Almost every galaxy, including our own, appears to have a black hole at its core. Most of the time these 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. For the first time, the new study shows convincing evidence of the onset, the “switching on,” of this active phase, in a black hole at the center of galaxy NGC 660, 42 million light-years away in the constellation Pisces.
NGC 660 is a stunning example of a polar ring system galaxy. Like most galaxies, it has a thick disk of stars and gas. But NGC 660 also hosts a larger and much less dense ring of stars and smaller star-forming clouds in orbit over its poles. This additional ring is thought to be the remnants of a past encounter with another galaxy that disrupted both systems.
In 2012, astronomers carrying out a survey with the Arecibo radio telescope in Puerto Rico noticed that NGC 660 had suddenly become hundreds of times brighter over just a few months. Normal galaxies do not change their brightness quickly as they are large systems made of many relatively small individual components in the form of stars, gas, and dust. Though all of these may change dramatically, the average brightness of a galaxy tends to be stable.
The earlier results mean that one particular object within the galaxy has undergone a significant change and become much brighter, and the likely culprit is either an exploding star or the central supermassive black hole. But the Arecibo observations could only hint at what was happening.
Over the last three years, a team of scientists led by Argo has been trawling through archived results from ground- and space-based telescopes. They then used three radio observatories: the United Kingdom’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.
Individual radio dishes have poor resolution, so astronomers link widely spaced telescopes together to simulate a much larger instrument — a technique known as interferometry. With the various networks of telescopes working in this way, the team could then look at NGC 660 in detail.
The new images show features about a light-year across, about a quarter of the distance between the Sun and Alpha Centauri, the next nearest star. They reveal a new bright radio source in the center of NGC 660, right where we expect to find the central supermassive black hole.
Inactive black holes do not emit large amounts of radiation, so we can only detect them by their gravitational effect on the orbits of stars around them. But the black hole in NGC 660 is now obvious, and it is many hundreds of times brighter than anything seen in the center of NGC 660 in the archive of radio images before 2010.
“Many examples of galaxies with active black holes are already known, often with massive jets stretching millions of light-years into intergalactic space,” said Argo. “But NGC 660 is special — for the first time we can see this activity starting up.”
The Westerbork observations let the team use this new strong radio source to probe the normally opaque clouds of hydrogen gas within the galaxy, shining a “torch” through the clouds to see what they are made of. 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.
Material — 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.
Material in the jet is fast, traveling at about 10 percent of the speed of light. “Nothing on Earth has anything like the energy of a jet leaving a giant black hole,” said Argo. “Because it moves so quickly, we should be able to watch the material traveling out over the next few years and measure its speed and energy. The big question is whether it has enough energy to overcome gravity and punch its way out of the galaxy or if it will fizzle out before getting that far.”
Studying the jet will give astronomers a clue about the initial eruption of the jet and how much material fell onto the black hole to cause the outburst in the first place.