Galactic Astronomy

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Galactic Astronomy

• Velocity fields of disks

Dong-hyun Lee 2007/08/23
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Velocity fields of disks

• Focus on the central freq. of the received
  radiation
• Central vel. dispersion line profile
• Fig. 8.36 collection of concentric, circular
  rings
• r center to point, n(r) unit vector normal
  to the ring of r
• Omega(r) ang. Vel. Of ring
• R unit vector to observer
• Flat disk

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Velocity fields of disks

• Fig. 8.31 rotation vel.
• Central part region of solid-body rotation
• Fig. 8.32 contours of const. v_los ? spider
  diagram
• Closed contours in spider diagram decline in
  rot.vel.
• Kinematic minor axis locus of pt.s having same
  sys. Vel.
• Kinematic major axis run through nucleus
  everywhere perpendicular to local const-vel
  contours
• Fig. 8.32 kin. Minor magor axes coincide
  apparent optical axes

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Circular-speed curves

• V_c (r ) deduce mass interior to rad.
• R_25 usually beyond ½ R_25
• Fig. 8.33 left panels in kpc / right ones
  func.of R_25
• Kinematic minor axis locus of pt.s having same
  sys. Vel.
• At large R/R_25 is correlated with lum.

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Circular-speed curves

• Sc at small R/R_25 max. rot. Speeds (lum. Than
  faint) at large R/R_25 flat or falling (lum.
  Than faint)
• Correl. b/w lum amp. In cir-sp. Curves ?
  Tully-Fisher rel
• Observed cir-sp.curves mass-to-light ratio
  Gamma(d) Gamma(b) (disk, bulge)
• Fig. 8.34 left disagreement of theory
  observation
• Fig. 8.35 at least 3 times as much mass within
  12R_d as within R_25 R_d disk sacle length
• Cir-sp.curves extend to more than 5R_d generally
  require significant masses to reside beyond R_25
  DM

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Circular-speed curves

• DH emits no light unknown spatial dist.
• Assume
• physically unmotivated
• N-body sim. NFW law
• M_0, a free parameters
• Gamma increases sharply as the surf. Brightness
  of gal.s falls below a frac of a percent of the
  brightness of night sky.

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Kinematic warps

• Tilted-ring model Fig. 8.36 right
• Each ring projects into an ellipse ellipse vel.
  contour (fig.8.32)
• Within each ellipse, kin. Axes will be mutually
  perpendicular
• K func.of r throug n(r) , kin.maj.axis
  will twist ? Fig. 8.37
• Fig. 8.38 l.o.s pass through disk twice ? line
  profile will normally have more than one peak
• Evidence of warps studies in p.512

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Oval distortions

• Observed spidergram kin. Axes arent mutually
  perpendicular Fig. 8.39 ? gas moving on
  elliptical rather than circular orbits
• Models complex rotation mat. model-dep. Rel.
  b/w disks shape vel. Field dep.on nature of
  driving non-axisym. Grav.potential
• Fig. 8.40 key points viewing angle
• Kin.axes are not perpendicular, although
  photometric axes are for face-on
• Kin. photo. Minor axes do not coincide
• Kin.maj.axis lies close to line of nodes, esp. at
  high inclinations