Dark matter deprivation in the field elliptical galaxy NGC 7507

Abstract

Context. 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 ΛCDM cosmologies, all galaxies should have a large dark matter component. Aims. Our aims are to determine the rotation and velocity dispersion profile out to larger radii than do previous studies, and, therefore, more accurately estimate of the dark matter content of the galaxy. Methods. We use penalised pixel-fitting software to extract velocities and velocity dispersions from GMOS slit mask spectra. Using Jeans and MONDian modelling, we then produce models with the goal of fitting the velocity dispersion data. Results. NGC 7507 has a two-component stellar halo, with the outer halo counter rotating with respect to the inner halo, with a kinematic boundary at a radius of ∼110 (∼12.4 kpc). 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 ΛCDM, although mildly anisotropic models that are completely dark matter free fit the measured dynamics almost equally well. Our MONDian models, both isotropic and anisotropic, systematically fail to reproduce the measured velocity dispersions at almost all radii. Conclusions. 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 than predicted by ΛCDM simulations. This indicates that NGC 7507 may be a dark matter free elliptical galaxy. Regardless of whether NGC 7507 is completely dark matter free or very dark matter poor, it is at odds with predictions from current ΛCDM 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.

Isolated elliptical galaxies in the local Universe

“…Regarding pure E galaxies in very isolated environments, the samples in these studies are usually composed of only a few bright (massive) objects (e.g., Colbert et al 2001;Marcum et al 2004;Reda et al 2004;Denicoló et al 2005;Collobert et al 2006;Hau & Forbes 2006;Smith et al 2010;Lane et al 2013Lane et al , 2015Richtler et al 2015;Salinas et al 2015). In contrast to these works, Smith et al (2004) and Stocke et al (2004) presented relatively large samples of 32 and 65 isolated E galaxies, respectively, but they did not consider the radial velocity separation of companion galaxies to classify an isolated candidate.…”

Isolated dark matter deprived galaxies in hydrodynamical simulations: real objects or artefacts?

“…However in this paper, we will not focus on dwarf galaxies, but more massive objects that are properly resolved in current large-scale hydro-simulations. A couple larger, potentially dark matter deprived, galaxies have been found observationally by studying the kinematics of their halos (Salinas et al 2012;Lane et al 2015) and strong-lensing studies of clusters (Monna et al 2017). Modern hydrodynamical simulations provide a possibility to study potential outliers of the stellar-to-halo mass relation and identify dark matter deprived galaxies.…”

What can isolated elliptical galaxies tell us about Cold Dark Matter?

“…Velocity dispersion versus galactocentric radius for NGC 7507. Filled squares are from GMOS longslit spectra (Salinas et al 2012), open squares are from GMOS slitmask spectra (Lane et al 2015). Left and middle panels: models which consider all the velocity dispersion points.…”

“…Furthermore, due to their environment the evolution of IEs should have stopped. However, many IEs have interacting dwarf companions (Lane et al 2013 or have dynamically young stellar populations (Lane et al 2015), and merging at the ∼ 10 7 M level was recently discovered in the M31 system (Amorisco et al 2014). These latter results are directly predicted by CDM simulations.…”