Dark Matter Deprivation in Field Elliptical Galaxy NGC 7507
  Previous studies have shown that the kinematics of the field elliptical
galaxy NGC 7507 do not necessarily require dark matter. This is troubling
because, in the context of LCDM cosmologies, all galaxies should have a large
dark matter component. We use penalised pixel fitting software to extract
velocities and velocity dispersions from GMOS slit mask spectra. Using Jeans
and MONDian modelling we produce best fit models to the velocity dispersion. We
find that NGC 7507 has a two component stellar halo, with the outer halo and
inner haloes counter rotating. The velocity dispersion profile exhibits an
increase at ~70" (~7.9 kpc), reminiscent of several other elliptical galaxies.
Our best fit models are those under mild anisotropy which include ~100 times
less dark matter than predicted by LCDM, although mildly anisotropic models
that are completely dark matter free fit almost equally well. Our MONDian
models, both isotropic and anisotropic, systematically fail to reproduce the
measured velocity dispersions at almost all radii. The counter rotating outer
halo implies a merger remnant, as does the increase in velocity dispersion at
~70". From simulations it seems plausible that the merger that caused the
increase in velocity dispersion was a spiral-spiral merger. Our Jeans models
are completely consistent with a no dark matter scenario, however, some dark
matter can be accommodated, although at much lower concentrations that
predicted by LCDM simulations. This indicates that NGC 7507 may be a dark
matter free elliptical galaxy. Whether NGC 7507 is completely dark matter free
or very dark matter poor, this is at odds with predictions from current LCDM
cosmological simulations. It may be possible that the observed velocity
dispersions could be reproduced if the galaxy is significantly flattened along
the line of sight (e.g. due to rotation), however, invoking this flattening is
problematic.
