This is a galaxy?

By Phil Plait | June 27, 2011 10:04 am

I love to post pretty pictures of galaxies and wax lyrical about their magnificent structure, complex history, and complicated internal compositions.

… and then there’s the Carina Dwarf galaxy. It’s so small and faint it wasn’t even discovered until 1977 even though it’s one of the closest galaxies in the sky! How did it avoid detection so long? This’ll make it obvious:

[Click to unendwarfenate.]

See it? Yeah, it’s that faint smattering of stars in the middle of the picture (the bright star near the center is in our Milky Way and coincidentally aligned with Carina). Not much to it, is there? It’s about 300,000 light years away, only 1/10th as far as the much brighter and more famous Andromeda Galaxy, and only about twice as distant as our two satellite galaxies, the Small and Large Magellanic Clouds, both of which are easily visible to the unaided eye.

Like the LMC and SMC, it is apparently a satellite of the Milky Way, but formed long after we did; studies of the stars in the Carina Dwarf indicate it’s only about 7 billion years old at most, while our galaxy is well over 10 billion years old. It probably formed from primordial gas orbiting the Milky Way, taking much longer due to its low mass and relatively quiet environment.

This image is a combination of observations taken with the 2.2 meter MPG/ESO and the Victor M. Blanco 4-meter telescopes in Chile. It shows that the galaxy has very little or no gas at all in it, and so its career in star-formation is long dead. But there’s still much to learn from such objects: they get eaten by bigger galaxies, for example. This cosmic cannibalism is one way galaxies like ours get so big, so studying these smaller bite-sized snacks in situ help us learn about ones we’ve already munched on.

Plus, galaxies like Carina might be the most common in the Universe! We just can’t see them because even at relatively small distances they fade away into the background. They may not be as flashy as spirals or as monstrous as giant elliptical galaxies, but they play an important role in building up such beasts. The more we know about them, the better we’ll understand the Universe itself.

Image credit: ESO/G. Bono & CTIO


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CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (37)

  1. icewings

    Awwww, wook at the cute widdle gawaxy…coochie coochie coo!

  2. Very cool. I’ve never heard of this little guy before. Any information on how big/how many stars are in it relative to the Milky Way?

  3. Wayne Conrad

    All my untrained eye sees are dots. Lots of them. Which ones are the Carina Dwarf galaxy?

  4. Zach

    Interesting. This brings up two questions for me.

    1. Does it have a supermassive black hole at the center? The way it formed makes me wonder if it didn’t just lump together thanks to density waves in the gas surrounding the Milky Way.

    2. If we can’t see them even when they’re relatively close, just how common are they? Could these little fuzzballs make up a relatively significant portion (greater than 0.1%) of the missing mass that we call dark matter?

  5. elgarak

    @Wayne,

    Look at the full res image. There’s a bluish ellipse behind/around the bright star in the center. This bluish ellipse is Carina.

  6. MattF

    Wayne Conrad: All my untrained eye sees are dots. Lots of them. Which ones are the Carina Dwarf galaxy?

    I think I see it.

    Look at the center. See how the bright star there has distortions to it that the rest of the bright stars in the image don’t? That kind of bluish fuzziness at an angle?

    I think that’s it.

  7. Steve

    A little help…is it right smack in the glare of the bright star in the middle? I can’t differentiate the Milky Way stars from the carina dwarf galaxy splat… I guess maybe that’s the point?

  8. It would be very disappointing to come be life on a planet in the galaxy only to discover your galaxy is small, empty, and far away from anything cool.

    edit: For those that can’t see it, I believe it’s the part where it’s more dense in the middle. You see a bunch of brigher (larger looking) stars around it and then in the middle these more densely packed dimmer stars in the gaps between. You have to look in between the brighter stars really. Where it’s more dense is where the galaxy be.

  9. Wayne Conrad

    Ah, got it. It’s nearly coincident with the horizontal portion of the lens flare. My brain had decided it was an optical effect, which is why I couldn’t see it. Thanks for the help, @MattF and @elgarak.

    I thought maybe Phil was demonstrating that people will say they see whatever you say you see. You know how tricky these astronomers are. Instead, my brain demonstrated how great it is at filtering out things that are “obviously” not what it is looking for. These brains, they’re tricker than even astronomers, sometimes.

  10. Jason

    I think I have actually heard of the Carina dwarf galaxy before, but I cannot remember where.
    Anyway, I can see how it is missed especially if it stays “hidden” behind that star for most observations from earth. I am guessing that over the relatively short term there is not enough parallax to move the start out of the way.

  11. No, I think it fills roughly a quarter of the field of view. If you look at the hi res image, the stars are more closely packed towards the middle than around the edges.

  12. Regner Trampedach

    To most commenters above: You all got it a bit wrong, I’m afraid. But in your defense, the BA wasn’t particularly clear. The dwarf galaxy fills the picture, it is centered about a tenth of the picture width above and to the left of the blue foreground star. The distribution of fainter stars belonging to the dwarf, is a broad and somewhat elliptical, diagonally (left,top) -> (right,bottom). The foreground stars from our own galaxy are generally brighter (and look a bit bigger in the picture).
    That none of you actually saw it, is good testimony to the difficulty of finding these guys, as they hide in plain sight.
    In edit: I see Vagueofgodalming@10 got it too :-)
    Almost forgot Lee Petersen @ 2: About 2 million stars (that is about a factor of 1000 less than the Milky Way), about 35 times more mass in it’s dark matter halo than in stars, and no gas or dust to speak of, according to S. Pasetto et al. (2011), Astronomy & Astrophysics, Vol. 525, A99.
    Cheers, Regner

  13. VinceRN

    If it were true that these were the most common type of galaxy in the universe, would all that previously unobserved mass appreciably reduce that fraction of dark matter that makes up the universe?

    @ #10 and #11 – thanks for clarifying that, I expected to to be smaller and partially obscured by the bright star, but couldn’t really make it out.
    I found another picture at http://users.westconnect.com.au/~sjastro/carinadwf.html that shows that better, though it’s not nearly as pretty.

  14. Douglas Troy

    Only 300,000 light years, well then, why don’t you just zip on over there and get some better pictures for us Phil. We’ll wait.
    :)

  15. Gary Ansorge

    12. VinceRN

    “If it were true that these were the most common type of galaxy in the universe, would all that previously unobserved mass appreciably reduce that fraction of dark matter that makes up the universe?”

    No!

    First off, if it shines, it isn’t “dark”. Second, it’s the excess momentum of stars around the galaxies that led to the “dark matter” hypothesis. Not enough visible/normal matter in the galaxies(by about five times) to account for the excess momentum.

    Gary 7

  16. Anchor

    Lotsa flotsam and jetsam out thar…

  17. chris j.

    i wonder what the milky way looks like, to someone observing from the carina dwarf. would a glorious dawn await?

  18. MikeS

    Ah, Carina, my old friend. The commenters above illustrate why these are so hard to find. This doesn’t look like a galaxy in the traditional sense. It’s more of an overabundance of stars. If you look at them on a big Schmidt or POSS plate, covering a huge area, they become more obvious, looking like a thumbprint. But our surveys have shown that they are VERY extended — Carina actually extends well off of the image Phil has above but it as such low surface brightness (stellar density) you can only detect it statistically or with very precise methods. Sloan has detected a bunch of these guys that look like ‘nuthin.

  19. Simon Green

    I’m interested in how it *was* finally discovered. I imagine someone (or some computer) with a lot of spare time made distance/velocity measurements on all the stars in the image? Perhaps as part of a larger survey? And then someone noticed a correlation?

    Or a slightly different sequence of events whereby a small sample of the stars were measured and someone came up with a distance of 300K lightyears. Checking a few more yielded the same result, and *then* a more in-depth study revealed the whole lot?

  20. VinceRN

    @14 Gary – Sorry if I misunderstood you, but if there’s not enough “by about five times” to account for the excess momentum, wouldn’t that reduce the fraction of dark matter by about a fifth? I mean that would finding that much more matter reduce the amount of dark matter from 23% to 18.4%(supposing the “by about five times” that you stated)? Or would the amount of matter in these abundant dwarf galaxies be negligible compared to the amount of dark matter?

    Perhaps you thought I was saying there would be no dark matter? Far from it, I’m just wondering about how all this previously unseen mass fits into the equation. Would it raise the amount of atoms in the universe by a percent or two, or would it be far less than that?

  21. Germs

    Here’s an overlay (animated GIF) of the image from #13 VinceRN’s link and the ESO image.
    http://tinypic.com/view.php?pic=2zhnx1c&s=7

  22. HvP

    VinceRN,

    As I understand it, the discovery of this galaxy isn’t a discovery of the actual stars themselves – we had already recorded them a long time ago. It’s the discovery that some of the stars are gravitationally interacting as a galaxy that is new.

    So no, this isn’t previously unidentified matter. The stars were seen and counted. It was simply assumed that they were part of our own galaxy instead of in their own galaxy.

  23. Grimbold

    @Simon #19
    I was shown transparent negatives of photographic plates on a tour of Siding Spring observatory and one of these had a dwarf galaxy on it (may in fact have BEEN Carina, I can’t remember). On this negative it was very obvious where the galaxy was. With foreground stars some appear large, others small. With the dwarf galaxy ALL of its stars are very small due to its distance and somewhat close together. It stood out pretty obviously once you knew what to look for. Carina would likely have been found by an experienced astronomer with a magnifying glass and a light table.

    It is not as obvious in the picture accompanying this article but I can still make it out.

  24. Checkmate1

    Okay, I see it now, thanks to the overlay photo (#21). But it begs the question: What IS the small blue smudge around the central star? None of the other bright stars seem to have this appearance.

  25. VinceRN

    @21 Germs – excellent.

  26. Messier Tidy Upper

    Not much to it, is there?

    Well, everything is relative. That’s still an awfully large region of space and number of stars and perhaps exoplanets. Even a tiny and comparitively unimpressive dwarf galaxy is an island universe of its own! :-)

  27. Ross Amans

    There should be a clear plot of galaxy diameter vs frequency oi galaxy, and similar galaxy mass/freq, galaxy number of stars/freq, galaxy brightness/ frequency. So an estimate of density of galaxies in starfields could be made, and also, density of visible galaxies in the starfield.
    But I cannot find any such plots.
    Anyone know where they are hiding?
    It would be bad astronomy if they did not even exist.

  28. Pete Jackson

    I have taken the image as published by BA and adjusted the brightness and contrast to show the Carina Dwarf Galaxy much more clearly, at least to me. Many browser/monitor combinations render dark images with too low brightness. Let me know if this image shows it more clearly to you:

    http://www.chinesemeridiansblueprintsoflife.com/Images/

    As I have mentioned before, you will see much more detail in an image if the background is gray than if the background is black.

  29. Jon Hanford

    @Simon #19, Grimbold #23,

    Grimbold, yes, astronomers working on the ESO/SRC Southern Sky Survey, using the 1.2m UK Schmidt Telescope at Siding Spring Australia, found the galaxy by visual inspection of one of the plates taken for the survey (and partially seen on an adjoining plate). The discovery paper, along with the discovery image, can be found here: http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1977MNRAS.180P..81C&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf

  30. Messier Tidy Upper

    @ ^ Jon Hanford – & @ Pete Jackson too : Thanks. :-)

  31. VinceRN

    My favorite thing about this blog, besides the blogger himself, is how much good information the commentators come up with.

  32. Messier Tidy Upper

    More info on the Carina dwarf on wikipedia here :

    http://en.wikipedia.org/wiki/Carina_Dwarf

    whilst this :

    http://en.wikipedia.org/wiki/Local_Group

    gives a list of the Local Group galaxies of which our inconspicuous Carina dwarf is but one member.

    Mind you it’s not the faintest, hardest or most recently spotted as this :

    http://kencroswell.com/UrsaMajorDwarf.html

    article by Ken Croswell illustrates. ;-) :-)

  33. Thameron

    So how many stars does it take to stop being a globular cluster and start being a dwarf galaxy?

  34. Captn Tommy

    Is it possible that this is a dispersed/dispersing Globular Cluster? It probably (from the looks of it) has as many stars. whose to say a catastrophic disrupttion could not overcome the gravity holding a Cluster together. It would also explain the lack of gas.

    Has the relative velocities and motions of the stars been studied to see if an origin point can be determined?

    Interesting picture.

  35. TMB

    @33 and 34: The faintest dwarf galaxies are indeed fainter than the brightest globular clusters, but the key difference is their dynamics: globular clusters are just bound together by the gravity of their stars, and so are very compact, while dwarf galaxies are bound together by the gravity of their parent dark matter halo, which contains much more mass so the galaxies are much more diffuse. But they can’t just be disrupted GCs because their velocities are random – they are not expanding, the stars are just whizzing around inside the dark matter halo (although some dwarf galaxies are tidally disrupting too – see the Sgr dSph, for the most dramatic example).

  36. Zach

    At 300,000 ly distant, we’re actually closer to this thing than we are to the center of our own galaxy!

  37. Pete Jackson

    No, Zach. The center of our Galaxy is about 30,000 ly distant, only one-tenth as far.

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