Marshaling a decade’s worth of data from telescopes around the world, scientists have captured new details of a gargantuan black hole feasting on a hapless star, watching as the black hole consumed its prey and burped out a jet of material moving at a significant fraction of the speed of light. The results are published in the June 13 edition of Science, and could help researchers better understand how black holes grow and influence their galactic surroundings.
“Never before have we been able to directly observe the formation and evolution of a jet from one of these events,” says study co-author Miguel Pérez-Torres of the Institute of Astrophysics of Andalusia in Spain.
The discovery’s first inklings emerged in January 2005, when a team led by astronomer Seppo Mattila of the University of Turku in Finland detected a brilliant pointlike source of infrared light from within Arp 299, a pair of merging galaxies some 150 million light-years from Earth. That July another team led by Pérez-Torres detected a bright source of radio waves from the same location.
Both had been hunting for supernovae in the vicinity of Arp 299’s colliding galactic cores, a dust-shrouded region filled with clouds of gas and newborn massive stars generated by the ongoing merger. And a supernova was, at first, exactly what they thought they found. These cataclysmic stellar explosions are particularly bright in visible light and in x-rays—except in Arp 299’s murky center most of that light would be absorbed by dust and reradiated in the infrared; the remnant would then leak out as radio waves. But follow-up infrared observations with NASA’s Spitzer Space Telescope showed the source was far too bright to be a supernova, blazing with light that would outshine a typical small galaxy by several 100-fold. That suggested the source was not a supernova at all, but rather a tidal disruption event (TDE), a star being torn apart by a supermassive black hole.
In a TDE roughly half of the ripped-up star is flung away from the black hole whereas the other half plunges to its doom, piling up around the hole’s maw in a whirling disk of glowing debris that can be mistaken for a supernova. There is no mistaking, however, the other signature of a TDE: twin jets of star stuff ejected from near the black hole’s poles at nearly light-speed by intense magnetic fields twisting and breaking like rubber bands. Ramming into the diffuse gas of the interstellar medium, the jets would produce copious radio waves potentially visible from Earth. So Mattila, Pérez-Torres and 34 additional collaborators organized an international campaign using a global network of radio telescopes to obtain high-resolution radio images of the source, patiently, periodically monitoring its size and shape in search of a jet.
In 2011 that patience began to pay off as the point source became lopsided at radio wavelengths, perhaps due to the emission of a jet. By the end of 2015, the point had expanded into a streak; the team clocked its growth at roughly 50,000 miles per second—a quarter of the speed of light. After years of scrutinizing their data and carefully modeling how light and jets from a TDE would propagate through Arp 299’s dusty core, the team was left “with one plausible explanation,” Mattila says. “The infrared and radio emissions came from the disruption of a hapless star being devoured by the supermassive black hole when it passed too close to this cosmic monster.” The TDE, in turn, pinpoints the 20-million-solar-mass supermassive black hole’s exact position within Arp 299’s core, and also reveals the size of the devoured star, which was between two and six times heavier than our sun.
The most surprising thing about their newfound TDE, Mattila and Pérez-Torres say, is it was discovered at all. Astronomers have detected and studied a handful of other TDEs in recent years, but most of these were found due to their brightness in visible light rather than via infrared and radio emissions. “Tidal disruptions are very rare events, and usually one needs to monitor an extremely large number of galaxies in order to detect them,” Pérez-Torres says. Finding one shrouded by dust in the merging galaxies of Arp 299, he adds, hints TDEs may be far more frequent in such places. In colliding galaxies, stars formed from gas funneled toward a central supermassive black hole might spark TDEs with relative regularity. The energy pouring out from those TDEs, in turn, could have profound effects, acting to stimulate or suppress star formation in the merging pair.
Additionally, Mattila notes, systems like Arp 299 were much more common in the distant universe, when the formation and evolution of galaxies were in earlier stages. “The event we have discovered could thus be just the tip of the iceberg of a hidden population of TDEs that were more common when the universe was much younger than today,” he says. Future infrared and radio observatories, he adds, could in the next decade allow astronomers to detect many more TDEs that are now “hidden by a curtain of dust,” lifting the veil to allow deeper studies of galactic assembly across cosmic time.