Dark matter content and internal dynamics of NGC 4697: NMAGIC particle models from slit data and planetary nebulae velocities

Abstract

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 nebulae (PNe) velocity data. The combined kinematic data set extends out to ~= 5' ~= 4.5 R_e and allows us to probe the galaxy's outer halo. For the first time, we model such a dataset with the new and flexible Chi^2-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 PNe 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 PNe measurements. We generate axisymmetric self-consistent models as well models including various dark matter halos. These models fit all the mean velocity and velocity dispersion data with Chi^2/N<1, both in the case with only luminous matter and in potentials including quite massive halos. The likelihood analysis together with the velocity histograms suggest that models with low density halos such that the circular velocity v_c ~< 200 km/s at R_e are not consistent with the data. A range of massive halos with v_c ~= 250 km/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 beta ~= 0.3 at the center, increasing to beta ~= 0.5 at radii >~ 2R_e.