Astronomers have recently observed an extraordinary event: the real-time awakening of a massive black hole in a distant galaxy. This rare phenomenon occurred in galaxy SDSS1335+0728, about 300 million light-years away in the constellation Virgo. Before 2019, this galaxy seemed quite ordinary. However, it suddenly began to glow brightly, catching the attention of scientists around the world.

What Happened?

In December 2019, the core of SDSS1335+0728 started to show dramatic changes in brightness, suggesting that a massive black hole at the galaxy’s center was becoming active. When this occurs in a galaxy, astronomers classify it as having an active galactic nucleus — a bright compact region powered by a massive black hole.

Massive black holes, which can have masses more than 100,000 times that of the Sun, are usually dormant and invisible. But they become extremely bright when they start consuming gas from their surroundings.

“Imagine you’ve been observing a distant galaxy for years, and it always seemed calm and inactive,” said Paula Sánchez Sáez, the lead author of the study accepted for publication in Astronomy & Astrophysics. “Suddenly, its core starts glowing brightly, unlike anything we’ve seen before.”

To understand what was happening, the research team used both old and new observations. They used special instruments, like the X-shooter on the European Southern Observatory’s (ESO) Very Large Telescope (VLT), to study the light from the galaxy in ultraviolet, optical, and infrared wavelengths. They found that SDSS1335+0728 was emitting significantly more light in these wavelengths than before. Additionally, the galaxy began emitting X-rays in February 2024, indicating that the black hole was active.

“These giant monsters usually are sleeping and not directly visible,” said co-author Claudio Ricci, from Diego Portales University. “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.”

Possible Explanations

The team considered other phenomena, such as supernova explosions or tidal disruption events, where a star is torn apart by a black hole. However, these events typically last only a few dozen to a few hundred days. In contrast, SDSS1335+0728 has continued to brighten for more than four years.

“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 co-author Lorena Hernández García, from MAS and the University of Valparaíso. “If so, this would be the first time that we see the activation of a massive black hole in real-time.”

To investigate further, the astronomers used various telescopes and instruments to gather more data. They analyzed old data from surveys like the Wide-field Infrared Survey Explorer, Two Micron All Sky Survey, Sloan Digital Sky Survey, and the Galaxy Evolution Explorer. These datasets provided a comprehensive view of the galaxy’s behavior before it brightened.

From 2021 onwards, new observations were made using the Swift Observatory’s Ultraviolet/Optical Telescope and X-ray Telescope, the Southern Astrophysical Research Telescope, ESO’s VLT, and the Keck Observatory. The team’s spectroscopic analysis revealed significant galaxy light emissions changes. The galaxy showed a much stronger ultraviolet light, typical of an accretion disk around a black hole. The amount of ultraviolet light has increased by four times since 2004. Additionally, there was a notable increase in mid-infrared light, suggesting the formation of a new dusty ring around the black hole.

A crucial piece of evidence for the black hole’s awakening was the detection of the 2024 X-rays. Before this, the galaxy had shown no X-ray activity. These new X-ray emissions were similar to those seen when a black hole starts to become active, indicating the formation of a region of high-energy particles around the black hole.

Although the current data strongly suggests that SDSS1335+0728 is undergoing a transformation due to its black hole waking up, more observations are needed to confirm this. Future studies will monitor the galaxy’s X-ray and ultraviolet emissions to see how it develops.

“Regardless of the nature of the variations, (this galaxy) provides valuable information on how black holes grow and evolve,” Sánchez Sáez says. “We expect that instruments like (the Multi-Unit Spectroscopic Explorer on the VLT or those on the upcoming Extremely Large Telescope) will be key in understanding.”