Nov 4 2019
For astrophysicists, black holes represent an integral part of how they interpret the universe. Black holes are so significant that researchers have been attempting to develop a census of all the black holes present in the Milky Way galaxy.
An artist’s rendering of the black hole astrophysicists identified in this study. The black hole (bottom left) is seen near a red giant star. The discovery shows there may be an entire class of black holes astronomers did not know existed. (Image credit: Jason Shults)
Yet, a recent study has revealed that researchers’ quest may not have included a whole class of black holes that were not known to them.
In a study that was recently reported in the Science journal, astronomers have provided a new method to look for black holes, and they demonstrated that a group of black holes tinier than the tiniest known black holes might exist in the universe.
We’re showing this hint that there is another population out there that we have yet to really probe in the search for black holes.
Todd Thompson, Study Lead Author and Professor, Department of Astronomy, Ohio State University
Thompson continued, “People are trying to understand supernova explosions, how supermassive black stars explode, how the elements were formed in supermassive stars. So if we could reveal a new population of black holes, it would tell us more about which stars explode, which don’t, which form black holes, which form neutron stars. It opens up a new area of study.”
One can envision a city’s census that counted only people who were 5′9″ and taller, and where the census takers did not even realize that people shorter than 5′9″ would also have existed. Data obtained from that census would not be complete and can give a wrong picture of the city’s population. That is exactly what has been taking place in the quest for black holes, added Thompson.
For a long time, astronomers have been looking for black holes, which have strong gravitational pulls to such an extent that nothing—not radiation, not matter—can escape. When certain stars die, they shrink into themselves and explode. This phenomenon results in the formation of black holes. Moreover, astronomers have been searching for neutron stars, which are dense, tiny stars that form when certain stars die and disintegrate.
Both black holes and neutron stars may contain interesting data regarding the elements found on Earth, and about the way stars live and die. However, in order to expose that data, astronomers first need to understand the location of the black holes. And to find out where the black holes are located, astronomers have to know what they are searching for.
One sign is that black holes are usually present in something known as a binary system. This merely indicates that a couple of stars are sufficiently close to each other, such that they are bound together by gravity in a common orbit around each other. If one of these stars dies, the other would still be alive and orbit the space where the dead star—currently a neutron star or black hole—once existed, and where a neutron star or black hole has formed.
For many years, researchers knew that all black holes had about 5 and 15 times the mass of the sun. Usually, the known neutron stars are not larger than about 2.1 times the sun’s mass; if these stars were above 2.5 times the mass of the sun, they would disintegrate into a black hole.
However, in the summer of 2017, a survey known as the Laser Interferometer Gravitational-Wave Observatory, or LIGO for short, observed a pair of black holes combining together in a galaxy that was approximately 1.8 million light-years away. One of those black holes had a mass that was around 31 times that of the sun; the other had a mass approximately 25 times that of the sun.
Immediately, everyone was like ‘wow,’ because it was such a spectacular thing. Not only because it proved that LIGO worked, but because the masses were huge. Black holes that size are a big deal—we hadn’t seen them before.
Todd Thompson, Study Lead Author and Professor, Department of Astronomy, Ohio State University
Along with other astrophysicists, Thompson had suspected for a long time that black holes may exist in sizes beyond the known range, and the discovery made by LIGO demonstrated that black holes can possibly be larger. However, a window of size remained between the smallest black holes and the biggest neutron stars. Thompson wanted to find out if that mystery could be solved.
He collaborated with other researchers and started to examine the data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), which gathered light spectra from about 100,000 stars across the Milky Way galaxy.
Thompson noted that the spectra may reveal whether a star is circling around another celestial object: Spectral changes—a shift toward bluer wavelengths, for instance, followed by a shift to redder wavelengths—can reveal that a star was circling a hidden companion.
Thompson started to comb through the data, searching for stars that exhibited that change, thus suggesting that they could perhaps be orbiting a black hole. He subsequently narrowed the APOGEE data to 200 stars that could be highly fascinating.
Thompson provided the data to Tharindu Jayasinghe, a graduate research associate from Ohio State University and who compiled an unlimited number of images of every promising binary system from the All-Sky Automated Survey for Supernovae (ASAS-SN). ASAS-SN has identified about 1,000 supernovae and operates from the Ohio State University.
The researchers’ data-crunching detected a massive red star that seemed to be circling something. However, that something, on the basis of the researchers’ calculations, could be relatively smaller than the identified black holes in the Milky Way galaxy, but much larger than the majority of the known neutron stars.
After further calculations and more data from the Tillinghast Reflector Echelle Spectrograph and the Gaia satellite, the researchers realized that they had identified a low-mass black hole, which could have 3.3 times the sun’s mass.
What we’ve done here is come up with a new way to search for black holes, but we’ve also potentially identified one of the first of a new class of low-mass black holes that astronomers hadn’t previously known about. The masses of things tell us about their formation and evolution, and they tell us about their nature.
Todd Thompson, Study Lead Author and Professor, Department of Astronomy, Ohio State University
Other Ohio State authors who contributed to this research are Christopher Kochanek, Kris Stanek, and Jennifer Johnson, all professors of astronomy; Jamie Tayar and Tom Holoien, former graduate students of the Ohio State University; and Katie Auchettl, a former Ohio State Center for Cosmology and Astro-Particle Physics (CCAPP) Postdoctoral Fellow.
The Research Corporation, the Simons Foundation Fellowship, and an IBM Einstein Fellowship from the Institute for Advanced Study at Princeton funded the study.