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Published August 20, 2008 | Published
Journal Article Open

Very broad [O III] λλ4959, 5007 emission from the NGC 4472 globular cluster RZ 2109 and implications for the mass of its black hole X-ray source


We present Keck LRIS spectroscopy of the black hole–hosting globular cluster RZ 2109 in the Virgo elliptical galaxy NGC 4472. We find that this object has extraordinarily broad [O III] λ5007 and [O III] λ4959 emission lines, with velocity widths of approximately 2000 km s^-1. This result has significant implications for the nature of this accreting black hole system and the mass of the globular cluster black hole. We show that the broad [O III] λ5007 emission must arise from material driven at high velocity from the black hole system. This is because the volume available near the black hole is too small by many orders of magnitude to have enough [O III]-emitting atoms to account for the observed L([O III] λ5007) at high velocities, even if this volume is filled with oxygen at the critical density for [O III] λ5007. The Balmer emission is also weak, indicating the observed [O III] is not due to shocks. We therefore conclude that the [O III] λλ4959, 5007 is produced by photoionization of material driven across the cluster. The only known way to drive significant material at high velocity is for a system accreting mass near or above its Eddington limit, which indicates a stellar-mass black hole. Since it is dynamically implausible to form an accreting stellar-mass black hole system in a globular cluster with an intermediate-mass black hole (IMBH), it appears this massive globular cluster does not have an IMBH. We discuss further tests of this conclusion, and its implications for the MBH-Mstellar and MBH-σ relations.

Additional Information

© 2008 University of Chicago Press. Received 2008 May 19; accepted 2008 July 16; published 2008 July 28. S.E.Z. acknowledges support for this work from NSF award AST 04-06891, A.K. acknowledges support from NASA LTSA grant NAG5-12975, and the work of D.S. was carried out at Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. We also thank the referee for a detailed and careful report. We wish to acknowledge the significant cultural role that the summit of Mauna Kea plays within the indigenous Hawaiian community; we are fortunate to have the opportunity to conduct observations from this mountain. Based on observations made at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.

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