Astronomers have witnessed never-before-seen changes in a galaxy, probably as a result of the sudden awakening of the enormous black hole at its core.
In late 2019, the previously unremarkable galaxy SDSS1335+0728 suddenly began to shine brighter than ever. To understand why, astronomers have used data from several space and ground-based observatories, including the European Southern Observatory’s Very Large Telescope (ESO’s VLT), to track how the galaxy’s brightness has varied.
“Imagine that you have been observing a distant galaxy for years and it always seemed calm and inactive,” says Paula Sánchez Sáez, an astronomer at ESO in Germany and lead author of the study accepted for publication in ‘Astronomy & Astrophysics’, in a statement. “Suddenly, his [núcleo] begins to show dramatic changes in brightness, unlike any typical event we have seen before.” This is what happened to SDSS1335+0728, which is now classified as an “active galactic nucleus” (AGN), a compact and bright light powered by a massive black hole, after it brightened dramatically in December 2019.
Some phenomena, such as supernova explosions or tidal disruption events (when a star gets too close to a black hole and breaks apart), can cause galaxies to suddenly light up. But these brightness variations typically last only a few dozen or, at most, a few hundred days. SDSS1335+0728 continues to shine brighter today, more than four years after it was first seen “lighting up.” Furthermore, the variations detected in the galaxy, located 300 million light years away in the constellation Virgo, are unlike anything seen before, pointing astronomers toward a different explanation.
The team attempted to understand these brightness variations using a combination of archival data and new observations from several facilities, including the X-shooter instrument on ESO’s VLT in Chile’s Atacama Desert. By comparing data taken before and after December 2019, they found that SDSS1335+0728 now radiates much more light in ultraviolet, optical, and infrared wavelengths. The galaxy also began emitting X-rays in February 2024. “This behavior is unprecedented,” highlights Sánchez Sáez, who is also affiliated with the Millennium Institute of Astrophysics (MAS) in Chile.
“The most tangible option to explain this phenomenon is that we are seeing how the [núcleo] of the galaxy begins to show (…) activity,” points out co-author Lorena Hernández García, from MAS and the University of Valparaíso in Chile. “If so, this would be the first time we will see the activation of a massive black hole in real time.
At the center of most galaxies, including the Milky Way, there are massive black holes, with masses more than a hundred thousand times that of our Sun. “These giant monsters are usually asleep and are not directly visible,” the co-author also explains. Claudio Ricci, from the Diego Portales University, also in Chile. “In the case of SDSS1335+0728, we were able to observe the awakening of the enormous black hole, [que] It suddenly began to feed on the gas available in its environment, becoming very bright. “
“[Este] process (…) has never been observed before,” states Hernández García. Previous studies reported that inactive galaxies become active after several years, but this is the first time that the process itself (the awakening of the black hole) observed in real time. Ricci, who is also affiliated with the Kavli Institute for Astronomy and Astrophysics at Peking University, China, adds: “This is something that could also happen to our own Sgr A star, the enormous black hole (. ..) located in the center of our galaxy”, but it is not clear how likely this is to happen.
Follow-up observations are still needed to rule out alternative explanations. Another possibility is that we are seeing an unusually slow tidal disturbance event, or even a new phenomenon. If this is indeed a tidal disturbance event, this would be the longest and weakest such event ever observed. “Regardless of the nature of the variations, [esta galaxia] provides valuable information about how black holes grow and evolve,” says Sánchez Sáez. “We hope that instruments like [MUSE en el VLT o los del próximo Telescopio Extremadamente Grande (ELT)] are key to understanding why the galaxy is lighting up.
