The shape of the dark matter halo in the early-type galaxy NGC 2974

TL;DR

This study uses HI and ionised gas kinematics to analyze the dark matter halo shape in NGC 2974, finding it to be consistent with an axisymmetric potential and revealing a significant dark matter component within five effective radii.

Contribution

The paper introduces a new method for correcting ionised gas velocities for asymmetric drift and combines HI and ionised gas data to model the galaxy's mass distribution and dark matter content.

Findings

The dark matter halo in NGC 2974 is consistent with an axisymmetric shape.

At least 55% of the mass within 5 Re is dark matter.

A pseudo-isothermal sphere best fits the observed rotation curve.

Abstract

We present HI observations of the elliptical galaxy NGC 2974, obtained with the Very Large Array. These observations reveal that the previously detected HI disc in this galaxy (Kim et al. 1988) is in fact a ring. By studying the harmonic expansion of the velocity field along the ring, we constrain the elongation of the halo and find that the underlying gravitational potential is consistent with an axisymmetric shape. We construct mass models of NGC 2974 by combining the HI rotation curve with the central kinematics of the ionised gas, obtained with the integral-field spectrograph SAURON. We introduce a new way of correcting the observed velocities of the ionised gas for asymmetric drift, and hereby disentangle the random motions of the gas caused by gravitational interaction from those caused by turbulence. To reproduce the observed flat rotation curve of the HI gas, we need to include a dark halo in our mass models. A pseudo-isothermal sphere provides the best model to fit our data, but we also tested an NFW halo and Modified Newtonian Dynamics (MOND), which fit the data marginally worse. The mass-to-light ratio M/L_I increases in NGC 2974 from 4.3 (M/L_I)sun at one effective radius to 8.5 (M/L_I)sun at 5 Re. This increase of M/L already suggests the presence of dark matter: we find that within 5 Re at least 55 per cent of the total mass is dark.