Addendum: "Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15: II. Kinematical Analysis and Dynamical Modeling"
Abstract
It has been reported that there is an error in the figure in Dull et al. (1997, hereafter D97) that shows the radial mass-to-light ratio (M/L) profile in Fokker-Planck models of M15. We discuss how this modifies the interpretation of the kinematical data presented in our paper. These data imply the existence of a dark and compact mass component near the center of M15. This could either be a single black hole (BH) or a collection of dark remnants that have sunk to the cluster center due to mass segregation. We previously showed that the latter interpretation is in conflict with the M/L profile published by D97, which supported the view that M15 harbors a BH. We repeat our analysis here with the corrected D97 profile. Models without a BH are now found to be statistically acceptable (within 1-sigma), although inclusion of a BH still provides a marginally better fit to the data. It does not necessarily follow that dark remnants are now the preferred interpretation of the data. The D97 models, as well as N-body models by Baumgardt et al. (2002), assume that all neutron stars are retained during cluster evolution. This conflicts with predictions of the neutron-star retention rate (typically below 10 per cent) based on pulsar kick velocities. Hence, existing cluster evolution models give an upper limit to the expected number of dark remnants near the center of M15. The presence of a single BH therefore continues to be a viable interpretation of the data. The best fit BH mass with the corrected D97 M/L profile is M_BH = 1.7^{+2.7}_{-1.7} x 10^3 solar masses, and with a constant M/L it is M_BH = 3.2^{+2.2}_{-2.2} x 10^3 solar masses. Both values agree with the correlation between velocity dispersion and BH mass that has been inferred for galaxies.[ABRIDGED]Keywords
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