Radio astronomers are watching a previously dormant black hole wake up in a dramatic display as material falls onto it for the first time in perhaps millions of years.
Almost every galaxy, including our own, appears to have a black hole at its core. Most of the time they are quiet, with just their invisible gravitational pull shaping their surroundings. But in about 10 percent of galaxies the central black hole is much more active, swallowing material and spitting out giant jets.
The new study shows, for the first time, convincing evidence of the onset, the “switching on” of this active phase, in a black hole at the center of the galaxy NGC 660—42 million light years away in the constellation of Pisces.
Wake Up Call
In 2012, astronomers noticed that NGC 660 had suddenly become hundreds of times brighter over just a few months. Because they are very large systems made of many (relatively) small individual components in the form of stars, gas and dust, normal galaxies don’t change their brightness very quickly.
Over the last three years, a team of scientists led by Megan Argo of the Jodrell Bank Centre for Astrophysics has been trawling through archived results from ground- and space-based telescopes. They then used data from three radio observatories: the UK’s e-MERLIN telescope operated from Jodrell Bank, the Westerbork array in the Netherlands, and the European VLBI network (EVN), which also includes telescopes in Russia, China, and South Africa combined to simulate a much larger instrument—a technique known as interferometry.
Sam Connolly, a postgraduate student at University of Southampton used X-ray astronomy to check the brightness of the source before and after the radio brightening, which helped rule out potential reasons for the brightening and to come to the conclusion that it is most likely to be a newly-awoken supermassive black hole.
“As supermassive black holes are so huge, they evolve very slowly, remaining dormant for thousands of years at a time, so to catch one waking up is really incredible.”
Gas, Dust, And Stars
The new images reveal a new, very bright radio source in the very center of NGC 660, right where the researchers expect to find the central supermassive black hole.
Inactive black holes don’t emit large amounts of radiation, so they can only be detected by their gravitational effect on the orbits of stars around them. However, the black hole in NGC 660 is now very obvious, and is many hundreds of times brighter than anything seen in the center of NGC 660 in the archive of radio images before 2010.
The parallel results from e-MERLIN show that the object is slowly fading, and is similar to other galaxies with more mature systems, and the highest resolution images from the EVN show evidence of a high-speed jet of material leaving the vicinity of the black hole.
Gas, dust and stars near a black hole can sit in stable orbits around the central massive object for a long time, but eventually it loses energy, spirals in, and falls onto the black hole. At the same time, some material is ejected and this seems to have created the outburst and jet now seen in NGC 660.
Scientists say that by studying the jet, they will get new clues about the initial eruption of the jet, and how much material fell onto the black hole to cause the outburst in the first place.
Researchers will present their findings at the National Astronomy Meeting in Llandudno, Wales.