First-ever "rogue" black hole discovered zipping through the galaxy
Astronomers believe they’ve detected the first “rogue” black hole, roaming the galaxy alone. The object made itself known when it passed in front of a background star, bending the light with its extreme gravity.
There’s a serious undercount between how many black holes we’ve found out in the cosmos, compared to how many are expected to exist. But that’s not too surprising really – these are the darkest objects possible, set against the inky void of space. Many black holes make their presence known through other means, such as hot bright disks of material orbiting them, flashes of light as they gobble up stars, or gravitational waves as they collide. But it’s long been predicted that many more are lurking out there far from other objects, essentially invisible.
There is however one way that these isolated black holes could reveal themselves – gravitational lensing. As they move in front of a background light source, the intense gravity warps that light ever so slightly, brightening and shifting it. For decades astronomers have been keeping watch for gravitational lensing events that can’t be explained by visible objects like stars and galaxies, which would suggest an invisible black hole is responsible. And now, this smoking gun may have finally been detected.
In 2011, two separate projects – Microlensing Observations in Astrophysics (MOA) in New Zealand and the Optical Gravitational Lensing Experiment (OGLE) in Chile – spotted a star that seemed to brighten. Over the next six years, researchers on the new study periodically checked in on the star using Hubble, to see how its light changed. Not only did it continue to change brightness, but its apparent position in the sky seemed to shift slightly, indicating a gravitational microlensing event.
The next step was to check whether the lensing object was indeed a black hole, or just a boring old star. Careful analysis showed that the object was giving off no light of its own, and the team calculated its mass to be about seven times that of the Sun. That’s a stock standard mass for a black hole to have, but is far too heavy for a neutron star or white dwarf. All together, the data points to a black hole as the culprit.
The researchers were also able to determine that the black hole is a little over 5,000 light-years from Earth, and is traveling at around 45 km (28 miles) per second. This suggests that it was probably “kicked” by the supernova that birthed it, sending it on its lonely journey through the galaxy.
The astronomers on the study say this is the first unambiguous detection of an isolated black hole. However, the paper has yet to be peer reviewed.
The research appears on the online preprint server arXiv.