Scientists at the University of Utah and the Smithsonian Astrophysical Observatory believe they have discovered the cause of massive black holes that exist at the center of many galaxies.
There have been numerous theories concerning how these black holes attain their huge masses. One theory is that they are a result of the consumption of a huge amount of primordial gas early in the development of the universe. Another theory is that they result from the merging of two black holes. Others propose that the holes were simply that large when created.
However, there is renewed interest in the possibility that these black holes gain their mass by stellar capture through tidal disruption events (TDEs). This theory suggests that when stars travel close to a black hole, they are consumed and obliterated. The star must be within the tidal radius of the black hole, which is the distance at which the tidal forces of the hole overwhelm the gravitational forces on the star, at which point the star cannot escape. Observations have shown that the rate of stellar capture by black holes is enough to develop fairly massive black holes over millions of years.
In their study published in The Astrophysical Journal Letters earlier this month, a team of scientists led by Benjamin C. Bromley of the University of Utah, expand on the stellar capture hypothesis by proposing a method of galactic black hole TDEs that disrupt binary star systems, which are stars that travel in pairs and have gravitational pull on each other.
Their research shows that one of the stars escapes as a hypervelocity star (HVS), while the other is sucked into the black hole and its mass is consumed. In fact, their paper is a follow-up of the 2005 discovery of HVSs. HVSs originated from binary systems with other stars, and were flung away at high speeds after interacting with central black holes that occupy most galaxies, including ours, the Milky Way. The stars travel at 1.5 million miles per hour after being
ejected by black holes with masses of 4.3 million times that of our Sun.
By measuring the properties of both the bound stars and the HVSs, scientists were able to accurately determine the frequency of stellar capture and the tidal disruption events that cause them, as well as of the rate of growth of black holes at galactic centers that were of interest to their study.
Because HVSs are already known to exist, and their characteristics have already been well-defined, the researchers were able to work backwards, effectively using the mass and speed of the ejected star to determine the mass of the captured star that it was previously paired with. As a result, the researchers could determine whether these binary systems could contribute to the growing mass of galactic black holes.
The alignment of calculations based on this model with actual observations concerning HVSs, TDEs and galactic black hole growth provides conclusive evidence that binary stellar capture is a method by which black holes grow in size.
The researchers’ model shows that the tidal disruptions caused by the pull of the black holes roughly equal the capture rate of stars from binary systems. In addition, as the rate of capture increases, significantly more mass is aggregated by the black hole from the disruption of stellar binary systems.
They hope that in the future, their research can be expanded to include bound stars of lower mass, and larger samples of HVSs and TDEs to more definitively back up their observations.
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