Emission Lines and Black-Hole Acitivity in Extragalactic Globular Clusters

Michigan State University, East Lansing MI

Investigators

Abstract

Dr. Zepf will follow up his serendipitous discovery of a violently variable X-ray source and a very broad optical emission line in the spectrum of a globular star cluster in elliptical galaxy in the Virgo Cluster of galaxies. It is difficult to interpret these two observations other than as signatures of a black hole into which gas is falling. The group will obtain wider spectral coverage to determine line ratios and hence whether the emitting gas is likely to be photo-ionized or shock-ionized. This bears on the mass of the likely black hole: a stellar-mass black hole is likely to produce shock-ionization, while a more massive black hole would photo-ionize the gas. Other globular star clusters would be searched for similar signatures. This work is potentially transformative, as it could provide the most solid evidence yet for intermediate-mass black holes: these objects are so massive that the explosion of a single dying star cannot make them. If intermediate-mass black holes are found to form in globular clusters, this will open a new area of research. Students will be trained as they participate in this research. Dr. Zepf will give public lectures on his work, and incorporate the results into his teaching of introductory survey classes.