In late 2019, the galaxy SDSS1335+0728, which had previously gone unnoticed, suddenly became significantly brighter. This unexpected phenomenon prompted astronomers to investigate the cause, utilizing data from various observatories, including the European Southern Observatory's Very Large Telescope (ESO's VLT). The study aimed to track the changes in the galaxy's brightness and understand the underlying reasons. The researchers concluded that they were witnessing a unique event: the sudden activation of a massive black hole at the galaxy's core. This marked an unprecedented observation in the field of astronomy.
Paula Sánchez Sáez, an astronomer at ESO in Germany and the lead author of the study, explained that the core of SDSS1335+0728 exhibited dramatic changes in brightness, unlike any typical events observed before. This significant transformation led to the classification of the galaxy as having an 'active galactic nucleus' (AGN). An AGN is a bright, compact region at a galaxy's center, powered by a massive black hole. The shift occurred in December 2019, when the galaxy's core began to shine more brightly, capturing the attention of astronomers worldwide.
Common astronomical phenomena, such as supernova explosions or tidal disruption events—where a star is torn apart by a black hole—can cause galaxies to light up suddenly. However, these brightness variations are usually short-lived, lasting only a few days to a few months at most. In stark contrast, SDSS1335+0728 has continued to grow brighter for over four years since it was first observed to 'switch on.' Additionally, the brightness variations detected in this galaxy, located 300 million light-years away in the constellation Virgo, are unlike any seen before. This suggested to astronomers that a different explanation was needed.
To understand these unusual brightness variations, the research team used a combination of archival data and new observations from several facilities. These included the X-shooter instrument on ESO's VLT in Chile’s Atacama Desert. By comparing data taken before and after December 2019, the team discovered that SDSS1335+0728 was emitting significantly more light at ultraviolet, optical, and infrared wavelengths. Remarkably, the galaxy also began emitting X-rays in February 2024. Sánchez Sáez noted that this behavior was unprecedented, adding a new dimension to our understanding of galactic activity.
The most plausible explanation for this phenomenon is that the core of SDSS1335+0728 is becoming active due to its massive black hole starting to consume the gas available in its surroundings, leading to increased brightness. This awakening of a massive black hole has never been observed in real time before. Lorena Hernández García, a co-author of the study, emphasized that if this explanation is correct, it would be the first time that the activation of a massive black hole has been witnessed as it happens. Claudio Ricci, another co-author, highlighted that massive black holes, with masses exceeding one hundred thousand times that of our Sun, typically lie dormant at the centers of galaxies, including the Milky Way. In the case of SDSS1335+0728, the black hole suddenly started to feast on nearby gas, becoming extremely bright.
While this process has never been observed before, it opens up the possibility that a similar event could occur with the massive black hole at the center of our own galaxy, Sgr A*. However, the likelihood of such an occurrence remains uncertain. Follow-up observations are essential to rule out alternative explanations. One possibility is that the observed brightness changes are due to an unusually slow tidal disruption event or even a new, yet-to-be-discovered phenomenon. If it is a tidal disruption event, it would be the longest and faintest ever observed. Regardless of the exact nature of these variations, SDSS1335+0728 provides valuable insights into how black holes grow and evolve. Sánchez Sáez noted that future instruments, such as those on the upcoming Extremely Large Telescope (ELT), will be crucial in understanding why this galaxy is brightening.
The unusual brightness variations of SDSS1335+0728 were initially detected by the Zwicky Transient Facility (ZTF) telescope in the United States. Subsequently, Chile's Automatic Learning for the Rapid Classification of Events (ALeRCE) system classified it as an AGN. The research team utilized archival data from several sources, including NASA's Wide-field Infrared Survey Explorer (WISE) and Galaxy Evolution Explorer (GALEX), the Two Micron All Sky Survey (2MASS), the Sloan Digital Sky Survey (SDSS), and the eROSITA instrument on the Spektr-RG observatory. Follow-up observations were conducted with ESO's VLT, the Southern Astrophysical Research Telescope (SOAR), the Keck Observatory, and NASA's Swift and Chandra observatories.
This groundbreaking research, detailed in the paper "SDSS1335+0728: The awakening of a ~ 106M black hole," was published in Astronomy & Astrophysics. The study involved a collaborative effort by researchers from various institutions, including ESO, the Millennium Institute of Astrophysics, the University of Valparaíso, the Diego Portales University, and many others. Their combined efforts have provided unprecedented insights into the dynamic processes occurring in the universe, particularly in the realm of black hole activity and galaxy evolution.