Astronomers are witnessing an unprecedented spectacle in the cosmos: the awakening of a supermassive black hole at the center of a distant galaxy.
In late 2019, a team of astronomers noticed an otherwise unremarkable galaxy called SDSS1335+0728, 300 million light-years away in the constellation Virgo. A sudden increase in the galaxy’s brightness was automatically detected by the Zwicky Transient Facility telescope at the Palomar Observatory in California.
With its extremely wide-angle lens, the camera scans the entire northern sky every two days, collecting data from celestial objects such as near-Earth asteroids and distant, bright supernovas.
An interdisciplinary team of astronomers and engineers followed up on Zwicky’s observation, using information from space and ground-based telescopes to see how the galaxy’s luminosity changed over time.
To their surprise, the researchers found that they witnessed a unique moment when a cosmic monster awoke. The results of their study have been accepted for publication in the journal Astronomy. & Astrophysics.
“Imagine you have been observing a distant galaxy for years and it always seems quiet and inactive,” said lead study author Paula Sánchez Sáez, an astronomer at the European Southern Observatory in Germany, in a statement. “Suddenly, its (core) shows dramatic changes in brightness, unlike any typical event we have seen before.”
The team classified the galaxy as one with an active galactic nucleus, that is, a bright, compact region powered by a supermassive black hole.
A number of celestial scenarios can cause a galaxy to suddenly brighten, such as supernova explosions or when stars get too close to a black hole and are torn apart in a so-called tidal flow event.
But such events last only dozens or hundreds of days—and SDSS1335+0728’s brightness is still increasing more than four years after researchers first observed a brightness increase that culminated like the flip of a cosmic light switch.
And the brightness variations in the galaxy are unlike anything astronomers have seen before, which confused them even more.
An unprecedented cosmic event
To find answers, the team consulted archival data from NASA’s Wide-field Infrared Survey Explorer and Galaxy Evolution Explorer, the Two Micron All Sky Survey, the Sloan Digital Sky Survey, and other observatories.
The researchers compared the data with follow-up observations made by the European Southern Observatory’s Very Large Telescope (VLT) in Chile, the Southern Astrophysical Research Telescope in Chile, the WM Keck Observatory in Hawaii, and NASA’s Neil Gehrels Swift and Chandra space-based X-ray observatories.
Together, the data sets provided a comprehensive picture of the galaxy before and after the December 2019 observation, showing that the galaxy has been emitting significantly more ultraviolet, visible and infrared light in recent years, and X-rays since February – unprecedented behavior, Sánchez Sáez said.
Because the galaxy is 300 million light-years away, the events astronomers are observing happened in the past – but the light from those events is only now reaching Earth after traveling through space for millions of years. A light-year is the distance light travels in one year, or 9.46 trillion kilometers.
“The most tangible way to explain this phenomenon is that we are seeing the (core) of the galaxy beginning to show (…) activity,” said study co-author Lorena Hernández García, an astronomer at the Millennium Institute of Astrophysics and the University of Valparaíso, both in Chile, in a statement. “If so, this would be the first time we have seen the activation of a massive black hole in real time.”
Sleeping sky giants
Supermassive black holes are those with a mass more than 100,000 times that of our sun. They are found in the center of most galaxies, including the Milky Way.
“These giant monsters are usually asleep and not directly visible,” study co-author Claudio Ricci, an associate professor at Diego Portales University in Chile, said in a statement. “In the case of SDSS1335+0728, we were able to observe the awakening of the massive black hole, which suddenly began to eat the gas present in its surroundings and became very bright.”
Previous research has pointed to dormant galaxies that appear to become active after a few years, which is usually triggered by the activity of black holes. But until now, the process of a black hole awakening has never been directly observed, Hernández García said.
The same scenario could play out with Sagittarius A*, the supermassive black hole at the center of the Milky Way, but astronomers are not sure how likely this is, Ricci said.
Astronomers cannot rule out that their observation could be an unusually slow tidal event or an unknown new celestial phenomenon.
“Regardless of the nature of the variations, (this galaxy) provides valuable information about how black holes grow and evolve,” said Sánchez Sáez. “We expect that instruments like (MUSE on the VLT or those on the upcoming Extremely Large Telescope) will be key to understanding (why the galaxy is getting brighter).”
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