Setting the bar

By Phil Plait | September 24, 2010 7:00 am

Galaxies come in a lot of flavors. And even in the major categories (spiral, elliptical) there are sub-flavors… like barred spirals, which are truly cool and weird and awesome. Behold!

eso_ngc1365

Yowza. Click to engalacticate.

That’s NGC 1365, a barred spiral about 60 million light years away in the Fornax cluster, as seen by the HAWK-1 camera on the Very Large Telescope in Chile. HAWK-1 is sensitive to infrared light, from just outside our human eye’s range to wavelengths about four times longer than we can see. Those wavelengths are pretty good (though not perfect) at penetrating dust clouds in galaxies, which block visible light. So mostly what you see in images like this one is light from stars, along with gas clouds. Where you see dark lanes is where the dust is so thick it blocks even the infrared light, too.

The two major spiral arms are obvious enough, as well as some smaller ones (called spurs) too. These are not physical spirals; stars near the center of the galaxy revolve faster around the center than ones farther out, and so if the arms were "real" they’d quickly (well, over a few hundred million years) wind up into a tight little curl. But we see spiral arms in galaxies of all ages, so we know they’re not transient, and must be stable features.

In the past few years the thinking is that the arms are traffic jams, literally regions where stars, gas, and dust pile up as they orbit the galaxy. Supporting this idea is that the arms are regions of intense star formation, which you’d expect if gas clouds are rear-ending each other in these jams. The clouds collapse after colliding and form stars. That also makes a lot of dust, and again you see a lot of that in the arms.

What causes the spiral pattern? That’s complicated and has to do with the way gravity works in a disk of star. Any sort of perturbation — the passing of a nearby galaxy, for example, or possibly even a wave of supernova explosions — can disturb the disk, causing the spirals to form.

eso_ngc1365_visIRThe same goes for bars. That’s the long, horizontal, relatively straight feature going across the galactic center. That’s a natural consequence of gravity in some galaxies, and in fact we are now pretty sure the Milky Way is a barred spiral, though our bar isn’t as prominent as the one in NGC 1365.

In the side-by-side image here (click it to get a much bigger picture), visible light from the galaxy is on the left and the IR image on the right. The dust is better at blocking visible light, so you can really see where the dust is… and once again, the bar is littered with it. Again, that makes sense since we’re seeing lots of star formation there.

Interestingly, the central bulge in NGC 1365 is elongated, and at an angle with respect to the bar. That bulge has mostly older, redder stars — that’s why it’s so prominent in the IR image. Those kinds of stars pour out infrared light. But it’s also lacking in dust and gas (the image is very smooth, indicating it’s the combined light of billions of stars; gas and dust would make it patchy like the bar and arms). That’s another indicator it’s old; all the gas has long ago been used up making stars.

It’s amazing what you can learn about a galaxy simply by comparing two images taken at different wavelengths. But that’s something astronomers have known for almost a century now; what we see with our eyes is not even close to the whole story, and it’s only by going outside our natural limitations that we can truly understand the Universe.

If there’s a morality lesson in that, I leave it to you to figure it out.

Tip o’ the dew shield to ESO_Observatory on Twitter. Images credit: ESO/P. Grosbøl


Related posts:

- Sculpting a barred galaxy
- Ten things you don’t know about the Milky Way galaxy
- Barred for life


CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (14)

  1. IVAN3MAN_AT_LARGE

    Phil Plait:

    If there’s a morality lesson in that, I leave it to you to figure it out.

    “Space… is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space, listen…” — Douglas Adams, The Hitchhiker’s Guide to the Galaxy.

  2. Our world, where you do not look everywhere you can see the vortices. Micro-and macro-vortices, bigger and smaller. It’s the same thing as in these pictures – spiral galaxies. Nature is the same.

  3. Another aspect to the morality lesson is that, if we use just our eyes to see, we get a very limited view of reality (profiling, anyone? Beuller? Beuller?)

  4. Gary Ansorge

    Life, the scum of the universe,,,it gets on EVERYTHING.

    ,,,kinda like my kitchen,,,

    Cool images.

    Gary 7

  5. chris

    the thing that trips me out the most is that there appear to be two distinct bars! the visible and IR images show the long “horizontal” bar, but you can also see that the spiral arms keep spiraling inward to attach to the elongated core, skewed about 60 degrees from the main bar and itself forming an apparently independent bar.

    one thing i love about pictures of galaxies is how many other, more distant galaxies you can see in the background. if you embiggen the IR photo, the reddish blob in the spiral arm near the top resolves into a near perfect pinwheel – another spiral galaxy in the background.

  6. Messier Tidy Upper

    What a magnificent, splendid, marvellous, fabulous, superluminous image. :-)

    Our own Milky Way is a barred spiral with “spurs” – our Sun is currently located in the Orion spur unless I’m very much mistaken.

    Galaxies come in a lot of flavors.

    Milky? Dark? Honeycomb? Caramel whirl? Blueberry fizz? Golden egg elliptical? ;-)

  7. Tribeca Mike

    “Any sort of perturbation … can disturb the disk, causing the spirals to form.

    “The same goes for bars.”

    Oh brother, tell me about it. (Places ice bag on head)

  8. Man phil, even if I never read a word you wrote, I’d still subscribe to your blog for the pictures.

  9. t-storm

    Near the top of the upper arm, just below it you can see another more distant spiral galaxy.

    I’m guessing the morality is more or less look at an issue through a different set of eyes and you’ll see a different point.

    Space is so weird. How is it even remotely possible that we are the only place that has life?

  10. Brian Too

    Any residents of this galaxy would have no idea how handsome their home is. Perhaps we should send them some snaps so they can appreciate it properly. Hey, that gives us an excuse to guilt them into returning the favor!

  11. “Here is my secret. It is very simple: It is only with the heart that one can see rightly; what is essential is invisible to the eye.” — Antoine de Saint Exupéry

  12. Gary Ansorge

    10. Brian Too

    “Perhaps we should send them some snaps so they can appreciate it properly”

    That has my vote. What better way to express our interest than to display our appreciation of their beauty.

    Of course, some paranoid could really screw up this idea,,,

    Gary 7

  13. Mitchell

    For analogy, consider travelling waves in highway traffic.
    http://math.mit.edu/projects/traffic/ http://trafficwaves.org/

  14. Gary Ansorge

    13. Mitchell

    Interesting, how the same thing keeps getting re-discovered over and over.

    I became quite attuned to traffic waves as a young driver in LA,,,50 years ago. Glad someone finally decided to formalize these observations.

    Gary 7

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