The PN.S Elliptical Galaxy Survey: a standard LCDM halo around NGC 4374?
  As part of our current programme to test LCDM predictions for dark matter
(DM) haloes using extended kinematical observations of early-type galaxies, we
present a dynamical analysis of the bright elliptical galaxy NGC 4374 (M84)
based on ~450 Planetary Nebulae (PNe) velocities from the PN.Spectrograph,
along with extended long-slit stellar kinematics. This is the first such
analysis of a galaxy from our survey with a radially constant velocity
dispersion profile. We find that the spatial and kinematical distributions of
the PNe agree with the field stars in the region of overlap. The velocity
kurtosis is consistent with zero at almost all radii. We construct a series of
Jeans models, fitting both velocity dispersion and kurtosis to help break the
mass-anisotropy degeneracy. Our mass models include DM halos either with
shallow cores or with central cusps as predicted by cosmological simulations -
along with the novel introduction in this context of adiabatic halo contraction
from baryon infall. Both classes of models confirm a very massive dark halo
around NGC 4374, demonstrating that PN kinematics data are well able to detect
such haloes when present. Considering the default cosmological mass model, we
confirm earlier suggestions that bright galaxies tend to have halo
concentrations higher than LCDM predictions, but this is found to be solved if
either a Salpeter IMF or adiabatic contraction with a Kroupa IMF is assumed.
Thus for the first time a case is found where the PN dynamics may well be
consistent with a standard dark matter halo. A cored halo can also fit the
data, and prefers a stellar mass consistent with a Salpeter IMF. The less
dramatic dark matter content found in lower-luminosity "ordinary" ellipticals
suggests a bimodality in the halo properties which may be produced by divergent
baryonic effects during their assembly histories.
