Now you see it, now you don't. A distant, energetic galaxy that mysteriously went dark was probably obscured by wind spurting from its black hole. The insight could help us better understand the mysterious relationship between galaxies and the black holes at their cores.
The galaxy in question is NGC 5548. It is so distant that its stars aren't detectable. We know it's there because its black hole emits non-visible light of various wavelengths when surrounding gas falls in. Recently, the light dimmed unexpectedly.
Now Gerard Kriss at the Space Telescope Science Institute in Baltimore, Maryland, and his colleagues say the culprits are the photons that normally light up the galaxy. Their momentum could push on the surrounding gas, creating a stream of material that obscures NGC 5548's light. But this could only happen if the surrounding gas blocks X-rays, which are also emitted by the black hole and would otherwise stop the gas being pushed around.
Kriss's team observed the galaxy between 2013 and 2014 and saw a dip in X-rays compared with 2002, meaning the gas is blocking X-rays (Science, doi.org/tbs). They conclude that a photon-powered wind is spurting from NGC 5548 – and caused the recent blackout.
"It comes squirting out like a lawn sprinkler," Kriss says.
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