Kinematical fingerprints of star cluster early dynamical evolution

Abstract

We study the effects of the external tidal field on the violent relaxation phase of star clusters dynamical evolution, with particular attention to the kinematical properties of the equilibrium configurations emerging at the end of this phase. We show that star clusters undergoing the process of violent relaxation in the tidal field of their host galaxy can acquire significant internal differential rotation and are characterized by a distinctive radial variation of the velocity anisotropy. These kinematical properties are the result of the symmetry breaking introduced by the external tidal field in the collapse phase and of the action of the Coriolis force on the orbit of the stars. The resulting equilibrium configurations are characterized by differential rotation, with a peak located between one and two half-mass radii. As for the anisotropy, similar to clusters evolving in isolation, the systems explored in this Letter are characterized by an inner isotropic core, followed by a region of increasing radial anisotropy. However for systems evolving in an external tidal field the degree of radial anisotropy reaches a maximum in the cluster intermediate regions and then progressively decreases, with the cluster outermost regions being characterized by isotropy or a mild tangential anisotropy. Young or old but less-relaxed dynamically young star clusters may keep memory of these kinematical fingerprints of their early dynamical evolution.

Star clusters in evolving galaxies

“…Despite this, the asymmetry of the tidal effects induces differential rotation in clusters as observed (e.g. van de Ven et al, 2006) and modelled (Boily and Pichon, 2001;Theis, 2002;Vesperini et al, 2014). This is strongly enhanced around core-collapse when the Coriolis effect alters the inward (in the contraction phase, respectively outward in the re-expansion phase) radial motions of stars by making them prograde (respectively retrograde).…”

“…Davies et al, 2011;Bianchini et al, 2013;Lardo et al, 2015). However, Vesperini et al (2014) noted that rotation, specially in the outer parts, of old globulars could results from the interplay of internal dynamics (relaxation) and external effects (tides, see Part III), whereas Gavagnin et al (2016) invoke mergers of clusters to induce rotation. Rotation of clusters, potentially set at birth, is thought to accelerate the pace of their evolution (Einsel and Spurzem, 1999;Hong et al, 2013) and modify their velocity dispersion profiles, thus potentially biasing the interpretation of observational data (Varri and Bertin, 2012;Bianchini et al, 2013).…”

The Strong Rotation of M5 (NGC 5904) as Seen from the MIKiS Survey of Galactic Globular Clusters

“…Hence, simple isotropic and nonrotating models (King 1966) are usually adopted to reproduce their observed surface brightness/star count profiles and to derive their structural parameters (e.g., Harris 1996). However, recent N-body simulations indicate that GCs do not attain complete energy equipartition (Trenti & van der Marel 2013; see also Bianchini et al 2016), and they may show differential rotation and complex behaviors of pressure anisotropy, depending on the degree of dynamical evolution suffered and the effect of an external tidal field (e.g., Vesperini et al 2014).…”

Merged or monolithic? Using machine-learning to reconstruct the dynamical history of simulated star clusters

“…Besides an elongated shape owing to residual pressure-tensor anisotropy that was left over by the merger or lingering rotation, in the case of off-axis mergers, the clues are probably subtle and difficult to parameterize. Moreover, deviations from spherical symmetry in clusters are actually observed (White & Shawl 1987;Chen & Chen 2010) but there is currently limited consensus as to their explanation because rotation and tidal effects may result in elongated profiles without the need for a merger (Davoust 1986;Bertin & Varri 2008;Varri & Bertin 2009, 2012Bianchini et al 2013;Vesperini et al 2014).…”