Every big galaxy in the Universe, as far as we can tell, has a supermassive black hole in its core. In most the black hole is somewhat isolated, so there’s no material for it to feed on. But for some there’s a lot of food just lying around in the form of interstellar gas. If that material falls in, it forms an accretion disk, a swirling flattened disk just outside the black hole’s event horizon. Material close in orbits at nearly lightspeed, while stuff farther out is slower. The rubbing of the faster and slower material creates friction on a literally cosmic scale, and that heats the material up…sometimes to millions of degrees.

Something that big (light-years across) and that hot glows. These accretion disks can be so luminous we can see them from literally billions of light-years away. These occur in the centers of galaxies, so we generically call these active galactic nuclei, or AGN. One of the first discovered, 3C 273, was only initially seen as a star in images, but was blasting out radio waves, so it was called a quasi-stellar radio source, shortened to quasar. It’s a subset of active galaxies, along with others like blazars, Seyferts, and more.

Quasars are pretty common, and surveys taken of the deep Universe have shown us millions of them. Yes, millions. They’re the subject of intense research, but there’s a problem: While there’s usually a whole galaxy surrounding the nucleus, the star light from the rest of the galaxy is not nearly as bright as the core, so it gets swamped (which is why 3C273 looked stellar at first).

This gets worse the farther away you see the quasar, because the nucleus is blazing away, but if we see the galaxy itself when it was young (a consequence of it being so far away, and the light taking billions of years to reach us) then there aren’t as many stars in the galaxy yet! That makes the galaxy proper even fainter, and harder to see.

JWST helps here, because it sees in infrared light. The expansion of the Universe redshifts the galaxy light, so if you look at galaxies at just the right distance, JWST can see them easily.

Now, for the first time, astronomers have been able to separate the starlight from the AGN light in extremely distant quasars using JWST [link to journal paper].

Now I want to be careful here. We’ve done this many times before, but (as far as I can tell) never for galaxies at this distance — 12.8 – 13 billion light-years away^*. The technique they used is a tried-and-true method as well. When light from a star is seen through a telescope, the optics inside the ‘scope spread that light out into a characteristic pattern. For JWST you’ve probably seen the weird hexagonal shape to the centers of stars speared by six diffraction spikes. That’s what I mean. There’s a mathematical relationship to how the light gets spread out from a point source (a teeny dot like a star seen from very far away), so we call this shape the Point Spread Function, or PSF.

Using stars in the same field as their quasars, they created a model of what the PSF looks like, then subtracted it from their quasar images. The idea is that the nucleus is an unresolved dot, but the galaxy itself is spread out. When you subtract the point source away, the remaining galaxy should be visible.

And it worked! They can see the star light from the galaxy surrounding the active quasar cores in six distant galaxies, revealing what the normal part of the galaxy looks like.

Why is this important? 

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