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SYFY WIRE Bad Astronomy

The lovely faces and dark hearts of two galaxies

By Phil Plait

Spiral galaxies are among the most beautiful objects in the Universe. Their grand, majestic nature is sweeping on a scale of hundreds of thousands of light years; their delicate arms are composed of a hundred billion stars blurred into a milky stream; and as for their cores... well, that's a different story.

Let me present to you two surpassingly beautiful galaxies, each with a dark secret in its heart.

First is NGC 4698, as seen by Adam Block using the 0.8 meter Schulman Telescope at Mt. Lemmon in Arizona:

[Click to galactinate.]

NGC 4698 is relatively close, at a distance of 60 million light years. This image is lovely, with the faint outer arms clearly visible, the inner arms lined with clouds of dust like black pearls on a string. The core looks odd, though, which I noticed right away. It's brighter than I would've expected, and appearing almost as if it's popping right out of the plane of the galaxy.

And here's a second spiral, M77, one with which I'm fairly familiar -- I spent a long evening photographing it for an observational astronomy class in grad school. This remarkable image, however, was constructed by Andre vd Hoeven, who downloaded dozens of Hubble images of M77 from the online archive, and painstakingly assembled them into this amazing shot:

[Click to embiggen.]

Wee see M77 a bit more face-on than NGC 4698, and by coincidence it's also roughly 60 million light years away. The red glow dotting the arms is indicative of star formation; those are vast gas clouds glowing from the heat of young, hot stars embedded in them. It too is thick with dust, and like NGC 4698 the core looks... odd. Too bright, too compact. In the high-res version you can also see a greenish glow off to the left of the core, like a searchlight shining in that direction.

So both of these galaxies look normal at a perfunctory glance, but clearly have something else going on, something not obvious that makes them special. A secret, if you will. But few secrets can be withheld from the prying eyes of astronomers and their tools. Especially spectrographs.

Stars give off light at all wavelengths, all colors. That's called a continuous spectrum. Use a prism to break up sunlight into a spectrum and you'll see wide bands of colors blending into one another. But a thin, hot gas (like inside the glass tubes of a neon sign) emits light at only specific colors, called emission lines. If you use a prism to break the light up from a neon sign, you see very thin lines with only a few colors represented.

Gas clouds in space are the same. They emit colors at specific wavelengths, depending on what's in the cloud. Oxygen glows green, hydrogen red, sodium yellow-orange. It's more complicated than this, but that's the gist of it. If you put a spectrograph -- a camera that can break up light and carefully measure its component colors -- on a telescope, you can determine what's emitting the light seen, what's in it, and even its temperature and speed.

Under a spectrograph, both NGC 4698 and M77 reveal complicated emission line spectra, meaning they have lots of hot, thin gas in their cores. That in itself is interesting enough, but raises another question: what could possibly be lighting those gas clouds up on a galactic scale?

And that's where astronomers have cracked the true secret of these galaxies: they have monstrous black holes in their cores. In fact, we think all big galaxies do, including our own. But the difference here is that the ones in these two galaxies are actively feeding. Matter is falling in to these black holes, but first it's piling up in huge disks around them. These disks get infernally hot and glow extremely brightly. This light escapes, and hits the gas clouds farther out in the galaxies... causing them to glow as well. In M77 the disk of matter is actually helping to focus a focused wind of light and matter that blasts away from the black hole, which is why it looks like there's a spotlight emanating from the galaxy's core. In a sense, there is.

Galaxies like this are said to be active. Our Milky Way's black hole isn't feeding right now, so it's quiescent. But a significant fraction of galaxies have active cores, and can be so bright they can be detected even from billions of light years away. And there you have it. Most galaxies seem normal enough on the surface, but for the monsters in their hearts. And in the case of at least these two island universes, it's the fierce light from those beasts that makes them so remarkable.

Beauty, as the saying goes, is only skin deep. But what lurks at the core is, sometimes, what makes that beauty shine.

Image credits: Adam Block/Mount Lemmon SkyCenter/University of Arizona; Andre vd Hoeven; both used by permission

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