The lovely faces and dark hearts of two galaxies

By Phil Plait | April 16, 2012 9:24 am

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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CATEGORIZED UNDER: Astronomy, Pretty pictures, Top Post

Comments (21)

  1. Martin

    Beautiful pix. If they have active nuclei with beaming, etc., there should be radio emission. Let’s see those pictures, too!

  2. gameshowhost

    The universe is so damned spiffy.

  3. Artor

    Do we know exactly what makes a galactic core active? Would one star falling into the black hole account for it, or does it have to consume a constant stream of matter? Is there anything that could activate the Milky Way? What would be the result of that, out here on the edge? Would we be irradiated, or would the beam be focused enough to not affect us?
    I hope my flurry of questions isn’t inane or boring, but this is so damn cool, I want to know more!

  4. Except that the one in our galaxy has this weird planet at the center after you get through the Great Barrier, and an angry two-faced (many-faced) Republican presidential candidate who has been waiting for an eternity for his chance. No, wait, that’s Mitt….

  5. HvP

    Jess, then I’m curious about what Romney’s plans would be for manned spaceflight.

    After all, what does Mitt need with a starship?

  6. Jim Johnson

    Speculation time: If the Milky Way were actively feeding (or if Earth was in a spiral arm of M77 or NGC 4698), how would it affect our view of the night sky? Would the side of the planet facing the heart of the galaxy be as bright as day? Would actual nightfall be as rare as a solar eclipse? Or would it be just a prettier, haloed version of our own view of the Milky Way?

  7. Wzrd1

    @2, Artor, pretty much nothing ever goes STRAIGHT into a singularity or even straight into a star. Typically, things orbit and may have close passes. Now, for a star, things could get a bit interesting, indeed, BA has talked about comets that were sun grazers finally fall into the sun.
    With singularities, it gets more complicated. Tidal forces tend to rip anything approaching apart LONG before it could reach the event horizon. So, instead, a cloud of gas would be spiraling in to the singularity, which is called the accretion disk, which heats up to insane temperatures- to the point where the ionized gas emits x-rays and accumulate intense magnetic fields.
    That is for a star, a planet or even the mythical Starship Boobyprize, which made a close approach and suffered spaghettification, then gasification, then ionization.
    Though, one MUST admit, said intrepid crew complained not, they didn’t quite get to know what happened to them. ūüėČ

    @5, Jim Johnson, the radiation LARGELY is “funneled” out in polar jets, the accretion disk is hidden from us by gas, dust and even stars, which would absorb a LOT of any radiation and the inverse square law would eliminate the rest, for dangerous or even REALLY bright things going on.
    But, the IR astronomers and radio astronomers would have a field day!

  8. Those pictures are so beautiful, and it’s so cool that we can figure these things out. And by “we” I mean, “someone really smart” and not me. :)

    I am probably wrong about this… I though it was called a “spectroscope” and that a “spectrograph” was what it produced. But maybe I have it the wrong way around.

  9. bullsballs

    if each galaxy is at the bottom of a very deep gravity well due to such massive black holes at their centers, the galaxies could be much closer than we could really tell, because of the distortion of space.
    and if the massive black hole is drawing the galaxy into the center, while also causing the galaxy to revolve around the core near the speed of light, the distortion would even be higher.
    so if the energy revolving around the core is traveling near the speed of light, would it transform from energy to matter? also, wouldn’t the time would be slowed to a snails pace?
    perhaps the universe is a little bit more complex than we are aware of?

  10. Sam

    I have a question Phil. If NGC 4698 is say 50,000 light years across, how does it appear so symmetrical to us when the light from the closer edge is 50,000 years younger than the light from the far edge? Why isn’t it distorted from moving or anything?

  11. Wzrd1

    @9, erm, no. First, for the “galaxy” to be orbiting near light velocity near the singularity, it would be below the event horizon.
    Gravitational distortion is NOT high enough to do what you suggest. If it did, the Earth would be flat and gravity changes space to make it appear round. Orbital studies, not to mention the math, prove that wrong.

    @10, I fail to see sufficient detail to determine ANY distortion of the galaxy. No spokes to consider, indeed, dust and gas diffuses the view enough to remove that resolution. Add to that the 50000 light year diameter and the distance between us and the galaxy and the lack of resolution from obstruction, I fail to see your idea.
    Especially when you consider galactic rotation velocities, which are rather slow in the time frame of 50000 years.

  12. Re 5 HvP: If someone managed to get past your bubble, wouldn’t you want to know what was outside? Of course nothing actually gets through the bubble. Interesting how one can go nowhere fast, and yet still have a warped view of reality, be completely lost and yet make great time. And time dilation does explain a LOT about conservatism. Talk about dragging one’s frame!

  13. warren

    Just gorgeous photos! They are examples of what has kept astronomy as a life-long hobby. Starting at age 12, its been 50 years. Its my belief that we have increased our knowledge of the Cosmos just as much or more than the evolution of the airplane from the Wright brothers to a 777 jumbo jet that can carry more than 500 people. For example, I remember reading in the 1962 or 63 Sky & Telescope artical about a strange new type of object called a Quasar! And science at that time as to what they where. The awareness that an object that far away could he seen blew everyone away because knowone knew what type of mechanism could power them. And think of what we know now. Thanks Phil for all the great photos.

  14. @9. bullsballs

    One of the weirdest things about galactic black holes is that they don’t really matter, until you get within a few hundred parsecs of them. They’re not the equivalent of the Sun in our solar system. The black hole may way a few MILLION solar masses (all compressed to a mathematical point), but the rest of the galaxy orbiting around it is BILLIONS of solar masses. The stars and gas vastly outweigh the giant black hole, so they just orbit the galactic disk.

    The disk of matter surrounding the black hole is probably not even aligned with the plane of the galaxy- those beams of radiation may be sweeping like a searchlight THROUGH the galaxy (as Phil points out in Galaxy #2 above)

  15. @10. Sam

    Perspective effects. If it’s 50,000 light years across and 5,000,000 light years away you won’t see a noticeable difference in brightness between the near side and the far side. As for distortions, galaxies don’t change all that much in 50,000 years, and stars don’t move all that far in 50,000 years. There should be some tiny distortion, but if it’s that far away we won’t see anything.

  16. Melissa

    Phil, as an astronomy instructor, I’d love it if you didn’t say that stars have continuous spectra. This is a misconception that I fight every semester. Astronomers extensively utilize the absorption lines is stars’ spectra to learn about characteristics like stars’ surface temperatures. Perhaps you could use an incandescent lightbulb as your example of a continous spectrum through a prism? Or mention that a typical prism doesn’t spread out the light enough to be able to see the Sun’s absorption lines.

    I know you work hard to communicate clearly and accurately to the public, and some of your comments have made me speak more clearly to my students. Please keep it up!

  17. Jeff

    I’ve always thought galaxies were god’s prettiest creation. They attracted my attention when I was 5 and still do today.

  18. These are lovely pictures. The question I have is “why spirals”? I understand why shells and fern-heads grow in spirals, but have not seen a simple cause for spiral galaxies. Any ideas?

  19. rabar

    The personification of black holes as ‘monsters’ or ‘beasts’ devouring stars, I think is a somewhat weird way to describe what is a perfectly natural process. In fact a complete galaxy can be understood as a whirlpool gradually going down a ‘drain’ into a somewhere else we do not yet understand.., maybe the same way water goes over a dam. Speaking of the ‘death’ of stars when they go supernova I also find irritating. Who is dying? They’re just undergoing a transformation into another state — a necessary one if we were ever to receive the more complex elements that make life possible.
    Just my humble point of view…

  20. Hellchylde

    sandsmith – spiral.. as they are spinning… .. but hers my thought?.. they tell us our black hole is not active.. what would happen if it woke up?.. started feeding as they put it?

  21. Matt B.

    The thin ring around NGC4698 is fascinating. Is it possible that it’s actually normal spiral arms, but the brightness had to be turned down so much because of the bright core?

    Can the brightness gradient be used to accurately determine the gas density gradient of the galaxy? (The problem being that less glow at any distance from the core could be from either low density there, meaning less surface to reflect, or high density further in, diverting the light through absorption and reflection.)


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