Subtracting black holes from quasars reveals the galaxy underneath

A tried-and-true astronomical technique uncovers galaxies in JWST images

May 23, 2024 Issue #725

Astronomy News

It’s a big Universe. Here’s a thing about it.

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).

An extremely bright “star”, actually the quasar 3C273, in a black field. A squiggly line near it is the jet blasting away form the center.

3C273 is the first identified quasar, seen here by Hubble. There’s a whole galaxy in that bright spot, completely overwhelmed by the intense nucleus. The squiggly line to the upper left is a jet of matter blasting away from the black hole. It’s longer than our entire galaxy. Credit: ESA/Hubble & NASA


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 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.

A field of thousands of galaxies as mostly small dots, with one highlighted. A zoom in shows the bright pattern of light from the JWST optics, and another below it shows the PSF subtracted, with the fainter galaxy light more obvious.

The field of one of the quasars (look at all the galaxies!) showing the quasar zoomed in (top inset) and again with the PSF subtracted and the fainter starlight visible (lower inset). Credit: Yue et al (2024), NASA


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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