BREAKING THE DEGENERACY BETWEEN ANISOTROPY AND MASS - THE DARK HALO OF THE E0 GALAXY NGC-6703

We have measured line-of-sight velocity profiles (VPs) in the EO galax y NGC 6703 out to 2.6R(e). Comparing these with the VPs predicted from spherical distribution functions (DFs), we constrain the mass distrib ution and the anisotropy of the stellar orbits in this galaxy. We have developed a non-parametric technique to determine the DF f(E, L-2) di rectly from the kinematic data. We test this technique on Monte Carlo simulated data with the spatial extent, sampling, and error bars of th e NGC 6703 data. We find that smooth underlying DFs can be recovered t o an rms accuracy of 12 per cent inside three times the radius of the last kinematic data point, and the anisotropy parameter beta(r) can be recovered to an accuracy of 0.1, in a known potential. These uncertai nties can be reduced with improved data. By comparing such best-estima te, regularized models in different potentials, we can derive constrai nts on the mass distribution and anisotropy. Tests show that, with pre sently available data, an asymptotically constant halo circular veloci ty upsilon(0) can be determined with an accuracy of +/- less than or s imilar to 50 km s(-1). This formal range often includes high-v, models with implausibly large gradients across the data boundary. However, e ven with extremely high quality data some uncertainty on the detailed shape of the underlying circular velocity curve remains. In the case o f NGC 6703, we thus determine the true circular velocity at 2.6R(e) to be 250 +/- 40 km s(-1) at 95 per cent confidence, corresponding to a total mass in NGC 6703 inside 78 arcsec (13.5 h(50)(-1) kpc, where h(5 0) = H-0/50 km s(-1) Mpc(-1)) of 1.6-2.6 x 10(11) h(50)(-1) M.. No mod el without dark matter will fit the data; however, a maximum stellar m ass model in which the luminous component provides nearly all the mass in the centre will. In such a model, the total luminous mass inside 7 8 arcsec is 9 x 10(10) M. and the integrated B-band mass-to-light rati o out to this radius is Upsilon(B) = 5.3-10, corresponding to a rise f rom the centre by at least a factor of 1.6. The anisotropy of the stel lar distribution function in NGC 6703 changes from near-isotropic at t he centre to slightly radially anisotropic (beta = 0.3-0.4 at 30 arcse c, beta = 0.2-0.4 at 60 arcsec) and is not well-constrained at the out er edge of the data, where beta = -0.5 to +0.4, depending on variation s of the potential in the allowed range. Our results suggest that also elliptical galaxies begin to be dominated by dark matter at radii of similar to 10 kpc.