BAN #340: If there’s a bright center to a galaxy…

[The planetary nebula M 2-9, winds from a dying star. Credit: NASA / ESA / Hubble Legacy Archive / Judy Schmidt]

Pic o’ the Letter

A cool or lovely or mind-bending astronomical image/video with a short description so you can grok it

Astronomy terminology can be confusing. Even for astronomers.

For example: Over the decades, astronomers found lots of galaxies that were weird. The nucleus, the very center of a galaxy, was brighter in this kind of galaxy than in others. Sometimes way brighter.

We now know that these galaxies have an actively feeding supermassive black hole in their center. All big galaxies have one of these beasts, but in some of them matter is falling into the black hole. It doesn’t fall straight in, but forms an enormous disk we call an accretion disk. That material orbits the black hole at fantastic speeds, and the speed depends very strongly on how far the material is from the black hole. Closer in stuff moves faster, and farther-out stuff orbits more slowly.

That gradient in speed is very steep for a black hole, so any bit of material rubs against the stuff closer in and farther out. This generates friction, so the matter heats up, and the disk gets extremely hot and extremely bright.

We also now know that due to geometry these galaxies appear very differently if we see the disk face-on or edge-on. The former are extraordinarily bright objects, and we see energy across the spectrum from them, from gamma rays down to radio. If the disk is more edge-on that high-energy stuff is blocked and the galaxy looks different.

So all these different phenomena — quasars, blazars, Seyferts, ULIRGs, and more — have the same engine driving them but we see them at different angles. This idea is called the unified model.I have a blog article from a while back where I talk about this in detail.

We lump all these objects into a category called active galaxies (our Milky Way is not active, so we say it’s quiescent). More specifically, they’re called AGNs, for active galactic nuclei.

Now maybe this is my own problem, but I do see people (including me) call these galaxies AGNs, when really we should say they have AGNs. This gets messy. If I say M87 is an AGN astronomers know what I mean, but really I should just say it’s an active galaxy. Yet I still call it an AGN. Habit.

What made me think of all this?

Another thing we say a lot is that the center of the galaxy, the actual AGN part, can outshine the rest of the galaxy’s light combined. All the stars in the galaxy don’t shine as brightly as the nucleus, so much energy is generated there. In a lot of images, even most, that’s hard to see. The center of a galaxy is large and bright anyway, and an AGN is kinda lost in all that glow.

But then there’s Markarian 817.

[The active galaxy Markarian 817. Credit: NASA, ESA, and the Hubble SM4 ERO Team]

Ha! Here you can definitely see just how bright the nucleus is compared to the rest of the galaxy. Of course, a trick of light is involved.

Hubble Space Telescope (and many ‘scopes like it) has a plus-shaped structure in its tube that supports a secondary mirror that reflects the light from a source down into the bottom of the ‘scope where the cameras sit. When light passes those thin metal strips it diffracts; the waves interfere with each other and spread out. That’s what makes those spikes you see in images like that, which are called diffraction spikes.

Only a point source, something no bigger than a dot like a star, can make those spikes. An extended source smears them out and they aren’t as bright. So when you see spikes like that in an image you know the source is very tiny and very bright.

In the case of Markarian 817 that’s the nucleus: The source of light is so small (the accretion disk, which can be dozens of light years across but still appears teeny from, in this case, over 400 million light years away) that it makes the spikes. And they’re bright, indicating the source is bright. The galaxy itself is rather faint compared to them, so you know this means the brightness of the galaxy’s active nucleus is on the same par as the rest of the galaxy itself.

[A closer look at Markarian 817. Credit: NASA, ESA, and the Hubble SM4 ERO Team]

Awesome. With Markarian 817 we so happen to be looking right down the throat of the AGN, so it appears especially bright.

And now when I claim the AGN can outshine a galaxy, I know where to point folks. I’ll still probably call the galaxy itself an AGN, too, but when I do you can sagely nod to yourself and think, “Ah, I know what he means.”

Et alia

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