ultra compact dwarfs nibbled by cannibals

Introducing Ultra Compact Dwarf Galaxies, the new galactic structures that are all the big-fish galaxies’ preferred snack. Our very own Milky Way is a bigger galaxy, in the process of eating Ultra Compact Dwarf galaxies as we speak.  Phil Plait’s Slate column and video has the gourmet details of galactic snacking; a must-read and watch for space geeks.

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60 thoughts on “ultra compact dwarfs nibbled by cannibals

  1. Very cool. Thanks, zic.

    Are the UCD galaxies that the Milky Way is “eating” the Magellanic clouds or something else?

    Could any Milky Way globular clusters be UCD nuclei or remnants?

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    • John Howard Griffin: Are the UCD galaxies that the Milky Way is “eating” the Magellanic clouds or something else?

      From the linked through submitted papers, the UCDs are orbiting galaxies Messier (M)-59 and M-85, which are each about 60 million light-years away. The paper also says these UCDs are on the young side as galactic objects go, so I would guess it’s unlikely they’re remnants (they may be precursors – the paper’s abstract says that these things may be common, but it’s just that nobody’s bothered to look for them as they didn’t fit in any boxes on stuff astronomers look for, and so were filtered out).

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      • Interesting. Thanks, Kolohe.

        So, if they are on the young side, then globular clusters are not UCDs, since most of them are composed of old stars with a lot of metal content.

        The Magellanic clouds, or the various dwarf galaxies around the Milky Way (and the Local Group) could be UCDs, I expect, but would be very old ones if they are.

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        • I would imagine that if any of the nearby galactic scale objects were UCDs, that fact would have been included prominently in any abstract or article about UCDs, instead of how it is presented – all of the objects fitting this category so far are part of far away supergroups, moreover, it was a previously undefined type of object.

          it also seems definitionally, the nearby objects may be too big, at thousands of light years across (like the Magellanic clouds) vice tens to a couple hundred light years across as the described UCD objects are.

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          • I agree with your analysis of the larger nearby objects like the Magellanic clouds, and that any nearby objects would have been included in the discussion.

            However, if I remember right, globular clusters are on the order of 50-200 light years in diameter, and some (perhaps many) of them are thought to have black holes or other dark matter at their center. This puts them closer in size and composition to the UCDs, though they are undoubtedly much older. I recall reading something recently about Dark Globular Clusters (I think) that discussed a Dark Matter connection with some of them. Unfortunately, I don’t recall exactly what I read about it, though.

            I suppose I’m just trying to seem intelligent by talking about astrophysics, which I am very interested in but only self-educated in. Please forgive my wondering aloud.

            These UCDs may show similarities with other small nearby objects after future research, I think, like globular clusters and other M (and NGC) objects.

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  2. One of the new UCDs just discovered has far fewer stars—something like 10 million—but it’s only about 20 light-years across!

    V=(4/3)*pi*r^3 = 4*r^3 (for blog math)

    -> 4*20^3=32,000 cubic light years

    -> 10*10^6/32*10^3 = 0.34*10^3 = 34 stars per cubic light year

    wow. That’s Manhattan / Hong Kong levels of density for stars

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      • Depends on the mass of the stars. If all the stars are yellow dwarfs like ours, stable orbits could exist. If they are all like Betelguese, or if there are a sizeable population of super massive stars floating around, then stable orbits could only exist in close to the stars (planets like Pluto, Uranus, & Neptune would likely get stripped off and either captured by the larger star, or made orphan to float in the dark).

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      • The Skeptics Guide to the Universe blog noted that whereas we can see about 3000 stars with the naked eye at night a hypothetical planet in that cluster would have around a million stars visible. The closest neighbors would on average be about 1/20th of a light year apart. It would be like having Proxima Centauri sitting in our Oort Cloud.

        Which means their stars likely don’t have Oort Clouds or, as noted, much in the way of planetary systems. I’m even more skeptical on that point than he is. A globular cluster isn’t anything like our galaxy. We circle the core along with our neighbors in decent orbits like cars on an interstate highway: we can all be moving pretty fast in absolute terms but our relative velocities are pretty low since we’re all going in generally the same direction at the same speed. Close encounters are uncommon, let alone collisions. Stars in globular clusters are “orbiting” every which way and they’re subject to so many gravitational interactions that they don’t so much orbit as wander aimlessly at a good clip. I would expect a LOT of close encounters over the course of a couple billion years and that would play hell with any planetary orbits. Maybe some “hot Jupiters” in really tight orbits would survive but not much else. Cataclysm would be the norm.

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      • Uh…yeah! “Paid”, sure!

        I’m not trying to get on anybody in particular, but the typo/grammar sloppiness goes straight to the top and has gotten really bad lately. Our Tod’s recent post was a mess in that regard.

        If someone has to be the Stannis Baratheon of House Grammarian around here, I guess it can be me.

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          • …it burns…

            Seriously, I don’t sweat grammar/punctuation/typos in comments (unless it’s for a joke, or the commenter is themselves calling out someone else’s mistake and then screws up), and one or two in an OP is inevitable in a fast-paced, unpaid environment and I will either let that slide or fix them quietly.

            But the more I see in an OP (especially if the OP is short) and the more glaring they are, the more agitated I get.

            This could well be my issue, but I know I view sites with lots of errors differently than I view those with few.

            It’s almost like that (possibly apocryphal) story about the extravagant, complicated tour riders some bands used to employ – the idea behind them was supposedly NOT that a band loved only brown M & M’s or whatever; it was that any venue that paid attention to the small weird details and got them right, would get the big basic things right too.

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            • I agree, and I say that being one of the worst offenders. I don’t know why I can’t see my own typos when proofreading, but often times that really is the case.

              I should probably have someone look over everything I write before publishing.

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              • I would be happy to be a typo-fixer, were I empowered to do so. All those books I read make me super-good at spotting them. And I have a light hand. And (unlike writing posts) it wouldn’t take me a lot of time.

                Not saying I’d catch every one, but I’d catch a lot…

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              • or others with a good editing eye might have better editing advice (or disagree with this entirely) but I heard that reading your piece backwards (that is to say, read the last sentence beginning to end, than the second-to-last-sentence beginning to end, etc.) is a good trick for finding minor spelling/grammar edits in your own writing because the disjointed reading does not allow you to “flow”. You are essentially reading decontextualized sentences — as opposed to entire paragraphs or a single, fluid piece — and therefore you will not gloss over errors that do not detract from understanding (especially if they are errors you make commonly and are then less attuned to).

                I’ve never actually tried it because I’m generally pretty good at catching my own errors (“Release the hounds, Smithers.”), but it might be worth a go for those who struggle with it.

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                • @kazzy I find I catch the most errors simply by reading it over again from the start. But I also always read it backwards a paragraph at a time… Then I read it forwards again. But I have a mild amount of dysgraphia and no dyslexia, so I think that gave me a LOT of practice at such things.

                  I can’t do a sentence at a time because I am too easily bored. Also, less embarrassingly, because I care about the rhythm of the paragraph and I don’t want to screw it up by fixing an error and then not noticing that the rhythm needs to be adjusted accordingly.

                  And of course the best way for me to find typos is simply to hit Publish and THEN reread the dang thing :D.

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        • Glyph:
          If someone has to be the Stannis Baratheon of House Grammarian around here, I guess it can be me.

          “I guess it could be me.”

          Use could (not can) to refer to conditional situations, in which something has to happen or be the case in order for someone to be able to do something or for something else to occur

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      • Heh…is that “you’re” intentional?

        Anyway, they weren’t simple spelling or grammar errors; they were missing or mis-chosen words, that made the meaning a little hard to parse.

        I inserted the words [that are] between “structures” and “all”; and I made “Our Milky Way” out of “I Milky Way”.

        If you’d prefer, I can leave them alone in future. But I don’t know that I am so altruistic as to fix work consistently for free, without *also* asking everyone to please, just try a little harder in the future to give posts a once-over. The pedantry is my payment! ;-)

        If I seem I lost patience with your post, trust me – yours just happened to be the latest straw that turned this camel (more) publicly-pedantic.

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        • I’m really dyslexic, and it’s made worse by barometric-change triggered migraine; and we’re in the Thunder Moon. I’m running on copy-editing fumes.

          as my witness, this was my major concern about becoming a contributor, and I beg your indulgence and support.

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          • Hey – I totally understand, and it was not my intention to make an example of you. I just corrected to one degree or another posts from Sam, Tod, and Aaron, and I griped in the comments on each.

            I wonder how hard it would be for CK to set up a system where the person that writes the post, can’t be the one to publish it; a second user has to hit a button before it posts. I assume WP’s native post versioning can capture/log the editing changes, in case any of them are objectionable to the author.

            This of course means that someone(s) will have to monitor the “pending-publish” queue and do that work, but it might be a small price to pay for cleaner copy…

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  3. Well, ok since we’re picking at nits

    “Our very own Milky Way is a bigger galaxy, in the process of eating Ultra Compact Dwarf galaxies as we speak.”

    I don’t think this statement is supported by Phil Plait’s article nor the academic papers used as sources for that article. No UCDs have been found in the vicinity of the Milky Way or the Local Group (yet), based on a cursory internet search.

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    • From the article (my bold):

      The Milky Way grew huge this way; our galaxy is in the top tier of spirals in the Universe. (Many are bigger, but the vast majority are far smaller.) It got that way by colliding and merging with smaller galaxies, enlarging its ranks over time. It’s actually in the process of eating several dwarf galaxies right now. Like, literally, at this very moment.

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      • From the original post “Our very own Milky Way is a bigger galaxy, in the process of eating Ultra Compact Dwarf galaxies as we speak.”

        #notalldwarfgalaxies

        Ultra Compact Dwarf galaxies are a subset of Dwarf galaxies. They are a newly classified subset of Dwarf galaxies, that was the entire point of Plait’s article. None of now labelled identified UCDs are in the neighborhood.

        This is why Gryfindor is overrated, getting points when they shouldn’t have. But what do you expect from the jock dorm.

        edit: and the other point of the BA post and the academic papers is that UCDs *aren’t* being eaten in the same way as other dwarf galaxies by big galaxies because of their star density and the probable existence of ‘overweight’ black holes at their centers.

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    • And quoting Plait:

      The Milky Way grew huge this way; our galaxy is in the top tier of spirals in the Universe. (Many are bigger, but the vast majority are far smaller.) It got that way by colliding and merging with smaller galaxies, enlarging its ranks over time. It’s actually in the process of eating several dwarf galaxies right now. Like, literally, at this very moment.

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