1. “An Inconvenient Galaxy” We discovered in ground-based photos: Outer pair winding outward clockwise (CW) Unusual inner single arm winding outward counter-clockwise (CCW). Our HST photo is shown here.

2. Previous HST photo analysis: Requires NGC4622’s disk be tilted from face-on and used dust clouds. In image blue is dark & dust clouds are lighter silhouettes. (1) The left is the near side. (2) Upper Doppler shift on line of nodes is away. Lower toward (1)&(2)=> disk must turn CW on the sky (curved arrow)

3. Previous results vs expectations. We find the disk turns CW (curved arrow top right). Outer pair thus leads, winding outward with disk rotation as shown. Astronomers were skeptical because result was against expectation of trailing arms which wind out opposite disk rotation (CCW curved arrow middle right). Astronomers were also skeptical because NGC4622 is only 19 o from face-on and dust might be on one side => misleading results.

4. Answering the skeptics with an independent method. Use Fourier frequency image analysis to reveal hidden arm components Independent of previous method. Works best for nearly face-on galaxies like NGC4622. Doesn’t use dust.

5. Newly discovered hidden inner Fourier pair One of known outer CW ( ) pair. One of new inner CCW (●) pair. Whichever way its disk turns, CW or CCW, NGC4622 MUST have a pair of leading arms.

6. Known single CCW arm and new outer CW single component (●). Disk’s orbital turning is CW or CCW on sky. Either the outer weak or strong inner arm must lead. Which lead? Inner CCW arms or outer CW arms.

7. Orbital angular rate versus radius in inner CCW arm region. Use with Fourier components. Orbital speed, V in km/s, is ~constant with radius, r, 18 thru 35 arc sec. Orbital angular rate, Ω O ( )=V/r in (km/s)/arc sec, declines with radius over this radius. Arms’ constant angular pattern speed Ω p ( ) = Ω O at co-rotation radius (CR,---)=21.5 arc sec. (5.16 arc sec= 1 kpc = 3.3x10 3 ly)

8. Fourier component IR, visual, blue (I,V,B) displacement outside CR. Left photo. Outside CR (----), arm pattern ( ) > CW than orbital motion ( ). Clouds ( ) enter yellow arm ( ) colliding to form massive stars. These later “light up” ( ) then die quickly behind (CCW outside) the arm. See right photo. Results in I, V, B CCW angle sequence of Fourier components outside CR.

9. Reversal of IVB Fourier arm angular sequence crossing CR. Inner single CCW arm crosses CR where arm rate (red) equals orbit rate (black). Recall that outside CR, orbit < arm rate →CCW IVB color/age sequence. Inside CR, orbit > arm rate → CW IVB color/age sequence.

10. I (---), V(…..).B( ____ ) sequence reversal crossing CR for inner single arm: 180 o jump also indicates CR. CCW IVB angle sequence outside CR => disk turns CW

11. Independent method results CW Orbital sense => Outer CW arm pair must lead and inner CCW trail Although the arms take ~160 million years to turn, Fourier components in different colors provide frames of a ``movie” showing that the outer pair leads. Previous result using dust effects of tilt is confirmed.

12. Why do inner arms trail and outer lead? Inner arm(s) trail where orbit angular rate (V/r) declines with radius. Leading outer arms also indicate that orbital angular rate rises with radius in outer disk.

13. If outer rotation curve speed rises with radius, NGC4622 may be in a much less centrally concentrated dark matter halo than normal.

14. Possibilities to better understand spiral arms and dark matter. Research team – T. Freeman (Bevill State Comm. Coll. Fayette, AL), –R. Buta (Univ. of Alabama, Tuscaloosa) –G. Byrd (Univ. of Alabama, Tuscaloosa), –Sethanne Howard, (U.S. Naval Observatory ret.). –Support from NASA STScI and NSF RUI –Astronomical Journal,Vol. 135, Number 1, 2008 Jan. p.408 –For images see http://bama.ua.edu/~byrd/AASJan08WebFiles.dochttp://bama.ua.edu/~byrd/AASJan08WebFiles.doc