The more we learn about the universe, the more evidence accumulates that not only is it beautiful and exciting, but it is an arena of incredible natural violence. In an April report, two scientists from Harvard University’s Department of Astronomy theorize that around 300 medium-size black holes may have been hurled into the Milky Way’s distant reaches.
[Artist David A. Aguilar of the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, Mass., depicted a “rogue black hole floating near a globular star cluster on the outskirts of the Milky Way.” Image courtesy of CfA.]
Black holes are believed to lurk at the middle of every galaxy. They pose a chicken-or-egg problem: some scientists think primordial dust collected around black holes that were spawned in the Big Bang, and the dust collapsed into stars and galaxies; another idea is that as stars formed, some inevitably collapsed into black holes, and these gravity sinkholes continued to grow as more stars blazed in the young galaxy and more were consumed.
A mature galaxy like ours may have formed by the coalescing of “galactic building blocks,” in the words of Ryan M. O’Leary and Abraham Loeb, published in the Monthly Notices of the Royal Astronomical Society, London. These building blocks amount to low-mass galaxies that merge in the Milky Way’s vicinity. When that happens, gravity waves sling black holes at their centers away from the main section of our larger galaxy.
“This velocity is sufficiently large to remove the remnant BH [black hole] from a low-mass galaxy but is below the escape velocity” needed to throw it out of the Milky Way altogether, they write. “If central BHs were common in the galactic building blocks that merged to make the MW [Milky Way galaxy], then numerous BHs that were kicked out of low-mass galaxies should be freely floating in the MW halo today.”
The article predicts that each of these black holes should drag along a compact cluster of stars from the original host galaxy. The captive stars are bunched together, making clusters that are far smaller than the familiar globular clusters of stars that orbit the Milky Way such as the Pleiades. The little variety would feature stars swiftly orbiting the black hole and would have a high ratio of mass compared to the amount of light emitted. The heavier mass is because of the central black hole.
It’s a compelling image: a black hole with its closest retinue of stars, cast out to the Milky Way’s farthest outposts; meanwhile, the remaining stars of the absorbed galaxies are pulled into the main Milky Way.
During the dance of merger, the smaller galaxies’ black holes would form a binary, the two circling each other. If the binary were surrounded by a disk of matter called an accretion disk, “the ejected BH would carry the disk with it and shine as a quasar.”
A quasar — the name comes from “quasi-stellar radio source” — is a celestial object that emits an incredible amount of energy, believed streaming from a black hole. The quasar may look like a star because of its relatively small size but it releases more light and energy than a star.
Today’s general sky surveys may not be sensitive enough to detect a small, distant star cluster orbiting a black hole. “Thus, the cluster will appear as a point source and might be confused as a single foreground star with unusual colors,” the authors report. However, the colors may be a tip-off.
Globular clusters are extremely old and their population of ancient stars is of particular colors. If what seems like a single star happens to have this coloration, maybe it’s really a cluster of old stars. Spectrograph studies might then indicate their true nature, the scientists write.
If this new type of cluster is discovered it would help in understanding the formation of the Milky Way and the dynamics of black hole mergers in the early universe, note O’Leary and Loeb.