A stellar census in globular clusters with MUSE: The contribution of rotation to cluster dynamics studied with 200 000 stars

Abstract

This is the first of a series of papers presenting the results from our survey of 25 Galactic globular clusters with the MUSE integral-field spectrograph. In combination with our dedicated algorithm for source deblending, MUSE provides unique multiplex capabilities in crowded stellar fields and allows us to acquire samples of up to 20 000 stars within the half-light radius of each cluster. The present paper focuses on the analysis of the internal dynamics of 22 out of the 25 clusters, using about 500 000 spectra of 200 000 individual stars. Thanks to the large stellar samples per cluster, we are able to perform a detailed analysis of the central rotation and dispersion fields using both radial profiles and two-dimensional maps. The velocity dispersion profiles we derive show a good general agreement with existing radial velocity studies but typically reach closer to the cluster centres. By comparison with proper motion data we derive or update the dynamical distance estimates to 14 clusters. Compared to previous dynamical distance estimates for 47 Tuc, our value is in much better agreement with other methods. We further find significant (> 3σ ) rotation in the majority (13/22) of our clusters. Our analysis seems to confirm earlier findings of a link between rotation and the ellipticities of globular clusters. In addition, we find a correlation between the strengths of internal rotation and the relaxation times of the clusters, suggesting that the central rotation fields are relics of the cluster formation that are gradually dissipated via two-body relaxation.

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

“…By fixing PA 0 to this value and using all of the observed stars, we finally obtain the diagnostic plots shown in Figure 8 and the values listed in Table 3 for the global rotation signatures of M5. A complementary analysis following the approach described in Kamann et al (2018) fully confirms these results and provides a 1σ uncertainty of 5°.5 for the rotation axis position angle (see the shaded region in Figures 4 and 7). This is one of the strongest and cleanest evidences of rotation found to date in a GC.…”

“…A very good agreement is also found for what concerns the position angle of the rotation axis. The only exception is the perpendicular direction found by Kamann et al (2018) in the innermost 10″ of the cluster. Higher spatial-resolution spectroscopy, with the enhanced version of MUSE operating at super-seeing conditions, or with the adaptive-optics corrected spectrograph ESO-SINFONI (see Lanzoni et al 2013), will shed new light on this intriguing feature.…”

“…Very recently, Kamann et al (2018) analyzed a large number of individual star spectra acquired at r<60″ with the integral-field spectrograph ESO-MUSE, and found a velocity gradient of 2.2 km s −1 . They measured the rotation axis position angle in different radial bins around the cluster center, finding PA~  -130 140 0 (once reported in our system) at r>10″, while the axis seems to be rotated by 90°in the innermost region.…”

“…Indeed, deviations from the sharply truncated King phase space distribution (e.g., see the cases of NGC 1851, as studied by Olszewski et al 2009;Marino et al 2014, NGC 5694 by Correnti et al 2011;Bellazzini et al 2015, and several others, as discussed, e.g., by Carballo-Bello et al 2018), spherical symmetry (e.g., Chen & Chen 2010), and pressure isotropy (e.g., van de Ven et al 2006;Bellini et al 2014Bellini et al , 2017Watkins et al 2015) are found in a growing number of GGCs. Also the observational evidence of systemic rotation is increasing (e.g., Anderson & King 2003;Lane et al 2009Lane et al , 2010Bellazzini et al 2012;Bianchini et al 2013;Fabricius et al 2014;Kacharov et al 2014;Kimmig et al 2015;Lardo et al 2015;Bellini et al 2017;Boberg et al 2017;Cordero et al 2017;Ferraro et al 2018b;Kamann et al 2018), possibly suggesting that, when properly surveyed, the majority of GCs rotate at some level. In particular, Ferraro et al (2018b) investigated the intermediate/external region of 11 clusters, demonstrating the presence of internal rotation in almost all of them.…”

“…In particular, Ferraro et al (2018b) investigated the intermediate/external region of 11 clusters, demonstrating the presence of internal rotation in almost all of them. Kamann et al (2018) surveyed the central regions of 25 GGCs, detecting signals of rotation in 60% of their sample. On the other hand, recent N-body simulations ) describing the long-term evolution of GC rotational properties suggest that the detection of (even modest) signals is crucial, as they may provide useful lower limits to the amount of angular momentum imprinted during their formation process.…”