News about black holes is usually accompanied by some fun description of them eating stuff. Stars, planets, even asteroids are on the galactic menu. But in the case of the supermassive black hole at center of NGC 253 (the Sculptor galaxy), the opposite is true. It’s not doing much at all. In fact, it appears to have taken leave from its supermassive duties of reigning gravitational terror over the matter inside its galactic core.
“Our results imply that the black hole went dormant in the past 10 years,” said Bret Lehmer of the Johns Hopkins University, Baltimore, and NASA’s Goddard Space Flight Center, Greenbelt, Md. “Periodic observations with both Chandra and NuSTAR should tell us unambiguously if the black hole wakes up again. If this happens in the next few years, we hope to be watching.” Lehmer is lead author of the new study published in the Astrophysical Journal.
Although the black hole is taking a nap, that doesn’t mean the galaxy isn’t picking up the slack. NGC 253 is one of the nearest “starburst” galaxies to the Milky Way, some 13 million light-years away, churning out newborn stars at an accelerated rate. It may seem surprising, then, that the black hole, with a mass of 5 million suns, is able to sleep through the star-forming commotion.
“Black holes feed off surrounding accretion disks of material. When they run out of this fuel, they go dormant,” said co-author Ann Hornschemeier of Goddard. “NGC 253 is somewhat unusual because the giant black hole is asleep in the midst of tremendous star-forming activity all around it.”
This apparent contradiction provides an opportunity for astronomers trying to understand the nature of starburst galaxies and the part their central black holes have to play in galactic evolution.
It is thought that the supermassive black holes that live in the hearts of the majority of galaxies grow at the same rate as their host galaxies. However, black holes are also known to extinguish star formation should they start feeding, generating a hellish environment near the galactic hub — intense radiation generated by an active black hole can cause incredible disruption.
In the case of NGC 253, astronomers cannot be sure whether the rate of star formation is increasing or decreasing, but they are keeping a close eye on the black hole.
In 2003, NGC 253′s black hole was an entirely different creature. It was highly active, generating X-ray radiation spotted by NASA’s Chandra X-ray Observatory, indicating it was consuming matter. But in followup studies in 2012 using Chandra and NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR), the black hole had fallen silent, indicating it had stopped accreting material.
During these observations, Chandra and NuSTAR spotted some of the supermassive black hole’s cousins near the core of NGC 253 — but these stellar mass black holes were wide awake, snacking on their host stars, producing X-rays. These “ultraluminous X-ray sources” (ULXs) are randomly distributed and consist of a small black hole (that has formed after the collapse of a massive star) feeding off stellar material.
So, by peering deep into NGC 253 with X-ray observatories, an ecosystem of black hole science has been exposed and scientists hope to periodically check in on the snoozing supermassive monster at the core, hoping to see it start to feed once more.
Image: The Sculptor galaxy is seen in a new light, in this composite image from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Southern Observatory in Chile. Image credit: NASA/JPL-Caltech/JHU