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Galactic Astronomy

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

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

  2. 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

  3. 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

  4. 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.

  5. 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

  6. 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.

  7. 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

  8. 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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