital angular rate decline with radius across the 21.4″ CR, the orbit sense must be CW and the strong outer CW arm pair must lead. We thus independently substantiate our earlier conclusions about NGC4622's unusual spiral pattern. Supported by NASA STScI GO 8707 and NSF AST 02-0177.
For images see http://bama.ua.edu/ byrd/AASJan08WebFiles.doc

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