Dr. Tom Maccarone is a coauthor of the paper, “Two stellar-mass black holes in the globular cluster M22”, and is Associate Professor,Department of Physics, at Texas Tech University at Lubbock Texas. The work is discussed in a popular form at "Physicists Find Black Holes In Globular Star Clusters, Upsetting 40 Years Of Theory" and it appeared in the journal http://www.nature.com/nature/journal/v490/n7418/full/nature11490.html. To help us at Physics Forums better understand this significant discovery, I asked Dr. Maccarone several questions about the black holes within globular clusters. Dr. Maccarone responded. The answers are informal, given without supporting citations, and includes the work of other scientists. They should guide us to a deeper understanding of globular clusters and black holes. Here is the exclusive interview for Physics Forums: Question1: If black holes are created, and then ejected from globular clusters, shouldn't there be renegade black holes flying around, possibly many thousands of them? Answer1: Yes -- but they would be hard to distinguish from black holes that just formed outright in the Milky Way halo, and would be a small fraction of the total number.In principle, we can find some fraction of these black holes through microlensing experiments, but it's not easy, and people are usually focused on other things with microlensing surveys. With the LSST, we should get a better handle on the number of stellar mass black holes that are in different parts of the Milky Way. Other than microlensing searches, black holes can normally be found only if they are in binaries. Question2: How common might black holes in globular clusters actually be? That is, what percentage of GCs have black holes? Answer2: My guess is that most or maybe all GCs have some black holes, but maybe typically 10 or so in a cluster, with a wide range on the number, and a much smaller fraction of them in close binaries where they will accrete mass from their companion star and be detectable in photometry. Question3: Are the black holes within globular clusters a result of normal star evolution, or are they, in some way, connected to the evolution of the GC? Answer3: It is uncertain whether the black holes in a cluster are entirely the result of normal stellar evolution. It's possible that compact objects can merge in star clusters. This can happen in close binary stars in field star populations as well, but probably for a lower fraction of the compact objects. It may, then, be that in some cases, merging two neutron stars makes a small black hole, or merging two black holes makes a heavier black hole, and that this may be more common in globular clusters, but how common this is is a fairly open question. Question4: Are there more black holes per capita within a GC than in the Milky Way? Or to ask it differently, does the old age of GC stars give us a different probability of finding black holes in GCs? Answer4: There are probably fewer black holes per capita in a GC than in the Milky Way. Age is relatively unimportant, because black holes normally should form only from very massive stars that evolve very quickly. We're reasonably confident that some dynamical ejection happens, which would reduce the number of black holes in globular clusters. The one thing that might work against the effects of the ejection is that globular clusters are often poor in metals, and the metals are important for the driving of stellar winds. With weaker stellar winds, stars will be more massive at the ends of their lifetimes, which may lead to more, heavier black holes forming. My expectation is that the dynamical ejection effect is more important, but the observational data right now are not good enough to give us strong tests. Question5: Is it possible that most globular clusters harbor black holes of significant mass? (this is in response to post #50 by Chronos) Answer5: I think post 50 was probably referring to the claims that there are intermediate mass black holes in a lot of globular clusters. The idea is that the mergers of black holes, or growth by accreting from other stars, or some other process, leads to the production of black holes of a few thousand to a few tens of thousand solar masses. These are more massive than can be made from normal stellar evolution, but much smaller than the supermassive black holes in the centers of galaxies. The evidence about these is mixed. The evidence in favor comes from dynamical studies, looking at the velocity distribution of stars around the centers of the clusters. In many clusters, there is evidence which is of marginal statistical significance that says that the mass-to-light ratio of the cluster increases toward the center. There are two major counterarguments. One is that the white dwarfs, neutron stars and stellar mass black holes should have very high mass-to-light ratios, and should sink to the centers of clusters because they are the heaviest objects in the clusters. The other is that there should be some gas in the star clusters, and the black holes should accrete some of the gas and emit X-rays and radio emission, but this has never been seen. I think it's widely, and maybe even generally, agreed by both camps that not *all* clusters should have intermediate mass black holes, because dynamical interactions between the black holes and the other stars would hold up core collapse. I would like to thank Dr. Tom Maccarone for his time and the kindness extended to Physics Forums, and to Greg Bernhardt for his approval to post. We encourage Physics Forum members to sound in with their thoughts and questions.