The sudden awakening of the previously quiet galaxy SDSS1335+0728 is the key to understanding the growth and development of supermassive black holes in the centers of galaxies
In late 2019, the previously unremarkable galaxy SDSS1335+0728 suddenly began emitting much more light than ever before. To understand the cause of these changes, astronomers used data from several space-based and ground-based observatories, including the European Southern Observatory's Very Large Telescope (ESO's VLT). In a published study accepted for publication in the journal Astronomy & Astrophysicists, they concluded that they had witnessed changes not previously observed in this galaxy, likely as a result of the sudden awakening of a massive black hole at its core.
«Imagine that you have been observing a distant galaxy for many years, and it has always seemed calm and inactive. Suddenly, its core begins to show dramatic changes in brightness, unlike any typical events we have seen before, — says Paula Sanchez Saez, astronomer at ESO in Germany and lead author of the study.
Some phenomena, such as supernovae or tidal disruptions, can cause sudden changes in the brightness of galaxies, but such changes usually last only a few tens or hundreds of days. However, SDSS1335+0728 continues to brighten today, more than four years after it was first seen «switched on». What's more, the changes detected in this galaxy, located 300 million light-years away in the constellation Virgo, are unlike anything seen before, indicating to astronomers that there must be another explanation.
The team analyzed archival data and new observations, including those obtained with the X-shooter instrument on ESO’s VLT in the Chilean Atacama Desert. By comparing data taken before and after December 2019, they found that SDSS1335+0728 began to emit much more light in the ultraviolet, optical, and infrared. The galaxy also began emitting X-rays in February 2024.
«This behavior is unprecedented,«, says Sánchez Sáez, who is also at the Millennium Institute for Astrophysics (MAS) in Chile. «The most plausible explanation for this phenomenon is that we are seeing the galactic core becoming active. If so, it would be the first time we have seen a massive black hole activate in real time,” adds co-author Lorena Hernandez Garcia of MAS and the University of Valparaiso in Chile.
Massive black holes, with masses more than one hundred thousand times the mass of our Sun, exist at the center of most galaxies, including the Milky Way. «They usually sleep and are not directly visible. In the case of SDSS1335+0728, we were able to observe the awakening of a massive black hole, which suddenly began to feed on the gas available in its vicinity, becoming very bright, — explains co-author Claudio Ricci of the University of Diego Portales, also in Chile.
«This process has never been observed before», — says Hernandez Garcia. Previous studies have shown that inactive galaxies become active after a few years, but this is the first time that the process of awakening a black hole has been observed in real time. Ricci, who is also a fellow at the Kavli Institute of Astronomy and Astrophysics at Peking University, adds: “This is what could happen to the black hole Sgr A*, located at the center of our galaxy. But it is unknown how likely it is that this will happen».
Follow-up observations are still needed to rule out alternative explanations. Another possibility is that scientists are observing an unusually slow tidal disruption or even a new phenomenon. If this is truly a tidal disruption, it will be the longest and weakest such event ever observed.
«Regardless of the nature of the changes, this galaxy provides valuable information about how black holes grow and evolve. We expect that instruments like MUSE on the VLT or the upcoming Extremely Large Telescope (ELT) will be key to understanding why the galaxy is getting brighter, — says Sanchez Saez.