A rogue black hole wandering the space lanes of our Milky Way galaxy alone could be the smallest black hole yet found, according to one estimate of its mass.
Earlier this year, astronomers led by Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland, announced the discovery of the first known isolated stellar-mass black hole.
The black hole is 5,000 light-years away and was discovered thanks to the power of its gravity to act as a gravitational lens, magnifying the light of a background star 19,000 light-years away. It was initially spotted by two ground-based surveys, the Polish-led Optical Gravitational Lensing Experiment (OGLE) which mostly uses the Las Campanas Observatory in Chile, and the Microlensing Observations in Astrophysics (MOA) project at the Mount John University Observatory in New Zealand.
Sahu's team used the Hubble Space Telescope to follow up on the discovery, and the degree of gravitational lensing allowed them to conclude that the black hole has a mass about 7.1 times greater than the sun's mass.
However, a second team has now come forward with a different mass calculation. The group, led by Casey Lam of the University of California, Berkeley, concluded that the object has a mass between 1.6 and 4.4 times the mass of the sun. If correct, then this could have intriguing implications.
Stellar-mass black holes are the product of the supernovae of stars with masses 20 times greater than the Sun. On the other hand, when stars with between 8 and 20 solar masses go supernovae, they leave behind a neutron star instead.
Neutron stars can theoretically have masses up to about 2.3 solar masses. Observations of stellar-mass black holes detectable in binary systems have not turned up any with less than 5 solar masses, creating a gap between the most massive neutron stars and the least massive black holes. If the black hole is at the upper end of Lam's mass range, it would help plug this gap. (Several candidate gravitational-wave events have also been detected involving objects that fall into this mass gap.)
"Whatever it is, the object is the first dark stellar remnant discovered wandering through the galaxy unaccompanied by another star," said Lam in a NASA statement.
Even though stars with more than 20 solar masses account for just 0.1% of all the stars in the Milky Way, there are so many stars in the Milky Way (an estimated 100–200 billion), and the Milky Way is so old (approximately 13 billion years) that there should now be 100 million or more stellar-mass black holes in our galaxy.
Many of these are found in binary systems, where their presence is evident from their gravitational pull on their companion star and their accretion of matter from their neighbor. One has even been found inside a star cluster, NGC 1850 in the Large Magellanic Cloud. However, many others will be wandering between the stars, going unnoticed until a chance alignment with a background star means we spot them creating a gravitational lens.
This discovery is just the tip of the iceberg. NASA's Nancy Grace Roman Space Telescope, which is planned for launch in 2027, will survey large swathes of the Milky Way and is expected to identify several thousand microlensing events, many of which could be black holes.
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Keith Cooper is a freelance science journalist and editor in the United Kingdom, and has a degree in physics and astrophysics from the University of Manchester. He's the author of "The Contact Paradox: Challenging Our Assumptions in the Search for Extraterrestrial Intelligence" (Bloomsbury Sigma, 2020) and has written articles on astronomy, space, physics and astrobiology for a multitude of magazines and websites.