Different Tracers Give Different Gravitational Mass Distributions

Context. Charting the extent and amount of dark matter (DM) in the Universe is highly appealing but is equally hard since it is only through the interpretation of its effect that we can track the DM distribution. Given the implementational problems, it is non-trivial to quantify the effects of DM on the motion of individual test particles in an elliptical galaxy, with the aim of identifying its total gravitational (i.e. luminous+dark) mass distribution; expectedly, this has caused controversy. Aims. Leaving such technical details aside, in this article we report on the danger of the very notion that test particle velocities can reliably imply total mass distribution in galaxies. Methods. We expose the fallibility of this mass determination route, by undertaking a Bayesian analysis of the observed line-of-sight velocities of individual test particles belonging two distinct types: planetary nebulae (PNe) and globular clusters (GCs) that span the outskirts of the galaxy NGC 3379. Results. The PNe and GC data are shown to be drawn from independent phase space distributions and total mass density distributions that are derived from implementations of the two kinematic data sets are found to be significantly different, leading to significant (at 1-σ level) differences in the resulting enclosed mass profiles. The assumption of isotropy in phase space is tested with a robust Bayesian test of hypothesis; the GC velocities are found to be much more supportive of the assumption of isotropy than are the PNe data. We find that this recovered difference in the state of isotropy between the phase space distributions that the data are drawn from, cannot be used to reconcile the differences in the recovered mass density distributions. Conclusions. The recovered dichotomy is indicative of the risk involved in the interpretation of mass distribution obtained from tracer kinematics, as the galactic mass distribution.