In December 2019, astronomers were stunned when the core of a relatively quiet galaxy 300 million light-years away suddenly lit up like a Christmas tree, shining brighter than we’ve ever seen.
A supermassive black hole that has sprung up and started gobbling up material from the space around it is the best explanation for such a faint galaxy suddenly bursting with light. Other events, such as exploding stars, can cause a distant galaxy to brighten – but these events usually fade over time.
SDSS1335+0728, as the galaxy is known, didn’t do the typical thing. In fact, it got even brighter and still shines today in optical, infrared, and ultraviolet light. As recently as February of this year, it even started emitting X-rays.
“The most tangible way to explain this phenomenon is that we see how [core] the galaxy is starting to show… activity,” says astronomer Lorena Hernández García of the Millennium Institute of Astrophysics (MAS) and the University of Valparaíso in Chile.
“If so, it would be the first time we’ve seen the activation of a massive black hole in real time.”
Black holes don’t sit around in space and sip all the time. They can only access material that is within their gravitational range. If there is nothing nearby, they remain relatively quiet or still. For example, the supermassive black hole at the center of the Milky Way is not classified as active because it feeds only minimally.
An active supermassive black hole, on the other hand, is one that causes the space around it to light up. Of course, the black hole itself does not shine; but if there is enough matter around it for a feeding frenzy, the forces at work, such as friction and gravity, cause the material to heat up to such an extreme that it glows with light as it spirals inexorably towards the black hole.
Galaxies with varying levels of activity in their cores have been observed across vast regions of space-time, informing models that suggest that gobbling up large amounts of material is one of the mechanisms by which supermassive black holes grow to such colossal sizes.
Evidence also suggests that supermassive black holes can change their activity levels. For example, the Milky Way has giant bubbles extending above and below the galactic plane from previous activity in the center. Other galaxies have similar features.
But seeing the transition from dormant to active is very strange indeed.
We’ve seen black holes suddenly blaze with light before; this is usually what is known as a tidal disruption event, in which a passing star or other ball of matter is captured by a lurking black hole, producing a flash of light as it is torn apart and swallowed.
A comprehensive analysis of the change in light produced by SDSS1335+0728 and its 1.5 million solar mass black hole reveals that a tidal disruption event is not the most likely culprit; such events are also usually short-lived.
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“In the case of SDSS1335+0728,” says astronomer Claudio Ricci of the University of Diego Portales in Chile, “we were able to observe the wake of a massive black hole, [which] it suddenly began to feast on the gas available in its vicinity and became very bright.”
Astronomers have recently spotted other examples of what may be a supermassive black hole entering an active phase, but more observations are needed to see if the increase in brightness continues.
This means that SDSS1335+0728 may represent a blueprint for what such a transition looks like. Because we don’t know what triggers a supermassive black hole to turn back on, studying the galaxy could help astronomers develop models.
This in turn could reveal the likelihood of it happening elsewhere – including right here in our own galaxy, the Milky Way.
However, it is possible that the brightening of SDSS1335+0728 could be the result of a very special type of tidal disturbance. Astronomers will continue to watch it closely to confirm what it does and what this behavior might mean for our understanding of the universe.
The research was published in Astronomy and astrophysics.