“an inconvenient galaxy” we discovered in ground-based photos: outer pair winding outward...
TRANSCRIPT
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• “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.
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• 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)
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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.
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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.
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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.
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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.
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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)
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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.
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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.
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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
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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.
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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.
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• 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
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