Kinematic signature of an intermediate-mass black hole in the globular cluster NGC 6388

Abstract

Context. Intermediate-mass black holes (IMBHs) are of interest in a wide range of astrophysical fields. In particular, the possibility of finding them at the centers of globular clusters has recently drawn attention. IMBHs became detectable since the quality of observational data sets, particularly those obtained with HST and with high resolution ground based spectrographs, advanced to the point where it is possible to measure velocity dispersions at a spatial resolution comparable to the size of the gravitational sphere of influence for plausible IMBH masses. Aims. We present results from ground based VLT/FLAMES spectroscopy in combination with HST data for the globular cluster NGC 6388. The aim of this work is to probe whether this massive cluster hosts an intermediate-mass black hole at its center and to compare the results with the expected value predicted by the M • − σ scaling relation. Methods. The spectroscopic data, containing integral field unit measurements, provide kinematic signatures in the center of the cluster while the photometric data give information of the stellar density. Together, these data sets are compared to dynamical models and present evidence of an additional compact dark mass at the center: a black hole. Results. Using analytical Jeans models in combination with various Monte Carlo simulations to estimate the errors, we derive (with 68% confidence limits) a best fit black-hole mass of (17 ± 9) × 10 3 M and a global mass-to-light ratio of M/L V = (1.6 ± 0.3) M /L .

Central kinematics of the globular cluster NGC 2808: upper limit on the mass of an intermediate-mass black hole

“…The effective velocity dispersion is derived using Eq. (1) in Lützgendorf et al (2011) and results in σ e = (13.4 ± 0.2) km s −1 . This is in very good agreement with the central velocity dispersion σ c = (13.4 ± 2.6) km s −1 of Pryor & Meylan (1993).…”

“…However, recent observations (e.g. Gebhardt et al 2000;Gerssen et al 2002;Gebhardt et al 2005;Noyola et al 2008;Lützgendorf et al 2011) have shown that the velocity-dispersion profiles of some globular clusters and dwarf galaxies are consistent with hosting a massive black hole at their center.…”

Understanding the central kinematics of globular clusters with simulated integrated-light IFU observations

“…Note, however, that simulations specifically designed to match particular cases are needed to fur-ther understand in details the discrepancies reported in the literature (e.g. see the case of NGC 6388, Lützgendorf et al 2011Lützgendorf et al , 2015Lanzoni et al 2013). …”

“…In particular, integrated light spectroscopy seems to measure rising central velocity dispersions, favouring the presence of IMBHs (see e.g., Noyola et al 2010 for ω Cen; Lützgendorf et al 2011Lützgendorf et al , 2015 for NGC 6388), while resolved stellar kinematics do not confirm the presence of this signature (see van der Marel & Anderson 2010 for proper motion measurements of ω Cen; Lanzoni et al 2013 for individual line-of-sight measurements in NGC 6388).…”

TheGaia-ESO Survey: Kinematics of seven Galactic globular clusters

“…Internal rotation may also play an indirect role in the open question of whether there are intermediate-mass black holes (IMBH) in some GCs. In fact, the detection of strong gradients in the velocity dispersion profile toward the cluster core is often interpreted as a hint of the presence of an IMBH (Baumgardt et al 2005), but the evidence gathered so far in support of the existence of IMBHs is inconclusive and controversial, and none of the published studies (van der Marel & Anderson 2010;Lützgendorf et al 2011;Lanzoni et al 2013) did consider differential rotation, which, together with anisotropy, can yield gradients in the velocity dispersion profiles Bianchini et al 2013). Finally, recent investigations indicate that rotation could be a key ingredient in the formation of multiple generations of stars in GCs (Bekki 2010;Mastrobuono-Battisti & Perets 2013).…”