Awakening a supermassive black hole

For about five years, the galaxy SDSS1335+0728, located 300 million light-years away, has been increasing in brightness, across almost the entire electromagnetic spectrum: it first began to shine in the ultraviolet range, then the visible, the infrared, and then, since February 2024, SDSS1335+0728 has also begun to emit X-rays. Paola Sánchez-Saez, an astrophysicist at the European Southern Observatory in Germany, and her colleagues sought to understand this explosion of galaxy brightness.

Most large galaxies have at least one black hole at their center, with a mass ranging from several million to a few billion times that of the Sun. Most of these black holes are quiet, showing no particular activity, and are therefore invisible. But if a supermassive black hole captures and coalesces the surrounding matter and gas under its gravity, an accretion disk forms around it. As the matter and gas fall toward the black hole, the resulting friction releases enough heat to ionize the matter. Under the influence of the spin of the supermassive black hole, some of this ionized matter is ejected in the form of jets, the radiation of which can be observed. This makes the center of the galaxy visible, and this is what we call an “active galactic nucleus.”

These active galactic nuclei are particularly abundant in the early Universe, because there would have been more gas in the cores of these young galaxies than those closer to us, but that doesn't rule out the possibility that some supermassive black holes were activated by a late encounter with a massive gas cloud. Could this be the case for SDSS1335+0728?

The first observations in the early 2000s of this galaxy, located in the constellation Virgo, did not show an active nucleus. No specific emission was received from the center of SDSS1335+0728. However, within a few years, the situation has changed. Since 2021, the ultraviolet emission has quadrupled compared to twenty years ago; since June 2022, the observed flux in the infrared has doubled, and since February 2024, the first X-ray emission has been recorded.

Such variations in light intensity have already been observed around other black holes, in the case of tidal rupture events. When a star gets too close to a black hole, it is torn apart by its gravitational force. The excess gas and matter that this torn apart star brings increases the brightness emitted from the black hole’s surroundings. “But these events are fleeting and last for a hundred days at most,” explains Paola Sánchez-Saez. “For it to last for several years as it did here, it would have to be a tidal rupture event of a hitherto unknown type.”

While this strange event cannot yet be ruled out, researchers consider it very likely that the supermassive black hole is located about 10⁶ The million-solar-mass galaxy SDSS1335+0728 has just been activated. “If this is indeed the case, this would be the first time we have observed such an event happening in real time,” the researcher concludes.