An Inconvenient Galaxy

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 NGC4622s 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)gt disk must turn CW on the sky (curved
  arrow)

Away. Line of nodes
Near
Far
Toward. Line of nodes
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 19o from face-on and dust might be on one
  side gt 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.
• Doesnt 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 (?).

• Disks 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 .
CR

• Use with Fourier components.
• Orbital speed, V in km/s, is constant with
  radius, r, 18 thru 35 arc sec.
• Orbital angular rate, OO( )V/r in (km/s)/arc
  sec, declines with radius over this radius.
• Arms constant angular pattern speed Op ( )
  OO at co-rotation radius (CR,---)21.5 arc sec.
  (5.16 arc sec 1 kpc 3.3x103 ly)

8

• Fourier component IR, visual, blue (I,V,B)
  displacement outside CR.
• Left photo. Outside CR (----), arm pattern (
  ) gt 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 lt arm rate ?CCW
  IVB color/age sequence.
• Inside CR, orbit gt arm rate ? CW IVB color/age
  sequence.

10
I (---), V(..).B( ____ ) sequence reversal
crossing CR for inner single arm 180o jump
also indicates CR. CCW IVB angle sequence
outside CR gt disk turns CW
0 deg.
CR
CR
90 deg.
180 deg.
11
Independent method results

• CW Orbital sense gt 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.
Normal Flat Curve
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/AASJan08We
  bFiles.doc

15
Study physics and astronomy at the University
of Alabama.