Pump up the galaxy

By Phil Plait | February 28, 2012 7:00 am

This is a galaxy?

Yup. It is! [Click to galactinate.]

This is the dinky Antlia Dwarf Galaxy (located in the southern constellation of Antlia, the "pump"), technically called a dwarf elliptical. It’s so faint and sparse that it wasn’t discovered until 1985 (and confirmed as being a galaxy in 1997), even though it’s only 4 million light years from Earth… not terribly farther than the Andromeda Galaxy, which is so big it’s visible to the naked eye! Antlia may be a member of the Local Group, a loose collection of a few dozen mostly small nearby galaxies; the Milky Way and Andromeda are the two biggest members.

This image is from Hubble, and shows just how dim a bulb this galaxy is. It only has a few million stars in it — our Milky Way has over a hundred billion, by comparison — and it’s only a few thousand light years across. The Milky Way is a full 100,000 light years in diameter, so if you put Antlia next to it you’d probably miss it entirely. Note that in this picture you’re only seeing the brightest stars in Antlia. At this distance, a star like the Sun in Antlia would be a tough object to see, even with Hubble. Most of the stars you see here are red giants, stars near the ends of their lives and thousands of times more luminous than the Sun.

But it’s an intriguing little bugger. For one thing, some of its stars are clearly very old, ten billion years or so. But other stars are just as clearly young, having been formed only a hundred million years ago or so (and I found a paper claiming it may have younger stars yet). That means Antlia has had more than one episode of star birth… but it doesn’t appear to be actively churning out stars now. If it did, the bright pinkish-red nebulae that form stars would be really obvious, especially in a galaxy this close by (like in, say, NGC 1427A).

Interestingly, there’s another galaxy near Antlia: NGC 3109, a patchy spiral somewhat smaller than the Milky Way. There’s some evidence the two have recently interacted in some way. The disk of the bigger galaxy is warped, which can happen when galaxies poke at each other gravitationally, for one. For another, both galaxies have neutral hydrogen gas in them — not visible in either picture here, since you need radio telescopes to see it in this case — and in NGC 3109 there is an extended plume of gas pointing more or less toward the Antlia galaxy, and moving at the same speed as Antlia. That’s pretty suspicious indeed, and the simplest explanation is they had a close encounter maybe a billion years ago.

It’s interesting that the encounter happened so long ago, but Antlia had an episode of star formation 100 million years ago. Sometimes galaxy collisions can trigger star birth, but that delay is long enough to make me think they’re unrelated. So what triggered the star birth?

Beats me. I did a literature search — I got kinda carried away looking into this galaxy while writing this post — and there’s very little work that’s been done on it. I’m not surprised; Antlia’s a recent discovery and faint. Maybe over time more astronomers will observe it, and we’ll get a better picture of this nearby diminutive galaxy, and get a better idea of its history.

Images credits: Antlia: ESA/Hubble & NASA; NGC 3109: Canada-France-Hawaii Telescope / Coelum

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CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (14)

Links to this Post

  1. Meet I Zw 18: A Dwarf Galaxy | Wired Cosmos | June 17, 2012
  1. zatytom

    What makes something a small galaxy, rather than a large cluster? Is it an arbitrary cutoff or is there a natural category difference?

  2. Messier Tidy Upper

    This is a galaxy?

    No, this is an image containing a number of galaxies – many distant spirals seen against and through that fascinating little Antlia dwarf! 😉 [/slightly facetiously.]

    Antlia may be a member of the Local Group, ..

    If it wasn’t would we be able to see it still? 😉
    In enough detail to study much at all? How much further would this Antlia dwarf have to be before we lost sight of it?

    Interestingly, there’s another galaxy near Antlia: NGC 3109

    How near both as seen in our skies and in real spatial terms please BA?

    EDIT : Should’ve checked the link first shouldn’t I? That notes that :

    Aparicio et al. (1997) calculate the physical separation of NGC 3109 and
    Antlia to be between 29 and 180 kpc, with a maximum separation of 37 kpc for the pair to be gravitationally bound.

    Sorry. Haven’t seen their separation as seen in our skies yet but it may be in there too – will have to leave that for tomorrow though. Half asleep already.

    Oh & great photo too – but what happened to the “Monday galaxy” idea? Not that I object to them anyday of the week, just wondering. :-)

  3. @zatytom I was about to ask the same thing.

  4. @1. zatytom – February 28th, 2012 at 7:27 am :

    What makes something a small galaxy, rather than a large cluster? Is it an arbitrary cutoff or is there a natural category difference?

    Good question – & one that Omega Centari, largest globular in the Milky Way’s collection and possible ex-dwarf galaxy, also raises.

    See :


    among other places – including this blog click onmy name for one example of that – for more on that hypothesis.

    Plus see :




    for info on Antlia one of the smallest and least known of the 88 official constellations.

    Personally, I’d rather have Felis instead! 😉

  5. With that heading, now I have the song by M|A|R|R|S running through my head.

  6. Steve J

    According to a quick search, galaxies have a dark matter halo, whereas clusters don’t.

  7. Digitalaxis

    Another item I’ve found is that galaxies have multiple star formation events, globular clusters don’t. Again, this suggests Omega Centauri (and others, as more accurate measurements are made) are actually dwarf spheroidal galaxies, not globular clusters.

  8. Methinks that I understand this system now. A ragtag fugitive fleet, having made a pit-stop a billion years ago to fill up on gas.

  9. TMB

    I’d argue it’s a dwarf spheroid, not a dwarf elliptical… too diffuse. :)

    As for the galaxy/cluster question… that’s a topic of very active research! Dark matter is definitely the biggest difference, but it’s not always easy to tell for the very small galaxies because the stars are moving so slowly that the difference between how fast they would move with or without dark matter isn’t that big compared to systematics like what kinds of orbits the stars are on (radial vs. tangential).

    Multiple stellar populations also isn’t distinguishing… there is actually evidence for multiple generations in a number of clusters, not all of which are so suspiciously ex-galaxies as omega Cen, and models suggest that it should be common for the most massive clusters.

    So there certainly are objects that get argued about. Which leads to an interesting situation where the names of some objects don’t make sense – newly-found globular clusters are numbered (in Arabic) by the survey they come from, while newly-found dwarf galaxies are numbered (in Roman) by the constellation they appear in. But some objects that we now think are galaxies, like Segue 1 and Willman 1, were thought to be globular clusters when they were first found.

  10. @5. Jim Saul : “With that heading, now I have the song by M|A|R|R|S running through my head.”

    You mean this one :


    complete with awesome astronomical space exploration videoclip?

    Youtubve also has a couple of other good clips Omega Centauri~wise like this mellow & informative animation of a HST image :


    Plus this one :


    from the Hubblecast series- and there’s also one from the IR-relevant Astronomy series the BA has posted a few installments of before linked to my name here. :-)

  11. Keith Bowden

    I have the second major Doctor Who theme running through my head now… I’m waiting to see if Tom Baker, Peter Davidson or Colin Baker’s head appears in the stars… :)

  12. Messier Tidy Upper

    Thinking of this :

    How much further would this Antlia dwarf have to be before we lost sight of it?

    There’s an interesting article here on the Starts with a Bang blog :


    on an new record breakingly distant dwarf galaxy – viewed from across a whopping ten billion (yes, that’s with a ‘b’) light years of spacetime! 😮

    However, it does cheat somewhat by being gravitationally lensed! 😉

    Hope this is okay netiquette~wise, please let me know if not.

    Off topic, sorry but I think this news item :


    is one the BA and others here may appreciate if they haven’t heard /seen it already.

    Finally, on the other major bolidal threat from the skies, this item :


    suggests some comets may be alien to solar system and have originated round other stars.

    PS. BA, the “older posts” tag that lets you go back through previous days posts beyond the current main page seems to have vanished from the end of the main page here. Not sure what’s happening with that, thought I’d just let you know. :-(

  13. Matt B.

    Ha! I almost missed the pun, Antlia being the Air Pump, but I noticed that fact due to looking in a spreadsheet I made that allows me to tell roughly where a constellation is, using the Cartesian coordinates of its alpha star.

    I divided the celestial sphere into 24 equal sectors, by dividing the octants into thirds along the perpendicular bisectors of their sides.

    The octants are named by taking the direction of the vernal equinox as the positive x axis, and the north celestial pole as the positive z axis, which I presume is standard among astronomers. Then a positive component becomes 0 and a negative one becomes 1, and they’re put in order “zyx” to form a binary number, translated into a decimal from 0 to 7. (So the low numbers are in the northern hemisphere and the high numbers are in the southern hemisphere.) I would have numbered them differently, but this system is the easiest to calculate in a spreadsheet. The divisions of an octant are named after which component is largest–x, y, or z.

    Alpha Antliae is in sector 5x. Therefore it’s “southwest” of the vernal equinox point.


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