Dark Matter Deprivation in Field Elliptical Galaxy NGC 7507

Abstract

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.

9 pages, 7 figures. Accepted for publication in A&A

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