Astronomers using NASA’s Hubble Telescope have come up with what they say is some of their best evidence for the presence of a rare class of “intermediate-sized” black holes that may be lurking at the heart of the closest globular cluster to Earth. , which is located 6000 light years away.
As intense gravitational dents in the fabric of the universe, virtually all black holes appear to come in two sizes: small and huge. It is estimated that our galaxy is littered with 100 million tiny black holes (several times the mass of our Sun) formed from exploding stars. The universe as a whole is awash with supermassive black holes that weigh millions or billions of times the mass of our Sun and are located at the centers of galaxies.
The long-sought missing link is an intermediate-mass black hole with a mass somewhere between 100 and 100,000 solar masses. How would they form, where would they hang out, and why do they seem so rare?
Astronomers have identified other possible intermediate-mass black holes using a variety of observational techniques. Two of the best candidates – 3XMM J215022.4−055108, which Hubble helped discover in 2020, and HLX-1, identified in 2009 – reside in dense star clusters on the outskirts of other galaxies. Each of these possible black holes has a mass of tens of thousands of suns and may have once been at the centers of dwarf galaxies. NASA’s Chandra X-ray Observatory has also helped make many possible transient black holes discoveries, including a large sample in 2018.
Looking much closer to home, a number of intermediate mass black holes have been detected in the dense globular clusters orbiting our Milky Way galaxy. For example, in 2008, astronomers from HST reported the suspected presence of a medium-mass black hole in the globular star cluster Omega Centauri. For many reasons, including the need for more data, these and other findings of intermediate-mass black holes still remain inconclusive and do not rule out alternative theories.
Hubble’s unique capabilities have now been used to target the core of the globular cluster Messier 4 (M4) to hunt for black holes with greater precision than previous searches. “You can’t do this kind of science without HST,” said Eduardo Vitral of the Space Telescope Science Institute in Baltimore, Maryland, lead author of the paper, which will be published in Monthly Notices of the Royal Astronomical Society.
Vitral’s team detected a possible intermediate-mass black holes with a mass of approximately 800 solar masses. The suspected object is not visible, but its mass is calculated by studying the motion of stars caught in its gravitational field, like bees swarming around a hive. Measuring their movement takes time and a lot of precision. This is where Hubble can do what no other current telescope can do. Astronomers looked at 12 years of observations of M4 from Hubble and resolved the exact stars.
Black hole in M4 could have up to 800 times the mass of our Sun
His team estimates that the black hole in M4 could have up to 800 times the mass of our Sun. The Hubble data tend to rule out alternative theories for the object, such as a compact central cluster of unresolved stellar remnants such as neutron stars, or smaller black holes swirling around each other.
“We’re pretty confident that we have a very small region with a lot of concentrated matter. It’s about three times smaller than the densest dark matter we’ve previously found in other globular clusters,” Vitral said. “This region is more compact than we can reproduce with numerical simulations when we consider a collection of black holes, neutron stars and white dwarfs segregated at the center of the cluster. They are not capable of creating such a compact concentration of matter.” “
A grouping of tightly connected objects would be dynamically unstable. If the object is not a single intermediate-mass black holes, it would require an estimated 40 smaller black holes crammed into a space only one-tenth of a light-year across to produce the observed stellar motions. The consequence is that they would merge and/or be thrown out in a game of interstellar pinball.
“We measure the motions of the stars and their positions and apply physical models that try to reproduce those motions. We end up measuring the extension of the dark matter in the center of the cluster,” Vitral said. “The closer to the central mass, the more randomly the stars move. And the larger the central mass, the faster these stellar velocities.”
Because intermediate-mass black holes in globular clusters have been so elusive, Vitral cautions, “While we can’t completely confirm that it’s a central gravitational point, we can show that it’s very small. It’s too small to be able to explain anything other than that it would be a single black hole. Alternatively, there may be a stellar mechanism that we simply don’t know about, at least within current physics.”
The Hubble Space Telescope is an international collaborative project between NASA and ESA. The telescope is managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts the Hubble and Webb science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.
Written by: Vaishali Verma
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