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

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Galactic astronomy

Galactic Astronomy

Velocity fields of disks

Dong-hyun Lee

2007/08/23


Velocity fields of disks

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


  • Velocity fields of disks1

    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


    Circular speed curves

    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.


    Circular speed curves1

    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


    Circular speed curves2

    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.


    Kinematic warps

    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


    Oval distortions

    Oval distortions

    • Observed spidergram : kin. Axes aren’t mutually perpendicular – Fig. 8.39  gas moving on elliptical rather than circular orbits

    • Model’s complex : rotation mat. & model-dep. Rel. b/w disk’s 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


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