Mapping the dark matter halo of early-type galaxy NGC 2974 through orbit-based models with combined stellar and cold gas kinematics

Abstract

We present an orbit-based method of combining stellar and cold gas kinematics to constrain the dark matter profile of early-type galaxies. We apply this method to early-type galaxy NGC 2974, using Pan-STARRS imaging and SAURON stellar kinematics to model the stellar orbits, and introducing HI kinematics from VLA observation as a tracer of the gravitational potential. The introduction of the cold gas kinematics shows a significant effect on the confidence limits of especially the dark halo properties: we exclude more than $95\%$ of models within the $1-\sigma$ confidence level of Schwarzschild modelling with only stellar kinematics, and reduce the relative uncertainty of the dark matter fraction significantly to $10\%$ within $5R_{\mathrm{e}}$. Adopting a generalized-NFW dark matter profile, we measure a shallow cuspy inner slope of $0.6_{-0.3}^{+0.2}$ when including the cold gas kinematics in our model. We cannot constrain the inner slope with the stellar kinematics alone.

13 pages, 11 figures, accepted for publication in MNRAS

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