• “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.

• 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)

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 19o from face-on and dust might be on one side => misleading results.

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.

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.

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.

Orbital angular rate versus radius in inner CCW arm region .

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• Use with Fourier components.

• Orbital speed, V in km/s, is ~constant with radius, r, 18 thru 35 arc sec.

• Orbital angular rate, ΩO( )=V/r in (km/s)/arc sec, declines with radius over this radius.

• Arms’ constant angular pattern speed Ωp ( ) = ΩO at co-rotation radius (CR,---)=21.5 arc sec. (5.16 arc sec= 1 kpc = 3.3x103 ly)

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.

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.

I (---), V(…..).B( ____ ) sequence reversal crossing CR for inner single arm: 180o jump also indicates CR.

CCW IVB angle sequence outside CR => disk turns CW

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.

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.0

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

• 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.doc