By Phil Plait | August 11, 2011 11:30 am

450 million light years away are two interacting galaxies. Both spirals, they are caught in each other’s gravitational claws. Already distorted and bound, eventually, to merge into one larger galaxy in a few million years, the view we have of them from Earth is both amazing and lovely… and hey: they’re punctuating their own predicament!

[Click to exclamatenate.]

Looking a lot like an exclamation point, the two galaxies together are called Arp 302 (or VV 340). This image is a combination of pictures from the Chandra X-Ray Observatory (purple) and Hubble (red, green, and blue). The bottom galaxy is a face-on spiral, while the upper one is seen more edge-on, giving the pair their typographical appearance.

They’re pretty nifty even if it weren’t for the funny coincidence of shape. The upper galaxy has a supermassive black hole in its core that’s actively feeding, but is obscured by thick layers of dust — the abundance of dust is clear in the upper picture, where in optical light it blocks the brighter material behind it (and the warped appearance is a dead giveaway the two galaxies are interacting; the plane of a disk galaxy gets distorted that way when affected by the gravity of another). That dust warms up, making the galaxy boom out infrared. The inset image is a combo of IR (shown in red) from Spitzer and ultraviolet (blue) by Galex, two other orbiting observatories. You can see how the core of the upper galaxy is bright in the IR, but the whole galaxy is strongly emitting, too.

The bottom galaxy, on the other hand, is much brighter in the UV, indicating it has a lot of active star formation, while the upper one doesn’t. Clearly, these are very different galaxies. Why? Perhaps they are different ages — older galaxies tend to have less star formation, so maybe the bottom one is younger. Maybe it simply has more gas in it with which to form stars. It’s hard to say. But studying such objects in multiple wavelengths, as was done here, provides critical clues to how they behave and why they do what they do.

Which is all well and good, but I still love how much it’s shaped like an exclamation point. Too bad it’s not part of the Comma Cluster!

Image credits: X-ray: NASA/CXC/IfA/D. Sanders et al; Optical: NASA/STScI/NRAO/A. Evans et al.; IR/UV: NASA/JPL-Caltech/J.Mazzarella et al.

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Collision of past and present
Evidence and theory collide with galactic proportions

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (36)

Links to this Post

  1. Beige Abc Blog | August 12, 2011
  2. Enlaces de Astronom√≠a: Semana 08-08-2011 « Campos de Estrellas | August 14, 2011
  3. Galactic Exclamation Point | August 14, 2011
  1. “exclamatenate” isn’t even a word. And I thought this was a political blog. What’s this astronomy stuff????

    Oh… and nice photo. Kind of makes me wish I could visit there.

  2. Joel

    I wonder if we can ever hope for a pair of galaxies distorted into a question mark?

  3. misha
  4. Dave

    How close are those galaxies? If we had a galaxy that close to us, how big would it look in the night sky? I can only imagine how amazing a sight that would be!

  5. Jamie

    I take the diffraction spikes on the star between them to mean that that happens to be a local star and not a lucky capture of a super nova or something generated by the collision?

  6. The interacting-galaxy pair MCG -325=19 is a pretty good question mark if inverted N-S. (Click my name for image link).

  7. Mr. C


    As a matter of fact, we do have galaxies that close to us, which look like this. The LMC and SMC are Dwarf galaxies, mind you, but still galaxies. Those are pretty interesting in themselves, but a full-on oblique spiral would be spectacular, I agree.

  8. Rick J

    How long before they might become 1? I am patiently waiting….he he

  9. Hey Phil: I’ve a quick question which this — and other posts — have sparked: when the combined-observatory images are produced, how is this done? Are they images taken at the same point in time — in which case presumably there is some parallax to be dealt with? Or are they taken at different points in time from more-or-less in the same location? And is the alignment for combining just a manually roughed process, or computed from a number of points?

  10. alfaniner

    I understand there are several galaxies to the left that spell out “holy” and “haleakala”.

  11. DrFlimmer

    @ #4 Dave

    That made me thinking for a moment:
    Since we are blurring out our own galaxy to near invisibility by our light pollution, near urban areas one would see almost nothing of the other galaxy.
    In a dark spot, however, the view must be fantastic. Probably one would see the band of one’s own galaxy, while the other one is medium bright diffusely glowing disk. I think this would, indeed, look spectacular: A diffuse disk overlaid with a relatively bright band of stars and dust. COOOOL!!!!

  12. Arthur Maruyama

    @ #8 Stuart Brown

    Parallax is useful/a problem only for very close objects in the sky. Even using the Earth’s orbit as the baseline astronomers can only directly calculate via parallax the distance to stars within about 200 light years, so any problems with parallax for objects 450 million light years from Earth can be all but ignored even for space telescope pictures taken many years apart in time.

    Hipparcos–an ESA mission launched in 1989–increased the range of parallax-derived distances to about 1600 light years, while Gaia–another ESA mission set to launch in 2013–is hoped to increase that range to tens of thousands of light years (with a 20% error at the extreme range).

  13. @ #12 Arthur Maruyama: Thanks. I suspect I have an exaggerated idea of the extent of parallax due to the fact that when I was child, and in the car with my mother driving, any suggestion to her that the speedometer needle was hovering rather higher than the speed limit was immediately refuted with the declaration that that was just parallax, and one needed to be sat in the driver’s seat to see the correct alignment. I suppose it introduced me to an important scientific concept whilst very young, but maybe mislead me as to the extent to which it applies.

  14. fastpathguru


    Parallax is ridiculously negligible for pretty much anything extra-solar.

    For some perspective, compare the diameter of the Earth’s orbit around the sun (0.00003 light years) to the distance between us and those galaxies (450,000,000 light-years)…

    (whoops, Arthur beat me to it. :)

  15. Sawdust Sam

    A super massive black hole in one galaxy and, presumably, another black hole in the other. Is it inevitable that they will collide? And what will be the result – one extra super massive black hole? And will the collision be impressive?

  16. Greg Dorais

    RE: 1. CafeenMan:

    “Exclamatenate” is a perfectly cromulent word.

  17. Chief

    You have to wonder how many solar masses will wind up in the black holes due to gravitational disturbance from the merging. It will most likely be the areas already near the center of the galaxies that will be affected but hope that no stars with inhabited words are wiped out. With the proximity of the solar matter, I can see where a lot of the cast off planets without a star come from. Our own galaxy has lots of the planet wanderers.

    Great picture and wonder if someone could build up a pic of what a similar view would look like with a similar galaxy hanging above the Milky Way.

  18. I have a question – what effect would a collision like this have, for example, on us as living beings? Would it affect us at all? I imagine the distances involved are so big anyway that from the point of view of living beings it may not make a difference if our galaxy merged with another one. Is this correct?

  19. Naked Bunny with a Whip

    And I thought this was a political blog.

    If it makes you feel more comfortable, just imagine that one of those galaxies is a collision denialist.

  20. Naked Bunny with a Whip

    @zzamboni: Phil discusses this briefly in his book.

  21. Gary Ansorge

    18. zzamboni

    Galaxies, like atoms, are 99% empty space(well, ALMOST empty. Just a bit of gas, dust and the occasional solar system). When the Milky Way collides with Andromeda(about 2 or 3 billion years or so), about all we’d see would be an increase in star formation(something which goes on all the time) and some clouds of interstellar gas/dust glowing from hard UV radiation.

    ,,,anyway, the effects on US would likely be minimal to zero(unless a nearby gas cloud turned into a massive star that then went super nova).
    Gary 7

  22. James

    Yay! New desktop!

  23. Jack M.

    Clever title of this blog post. I don’t think it’s common knowledge that “bang” is slang for “exclamation point.”

  24. Messier Tidy Upper

    @^ Jack M. I thought “bang” was slang for soemthing else? ūüėČ

    Exclaimation mark (resembling) worthy indeed. 8)

    Awesome superbly magnificent image here. Thanks. :-)

  25. Kassul

    Have observations of these & similar colliding galaxies helped eliminate possible dark matter candidates? The existance of [this many/number/type of] MACHOs for instance would predict that [blah blah etc wooo] would be seen when you had two galaxies merge together, and we don’t observe that, so MACHOs can/can’t be of [whatever type/size/qty]?

  26. Clive DuPort

    Beautiful picture.

    So, 450 million light-years away & will take a few million years to merge. Consequently the coming together occured over 400 million years ago. When’s the next bus so I can go & have a look?

  27. Aubri

    That’s a really…
    *puts on dark glasses*
    big bang.

  28. fastpathguru


    On a fast-enough spaceship, flying towards the pair of galaxies would A) close the time-gap between you and it, fast-forwarding the merger process from your perspective, and B) time-dilation from moving near-light-speed would also have you “aging” more slowly, further fast-forwarding the process…

    That would make for one really cool youtube video.

  29. I thought only Linux nerds called the exclamation point “bang”.

    Not that I would know anything about that….

  30. Brian

    A few million years? That sounds suspicious to me. Assuming these galaxies are about 100,000 light years apart (which is roughly the diameter of the milky way’s main disk), moving that far in a few million years would require the galaxies to be falling towards each other at several percent of the speed of light.

    Did you mean a few billion years?

  31. Thespis

    #29 ShoeShine Boy: And people who used a typewriter or know Printer’s Slang (or worked with typesetting in the murky past…)

    For even MORE fun, look up interrobang. That’s a *good* Scrabble word.
    Or go here for the transcript of the TIME magazine article from 1967.,9171,941558,00.html

  32. @34 Thespis: Thanks for the info. Now, where is the interrobang key on my keyboard? :)

  33. mike

    Everywhere we look in the sky we see galaxies.

    The number of places possible for life simply boggles the mind.

    At least it boggles my mind.


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