What have We Learned?

The results of the previous section can be summarized as follows:

• For some disk geometries (eg., an exponential disk), can fall faster than and still be physical. An unambiguous ``faster-than-Keplerian decline'' in is thus a sufficient, but not necessary, signature of a flattened geometry for the total mass of a galaxy.
• Kinematic noise at the 5-10% level has a very small statistical effect and no systematic effect on mass estimates derived from rotation curve inversion.
• Kinematic extrapolations beginning at R affect mass estimates beginning at about R/2. Interior to the last measured point, the enclosed mass is uncertain by 30% and surface mass density by as much a factor of 3 for flattened geometries; the certainty of both estimates deteriorate very rapidly beyond the measured kinematics.
• Mass estimates from rotation curve fitting are subject to these same uncertainties beginning at half the kinematic radius since, by assuming a functional form for the radial mass distribution, they have implicitly adopted an extrapolation for the kinematics of the galaxy.
• Over the radial range of HI rotation curves, isothermal halos have exponential surface mass densities with scale lengths a few times the optical scale length. Rotation curve kinematics are thus equally consistent with an exponential and isothermal dark matter distributions.

A fuller description of the inversion technique and its advantages, as well as its application to a sample of well-defined rotation curves, will appear elsewhere (Sackett, in preparation).

© Copyright Astronomical Society of Australia 1997