Could we be next on the menu?
A few years ago, in 2019, astronomers in California intercepted a mysterious signal from another galaxy. The sudden flare of light came from an otherwise unremarkable galaxy in the Virgo constellation, and continued fluctuating for the next few years, including to this day. Now, researchers suspect that they’ve found the cause: a giant black hole waking up, and starting to devour the galaxy around it.
In a recent statement on the discovery, Paula Sánchez Sáez, an astronomer at ESO in Germany, says that the dramatic changes in the brightness of the galaxy’s core are “unlike any typical events we’ve seen before”. Alongside a team of researchers, Sáez embarked on a quest to learn more after these changes were first observed, using archival data and new observations from several research facilities.
As a result, the team have concluded that the dazzling behaviour of the galaxy is, in fact, “unprecedented”. Catchily named SDSS1335+0728, the galaxy itself is now classified as having an “active galactic nucleus”, meaning there’s a bright region at its core that’s being powered by a massive black hole. (We’re talking 1 million times the mass of our sun.) What’s really interesting, though, is how it started shining from out of nowhere.
It isn’t unheard of for galaxies to suddenly light up. Supernovae (exploding stars) or tidal disruption events (when a star passes too close to a black hole, and gets torn apart) have been observed before. However, the visible effects usually only last a few hundred days, at most. The galaxy in question is unusual because it’s been flashing us for more than four years, and continues to get even brighter across all wavelengths: ultraviolet, visible, and infrared.
“The most tangible option to explain this phenomenon is that we are seeing how the [core] of the galaxy is beginning to show activity,” says Lorena Hernández García, co-author of the new study. “If so, this would be the first time that we see the activation of a massive black hole in real time.”
What does this mean exactly? Are we about to get spaghettified from 300 million light-years away? Thankfully, no. As far as we know, most galaxies have massive black holes at their centre, and many – like Sagittarius A* at the centre of the Milky Way – are dormant. “These giant monsters usually are sleeping and not directly visible,” explains Claudio Ricci, another of the study’s co-authors. But, in the case of SDSS1335+0728: “We were able to observe the awakening of the massive black hole, [which] suddenly started to feast on gas available in its surroundings, becoming very bright.”
The black hole itself doesn’t actually get bright, of course, since even light can’t escape its gravitational pull. As it ‘wakes up’, however, its strong effects on surrounding material could have released a lot of energy, resulting in the glow we’ve witnessed here on Earth. If so, it’s the first time this has been observed in real-time, marking a significant astronomical milestone.
You might be wondering: could our own black hole wake up one day, and start devouring its surroundings? The answer is: technically, yes, but we’re not sure how likely it is to happen, or how frequently. According to NASA, it might actually have occurred in recent history, with a 2023 study suggesting that Sagittarius A* last woke up from its interstellar slumber as recently as 200 years ago, to chomp on some cosmic debris before nodding off again.