2008MNRAS.385.1729D

We present a dynamical study of NGC 4697, an almost edge-on, intermediate-luminosity, E4 elliptical galaxy, combining new surface brightness photometry, new as well as published long-slit absorption-line kinematic data, and published planetary nebula (PN) velocity data. The combined kinematic data set extends out to ≃5arcmin (≃4.5R_e) and allows us to probe the galaxy's outer halo.

For the first time, we model such a data set with the new and flexible χ²-made-to-measure particle code nmagic. We extend nmagic to include seeing effects, introduce an efficient scheme to estimate the mass-to-light ratio, and incorporate a maximum-likelihood technique to account for discrete velocity measurements.

For modelling the PN kinematics, we use line-of-sight velocities and velocity dispersions computed on two different spatial grids, and we also use the individual velocity measurements with the likelihood method, in order to make sure that our results are not biased by the way we treat the PN measurements.

We generate axisymmetric self-consistent models as well models including various dark matter haloes. These models fit all the mean velocity and velocity dispersion data with χ²/N < 1, both in the case with only luminous matter and in potentials including quite massive haloes. The likelihood analysis together with the velocity histograms suggest that models with low-density haloes such that the circular velocity v_c≲ 200km/s at 5R_e are not consistent with the data. A range of massive haloes with v_c≃ 250km/s at 5R_e fit the PN data best. To derive stronger results would require PN velocities at even larger radii. The best-fitting models are slightly radially anisotropic; the anisotropy parameter β ≃ 0.3 at the centre, increasing to β ≃ 0.5 at radii ≳2R_e.