2017MNRAS.471..383S

We analyse the stellar kinematics of the z = 0.169 brightest cluster galaxy in Abell 1201, using integral field observations acquired with the Multi-Unit Spectroscopic Explorer on the Very Large Telescope. This galaxy has a gravitationally lensed arc located at unusually small radius (∼5 kpc), allowing us to constrain the mass distribution using lensing and stellar dynamical information over the same radial range. We measure a velocity dispersion profile which is nearly flat at σ ≃ 285 km s^–1 in the inner ∼5 kpc, and then rises steadily to σ ≃ 360 km s^–1 at ∼30 kpc. We analyse the kinematics using axisymmetric Jeans models, finding that the data require both a significant dark matter halo (to fit the rising outer profile) and a compact central component, with mass M_cen ≃ 2.5 x 10¹⁰ M_☉ (to fit the flat σ in the inner regions). The latter component could represent a supermassive black hole, in which case it would be among the largest known to date. Alternatively M_cen could describe excess mass associated with a gradient in the stellar mass-to-light ratio. Imposing a standard Navarro-Frenk-White (NFW) dark matter density profile, we recover a stellar mass-to-light ratio U, which is consistent with a Milky Way-like initial mass function (IMF). By anchoring the models using the lensing mass constraint, we break the degeneracy between U and the inner slope γ of the dark matter profile, finding γ = 1.0 ± 0.1, consistent with the NFW form. We show that our results are quite sensitive to the treatment of the central mass in the models. Neglecting M_cen biases the results towards both a heavier-than-Salpeter IMF and a shallower-than-NFW dark matter slope (γ ≃ 0.5).