Astronomers have found a smaller, stellar-mass black hole lurking in a nearby satellite galaxy of our own Milky Way.  The black hole has been hiding in a star cluster named NGC 1850, which is one of the brightest star clusters in the Large Magellanic Cloud. The black hole is 160,000 light-years away from Earth, and is estimated to be about 11 times the mass of our Sun.

Using the European Southern Observatory’s Very Large Telescope in Chile, astronomers found the black hole when they noticed a star with a peculiar motion among the star cluster, where other stars weren’t behaving the same way. Further investigation revealed the gravitational influence came from a stellar mass black hole.

This is the first time astronomers have used this detection method to reveal the presence of a black hole outside of our galaxy. Astronomers say this method could be helpful in locating other hidden black holes in the Milky Way and nearby galaxies, and it can also help shed light on how these mysterious objects form and evolve.

Usually, black holes can be detected by the X-ray glow they emitted as they swallow matter, or from the gravitational waves generated as black holes collide with one another or with neutron stars.

But most smaller stellar-mass black holes don’t give away their presence through X-rays or gravitational waves.

“The vast majority can only be unveiled dynamically,” says Stefan Dreizler, one of the team members who contributed to the new paper, published in the Monthly Noticed of the Royal Astronomical Society. “When they form a system with a star, they will affect its motion in a subtle but detectable way, so we can find them with sophisticated instruments.”

The team said they have been acting as detectives, trying to track down a criminal gang from their missteps.

“We are looking at every single star in this cluster with a magnifying glass in one hand trying to find some evidence for the presence of black holes but without seeing them directly,” said Sara Saracino from the Astrophysics Research Institute of Liverpool John Moores University in the UK, who led the research. “The result shown here represents just one of the wanted criminals, but when you have found one, you are well on your way to discovering many others, in different clusters.”


NGC1850 as seen with the Very Large Telescope and the Hubble Space Telescope. Credit: ESO, NASA/ESA/R. Gilmozzi/S. Casertano, J. Schmidt.

NGC 1850 is an unusual double star cluster that lies in the bar of the Large Magellanic Cloud, a neighbor galaxy of our own Milky Way.  NGC 1850 is the second brightest star cluster in the LMC, and contains thousands of stars.

A specialized instrument on the VLT, the Multi Unit Spectroscopic Explorer (MUSE) allowed the researchers to observe the very crowded star cluster and analyze the light of every single star in the vicinity.

“The net result is information about thousands of stars in one shot, at least 10 times more than with any other instrument,” said co-author Sebastian Kamann, also from Liverpool’s Astrophysics Research Institute.

ESO’s Extremely Large Telescope in Chile, set to start operating later this decade, will allow astronomers to find even more hidden black holes.

“The ELT will definitely revolutionise this field,” says Saracino. “It will allow us to observe stars considerably fainter in the same field of view, as well as to look for black holes in globular clusters located at much greater distances.”

Read the team’s paper
Press release from ESO

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