At the very center of every large galaxy, including our own Milky Way, lies a supermassive black hole. These monsters can be huge, millions or billions of times the Sun’s mass. Some, like ours, aren’t actively feeding on material, so they’re quiescent.

Others, though, have material falling into them at a high rate. This material piles up outside the black hole to form an accretion disk. That stuff gets extremely hot due to friction, and can glow with the power of millions of stars. From Earth we see this a normal-looking galaxy with an extremely luminous core, and call them active galaxies.

Many of these black holes also power twin beams of matter and energy that are emitted over their poles (up and down, away from the accretion disk). The mechanism behind these jets, as they’re called, is still not hugely well understood, but it’s most likely magnetic fields in the disk getting wound up like a vortex at its inner edge, close to the black hole. These fields can store immense amounts of energy, and as they get twisted up they can launch the jets away. The jets can extend for a long way, and emit colossal amounts of energy.

Surrounding all this are clouds of dust — made up of tiny grains of material that are rocky (made up of silicates) and/or sooty (carbonaceous). The dust is known to be warm, but what’s the heating mechanism? Is it energy from the glowing disk, or from the jet? The answer will help astronomers understand what’s going on in this hellish environment better.

So astronomers pointed JWST at the relatively nearby spiral galaxy ESO 428-G14, which is about 75 million light-years from Earth [link to journal paper]. It’s an active galaxy, with a central black hole of roughly 20 million solar masses. It’s also known to have a jet, seen in radio waves.

JWST sees in the infrared, the kind of light emitted by warm dust. If the jet is heating the dust, the glow from the dust should be seen aligned along the jet, If it’s from the disk, the glow should be more diffuse (think of it like lighting up a room with a flashlight beam as opposed to a lamp).

What they found is that the dust glows pretty brightly along the jet, indicating the jet is the source of the heat. The dust is at a temperature of about -160°C, which is frigidly cold by human standards but warm by astronomical ones.

This is pretty interesting! The environment of a jet is brutal; the material is screaming outward at a decent fraction the speed of light. It slams into surrounding matter, creating extremely powerful shock waves (sonic booms, if you like), which can heat the matter. It’s surprising the dust is warmed by this because we’d expect the dust to be annihilated by these power shocks (like windows in a building shattering if a jet flies past at supersonic speeds). Something is protecting the dust, and it’s not clear what. Maybe there are gas clouds around the dust shielding it. Maybe dust lasts longer than we thought, or that it does get destroyed but is good at reforming in areas farther out from the jet where the environment is more clement. 

Active galaxies are among the most energetic and powerful objects in the cosmos, and play a big role in how the Universe behaves. There’s still a lot we don’t understand about them, but we’re getting better at this all the time.

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