An image showing M15 and the location of the mysterious X-ray source, now confirmed to be a black hole.X-ray: NASA/CXC/Univ. of Alberta/B.Tetarenko et al; Optical: NASA/STScI; Radio: NSF/AUI/NRAO/Curtin Univ./J. Miller-Jones
Astronomers have discovered that a distant source of radio waves once thought to be a galaxy could in fact be a binary star system containing a low-mass star and a black hole.
The discovery has opened up the possibility that there could be thousands more black holes in the Milky Way that have gone unnoticed.
An object named VLA J213002.08+120904 had been known about for about two decades, appearing close to globular cluster M15 as seen from Earth, but astronomers had predicted the object was probably a distant galaxy.
However, using data from the Chandra X-ray Observatory, the Hubble Space Telescope and the Karl G. Jansky Very Large Array, it is now been revealed that this object is in fact just 7,200 lightyears away, placing it well within our own Galaxy and five times closer than M15.
VLA J2130+12 is in fact a black hole a few times the mass of our Sun that is feeding off material from a companion star.
However, it is doing so incredibly slowly and has not been giving off the usual telltale signs that point to a binary system.
"Usually, we find black holes when they are pulling in lots of material.
Before falling into the black hole this material gets very hot and emits brightly in X-rays," says study lead Bailey Tetarenko of the University of Alberta, Canada.
"This one is so quiet that it's practically a stealth black hole."
Because the study that uncovered the black hole only scanned a relatively small patch of the sky, astronomers believe there could be many more unnoticed, quiet black holes in the Milky Way.
In fact, there could be tens of thousands or even millions of these objects in the Galaxy; potentially thousands of times as many as thought.
"Some of these undiscovered black holes could be closer to the Earth than we previously thought," says co-author Robin Arnason from Western University, Canada.
"However there's no need to worry as even these black holes would still be many light years away from Earth."