Stellar dynamics in the strong-lensing central galaxy of Abell 1201: A low stellar mass-to-light ratio, a large central compact mass, and a standard dark matter halo

Abstract

We analyse the stellar kinematics of the z=0.169 brightest cluster galaxy (BCG) in Abell 1201, using integral field observations with VLT/MUSE. This galaxy has a gravitationally-lensed arc located at unusually small radius ($\sim$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 $\ddot{I}ƒ$ $\approx$ 285 km/s in the inner $\sim$5 kpc, and then rises steadily to $\ddot{I}ƒ$ $\approx$ 360 km/s at $\sim$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 $M_{\mathrm{cen}}$ $\approx$ 2.5$\times$10${}^{10}$ $M_{\odot }$ (to fit the flat σ in the inner regions). The latter component could represent a super-massive black hole, in which case it would be among the largest known to date. Alternatively $M_{\mathrm{cen}}$ could describe excess mass associated with a gradient in the stellar mass-to-light ratio. Imposing a standard NFW dark matter density profile, we recover a stellar mass-to-light ratio $\hat{I}\yen$ 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 $\hat{I}\yen$ and the inner slope ${\hat{I}}^3$ of the dark matter profile, finding ${\hat{I}}^3$=1.0$\pm$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 $M_{\mathrm{cen}}$ biases the results towards both a heavier-than-Salpeter IMF and a shallower-than-NFW dark matter slope (${\hat{I}}^3$ $\approx$ 0.5).

MNRAS accepted