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MUSE crowded field 3D spectroscopy of over 12 000 stars in the globular cluster NGC 6397. II. Probing the internal dynamics and the presence of a central black hole

S. Kamann, T.-O. Husser, J. Brinchmann, E. Emsellem, P. Weilbacher, L. Wisotzki, M. Wendt, D. Krajnović, M. M. Roth, S. Dreizler

Abstract
We present a detailed analysis of the kinematics of the Galactic globular cluster NGC 6397 based on more than ~18 000 spectra obtained with the novel integral field spectrograph MUSE. While NGC 6397 is often considered a core collapse cluster, our analysis suggests a flattening of the surface brightness profile at the smallest radii. Although it is among the nearest globular clusters, the low velocity dispersion of NGC 6397 of < 5 km s-1 imposes heavy demands on the quality of the kinematical data. We show that despite its limited spectral resolution, MUSE reaches an accuracy of 1 km s-1 in the analysis of stellar spectra. We find slight evidence for a rotational component in the cluster and the velocity dispersion profile that we obtain shows a mild central cusp. To investigate the nature of this feature, we calculate spherical Jeans models and compare these models to our kinematical data. This comparison shows that if a constant mass-to-light ratio is assumed, the addition of an intermediate-mass black hole with a mass of 600 M brings the model predictions into agreement with our data, and therefore could be at the origin of the velocity dispersion profile. We further investigate cases with varying mass-to-light ratios and find that a compact dark stellar component can also explain our observations. However, such a component would closely resemble the black hole from the constant mass-to-light ratio models as this component must be confined to the central ~5″ of the cluster and must have a similar mass. Independent constraints on the distribution of stellar remnants in the cluster or kinematic measurements at the highest possible spatial resolution should be able to distinguish the two alternatives.

Keywords
globular clusters: individual: NGC 6397, stars: kinematics and dynamics, techniques: radial velocities, techniques: imaging spectroscopy, black hole physics

Notes
Based on observations obtained at the Very Large Telescope (VLT) of the European Southern Observatory, Paranal, Chile (ESO Programme ID 60.A-9100(C))

Astronomy and Astrophysics
Volume 588, Article Number A149, Number of pages 12
2016 April

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia