Artist’s animation of the awakening black hole at the center of SDSS1335+0728 in real time – a first for astronomers.
In December 2019, astronomers were surprised to observe a long-quiet galaxy 300 million light-years away that suddenly burst into life, emitting ultraviolet, optical and infrared light into space. In February of this year, the galaxy began to emit X-ray light; it happens more active. Astronomers believe that it is most likely an active galactic nucleus (AGN) that receives energy from supermassive black holes at the center of the galaxy and/or from the rotation of the black hole. That’s the conclusion of a new paper accepted for publication in the journal Astronomy and Astrophysics, although the authors acknowledge the possibility that some kind of rare tidal disturbance (TDE) could also be involved.
After decades of silence, the brightening of SDSS1335_0728 in the constellation Virgo was first captured by the Zwicky Transient Facility telescope. Its supermassive black hole is estimated to be about 1 million solar masses. To better understand what might be going on, the authors combed through archival data and combined it with data from new observations from a variety of instruments, including the X-shooter, part of the Very Large Telescope (VLT) in Chile’s Atacama Desert.
There are many reasons why a normally quiescent galaxy might suddenly brighten, including supernovae or TDEs, in which some of the original mass of the torn apart star is ejected violently outwards. This, in turn, can create an accretion disk around the black hole that emits strong X-rays and visible light. However, these events don’t last nearly five years – usually no more than a few hundred days.
So the authors concluded that the galaxy has woken up and now has an AGN. First discovered by Carl Seyfert in 1943, the glow is the result of cold dust and gas surrounding a black hole that can form orbiting accretion disks. Gravitational forces compress matter in the disk and heat it to millions of degrees Kelvin, creating radiation across the electromagnetic spectrum.
Alternatively, the activity may be due to a particularly long and faint TDE – the longest and faintest yet detected, if so. Or it could be a completely new phenomenon. So SDSS1335+0728 is a galaxy to watch. Astronomers are already preparing for follow-up observations with, among others, the Multi Unit Spectroscopic Explorer (MUSE) and the Extremely Large Telescope VLT, and perhaps even the Vera Rubin Observatory, due to launch next summer. Its Large Synoptic Survey Telescope (LSST) will be able to continuously image the entire southern sky and potentially capture even more galaxy awakenings.
“Regardless of the nature of the variations, [this galaxy] it provides valuable information about how black holes grow and evolve,” said co-author Paula Sánchez Sáez, an astronomer at the European Southern Observatory in Germany. “We expect that tools like [these] will be the key to understanding [why the galaxy is brightening].”
There is also a supermassive black hole at the center of our Milky Way Galaxy (Sgr A*), but there is not yet enough material accreting for astronomers to pick up any emitted radiation, even in the infrared. So its galactic core is considered inactive. It may have been active in the past, and it is possible that it will reawaken in a few million (or even billion) years, when the Milky Way merges with the Andromeda galaxy and their respective supermassive black holes merge. Only time will tell.
Astronomy and Astrophysics, 2024. DOI: 10.1051/0004-6361/202347957 (About DOI).
Image listing from ESO/M. Kornmesser