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Kinematics

The velocity dispersion profiles of the galaxies are also measured using a method faithful to current observational techniques. A slit 3.2 kpc in width is laid along the apparent major axis in three independent directions. This corresponds observationally to a 1.5 arcsecond slit laid across a galaxy at a distance of 100 Mpc. Particles are binned in squares 3.2 kpc on a side and the mean line-of-sight velocity and velocity dispersion is measured in each bin. Like real ellipticals, the galaxy rotates slowly about its minor axis with tex2html_wrap_inline883 km/s (Franx, Illingworth & Heckman 1989)[24]. Figure 9 shows the velocity dispersion profile along the apparent major axis for the three lines of sight down each of the principal axes.

  


Figure 9: Velocity dispersion profile measured along a slit laid on the major axis for the three principal axis projections of the galaxy. The velocity dispersion declines slowly with distance from the centre. There is no sign of an upturn at large distances.

The central value peaks between 300 and 450 km/s depending on the line of sight. The large value of 450 km/s occurs when looking exactly down the long axis of the galaxy showing the anisotropy of the velocity ellipsoid in this flattened triaxial stellar system. These central values again are in accord with real giant ellipticals although the value of 450 km/s might be considered too large (Fisher et al. 1995)[23]. The velocity dispersion only declines gradually out to 60 kpc (about 3 effective radii) again in similar fashion too many elliptical galaxies. There is no sign of an upturn in the velocity dispersion at large radii as seen in the exceptional case of the cD galaxy in A2029 (Dressler 1979)[17].

In three dimensions, the measured velocity dispersion is nearly isotropic to the centre but becomes radial anisotropic with a radial anisotropy parameter (Binney & Tremaine 1987)[8], tex2html_wrap_inline885 at 3 tex2html_wrap_inline837. The density profiles of Figure 7 for the dark matter and stars were fit with Hernquist (1990) models and used to solve the spherical Jeans equations for the velocity dispersion profile of the stars using constant values for the anisotropy parameter, tex2html_wrap_inline889. Figure 10 shows that the velocity profile is consistent with the mass model for values of tex2html_wrap_inline891 within r < 20 kpc (tex2html_wrap_inline895). The best fit spherical Jeans model in Figure 10 is one where the anisotropy grows monotonically from the center with tex2html_wrap_inline897 to tex2html_wrap_inline885 at 3 tex2html_wrap_inline837.

  


Figure 10: The spherically averaged radial velocity dispersion profile compared to anisotropic spherical model predictions from the fitted density profile. Each line is labelled with the anisotropy parameter used in the model. The anisotropy of the model rises from about 0.2 in the center to 0.5 at 100 kpc (tex2html_wrap_inline903). The dashed line represents a best fit model with tex2html_wrap_inline905 growing monotonically from 0.0 to 0.5 from the center to 100 kpc.


next up previous
Next: Comparison with other merger Up: Analysis of the Merger Previous: Density

John Dubinski
Tue Feb 17 16:03:05 EST 1998