Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15. II. Kinematic Analysis and Dynamical Modeling

By J. Gerssen, R. P. van der Marel, K. Gebhardt, P. Guhathakurta, R. Peterson, C. Pryor. 2002

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We have used the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) to obtain high spatial resolution spectroscopy of the central region of the dense globular cluster M15. The observational strategy and data reduction were described in Paper I of this series. Here we analyze the extracted spectra with a cross-correlation technique to determine the line-of-sight velocities of individual stars. Our final STIS velocity sample contains 64 stars, two-thirds of which have their velocity measured for the first time. The new data set triples the number of stars with measured velocities in the central projected R ≤ 1″ of M15 and doubles the number in the central R ≤ 2″. We combine our data with existing ground-based data to obtain nonparametric estimates of the radial profiles of the projected rotation velocity, velocity dispersion, and rms velocity σrms. The results differ from earlier work in the central few arcseconds in that we find that σrms rises to ∼14 km s-1, somewhat higher than the values of 10–12 km s-1 inferred previously from ground-based data. To interpret the results we construct dynamical models based on the Jeans equation for a spherical system. If the velocity distribution is isotropic, then M15 must have a central concentration of nonluminous material. If this is due to a single black hole, then a fit to the full velocity information as function of radius implies that its mass is MBH = (3.9 ± 2.2) × 103 M⊙. The existence of intermediate-mass black holes in globular clusters is consistent with several scenarios for globular cluster evolution proposed in the literature. The inferred mass for M15 is consistent with the extrapolation of the relation between MBH and σrms that has been established for galaxies. Therefore, these results may have important implications for our understanding of the evolution of globular clusters, the growth of black holes, the connection between globular cluster and galaxy formation, and the nature of the recently discovered "ultraluminous" X-ray sources in nearby galaxies. Instead of a single intermediate-mass black hole, M15 could have a central concentration of dark remnants (e.g., neutron stars) due to mass segregation. However, we argue that the best-fitting Fokker-Planck models that have previously been constructed for M15 do not predict a central mass concentration that is sufficient to explain the observed kinematics. To fit the M15 data without any central dark mass concentration one must assume that the velocity distribution is significantly radially anisotropic near the center, which contradicts predictions from both Fokker-Planck and N-body calculations.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/8c502a68b6561d33cff688299256a0c69f7073fb

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Citations in the corpus

Formation of Massive Black Holes in Dense Star Clusters. I. Mass Segregation and Core Collapse (2003, result): “In particular, recent HST observations and dynamical modeling of M15 by Gerssen et al. (2002, 2003) yielded results that are consistent with the presence of a central massive BH in this cluster.”
NEW LIMITS ON AN INTERMEDIATE-MASS BLACK HOLE IN OMEGA CENTAURI. I. HUBBLE SPACE TELESCOPE PHOTOMETRY AND PROPER MOTIONS (2009, background): “Kinematical evidence for IMBHs on the basis of an in- crease in the line-of-sight velocity dispersion towards the center has been presented for the globular clusters M15 and G1 (van der Marel et al. 2002; Gerssen et al. 2002; Gebhardt, Rich, & Ho 2002, 2005).”
An Intermediate-Mass Black Hole in the Globular Cluster G1: Improved Significance from New Keck and Hubble Space Telescope Observations (2005, background): “The two claims for dynamical evidence for intermediate-mass black holes are for G1 inM31 (Gebhardt et al. 2002) andM15 (van der Marel et al. 2002; Gerssen et al. 2002), both of which have been challenged by Baumgardt et al. (2003a, 2003b).”
Dynamical evolution of star clusters in tidal fields (2002, background): “Dark matter could be present either in the cluster centers in the form of intermediate mass black holes (Gerssen et al. 2002, Gebhardt, Rich & Ho 2002), or in the form of faint stellar mass white dwarfs and neutron stars.”
Massive Black Holes in Star Clusters. II. Realistic Cluster Models (2004, methodology): “This seems possible both for radial velocity (Gerssen et al. 2002) or proper motion studies (McNamara et al. 2003) with HST.”
Binary Mergers and Growth of Black Holes in Dense Star Clusters (2005, background): “Observations and dynamical modeling of the globular clusters M15 in the Milky Way and G1 in M31 appear to be consistent with such a central massive BH (Gerssen et al. 2002, 2003; Gebhardt et al. 2002).”
Intermediate - mass black holes (2003, background): “…supermassive black holes in galaxies (Ferrarese & Merritt 2000; Gebhardt et al. 2000a; Merritt & Ferrarese 2001a,b; Tremaine et al. 2002) also applies to globulars (Gebhardt et al. 2002; van der Marel et al. 2002; Gerssen et al. 2002), this has major consequences for the origin of the correlation.”
TIDAL DISRUPTION AND IGNITION OF WHITE DWARFS BY MODERATELY MASSIVE BLACK HOLES (2008, background): “…tens of kiloparsecs, a number of globular clusters (Brodie & Strader 2006; Moehler & Bono 2008) have a high enough central density to let white dwarfs interact and collide with other stars, or, if developed, with a central massive black hole (Gerssen et al. 2002, 2003; Gebhardt et al. 2002, 2005).”
Observational evidence for intermediate-mass black holes (2017, methodology): “…cluster based on kinematic measurements and dynamical modeling was M15 (Bahcall & Wolf 1976; Peterson et al. 1989), which presented a pronounced rise in its velocity dispersion profile and for which a BH mass of (3.9± 2.2)× 103 M was estimated (Gebhardt et al. 1997, 2000a; Gerssen et al. 2002).”
NEW LIMITS ON AN INTERMEDIATE-MASS BLACK HOLE IN OMEGA CENTAURI. II. DYNAMICAL MODELS (2009, methodology): “Gerssen et al. (2002) used line-of-sight velocity data for individual stars in the core-collapsed cluster M15 (NGC 7078) to argue for the presence of ∼ 3000M⊙ in dark material near the center, possibly in the form of an IMBH.”
N-body modelling of globular clusters: masses, mass-to-light ratios and intermediate-mass black holes (2016, background): “…Cordero et al. (2015) LOS Drukier, Fahlman & Richer (1992) SD NGC 7078 Peterson, Seitzer & Cudworth (1989) LOS Drukier et al. (1998) LOS Gebhardt et al. (2000) LOS Gerssen et al. (2002) LOS Carretta et al. (2009b) LOS Lardo et al. (2015) LOS Kimmig et al. (2015) LOS McNamara, Harrison & Anderson…”