A galaxy that's all hat and no head

By Phil Plait | February 4, 2011 6:30 am

Hmph. I’m no genius, and I know there’s lots of astronomy-related things I don’t know that much about. But what surprises me is that there still are complete surprises for me… like a type of galaxy I’ve never heard of!

So here’s NGC 3621, as seen by the 2.2 meter MPG/ESO telescope in La Silla, Chile:

[Click to galactinate to the 3500 x 3100 pixel version.]

Pretty cool, right? This is a near true-color image, using three filters that come close to mimicking the eye’s blue, yellow-green, and red sensitivity, as well as a filter that selects the light from warm hydrogen gas (shown as pinkish-red). As usual, that last bit shows where stars are actively being born.

At 22 million light years away, NGC 3621 looks like your usual big spiral galaxy: flat disk, arms sweeping out majestically, central bulge… hey, hold on there a second. Where’s the central bulge?

Turns out, this galaxy doesn’t really have one. There’s a brightening to the center, sure, but no actual spheroidal region of old stars like in most spirals. That’s weird, and something I hadn’t heard of before! A galaxy that’s all disk.

Spirals can have all manners of central bulges. Andromeda, for example, has a nice puffy one. The Milky Way has a compact core but has a rectangular bar going across it. Some have huge bulges, some tiny. But I thought there always was one. But that’s not the case.

Turns out, current thinking is that central bulges in galaxies grow as galaxies collide. The galaxy itself grows, but gravitational instabilities in the collision lead to a build-up of stars in the center — and that forms the bulge. Apparently, though collisions are what made the Milky Way and Andromeda the giants they are today, NGC 3621 has never suffered through one, or at least not a big one. That’s a bit weird, but apparently more common than previously thought. Certainly more common than I thought, since I would’ve thought there’d be zero like that.

Another odd surprise too is that there’s evidence of a supermassive black hole in the center of this galaxy, and that it’s somewhat active– it’s weakly emitting energy as it gobbles down matter. All big galaxies have big black holes in their hearts (they form together, actually), but in general having an active one is something I would normally associate with galaxies that have cannibalized other galaxies. It doesn’t have to be that way, as NGC 3621 shows us, but still, it’s just another way this galaxy surprised me (even if it’s a mild one).

So look at that: I learned something, and not just anything: something surprising. That’s my favorite kind of thing to learn, too!

Related posts:

Revisiting the Whirlpool
Hubble pokes at a galactic bulge
Hub of beauty
A Swift view of Andromeda

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: ESO, NGC 3621, spiral galaxy

Comments (26)

  1. I like learning. I think I may have a slight addiction to it. Thanks for feeding my addiction.

  2. Jeff J

    “Another odd surprise too is that there’s evidence of a supermassive black hole in the center of this galaxy…”

    There are no dumb questions, right? So here goes: Why is this a surprise? If the galaxy is all disk and maintains an otherwise normal spiral structure, shouldn’t it stand to reason that there is something massive in the middle to replace the mass of the missing central bulge?

  3. Jeff J, I think the part that’s surprising is the other part of that quote:

    “and that it’s somewhat active”

  4. Nigel Depledge

    The BA said:

    Where’s the central bulge?

    The CIA have moved it!!!!


  5. Jeff J — I don’t know the mass of the central black hole in this galaxy, but I suspect it’s tiny compared to the mass of even a very modest central bulge.

    Most (probably “nearly all”) galaxies with a central bulge have a supermassive black hole at the center of the bulge. This includes elliptical galaxies, which are “all bulge”. The observation is that the mass of the black hole is about 0.15% of the mass of the bulge. In other words, while the black hole is very important to stars right near it, it’s completely insignificant to the bulge as a whole, or to the galaxy as a whole.

    So… I suspect that the supermassive black hole here doesn’t really do anything significant in terms of replacing the gravitational effect of a galaxy central bulge. Also, the observation of the correlation between bulge size and black hole size means that a galaxy with a black hole but no central bulge is deviating from that observed correlation. Hence, I’m guessing, Phil’s surprise.

  6. Is this one of those galaxies that ran off on its own or is it part of a cluster and just managed to avoid collisions by sheer luck in a cluster?

  7. chris j.

    time for another game of “find the ring galaxy in the background.” i think i found one in the lower left.

    but check out that barred spiral at bottom center! the bar looks like it touches an otherwise empty ring, and the spiral arms seem to emanate from the ring, not the bar.

  8. Thanks! I was just looking at this galaxy last weekend in my 18″, I noted “slightly brighter at center”. Fun to get this sort of info a few days later.

  9. JLE

    Isn’t NGC 3621 not part of any group of galaxies so its very possible that there is nothing near it for it to collide with? Just wondering, as I could be wrong on this. Nice to learn about new things and see unique things out there. Here’s a thought. IF there was a technological advance civilization out there who had built a telescope capable of seeing planets in another galaxy (yes, highly unlikely) and they were looking at us, they would be looking back 22 million years to the early Miocene epoch on earth. I thing the ape creatures we are descended from didn’t show up until about 15 million years ago but I’m not positive on that.

  10. Acronym Jim

    “Some have huge bulges, some tiny. But I thought there always was one. But that’s not the case.”

    It could simply be that NGC 3621 is a grower, not a shower. Occam’s Razor.

  11. Jamey

    @Chris j. – There’s a couple of nice colliding galaxy pairs, too, and other shapes in there too – it’s a really nice field!

  12. Gary Ansorge

    “Hmph. I’m no genius, and I know there’s lots of astronomy-related things I don’t know that much about. ”

    I’ve never met a Genius that knew everything, so MY question is, how do you know you’re no genius?

    Genius simply means “One who creates something new.” (origin; Latin, tutelary spirit, natural inclinations, from gignere to beget). As such, your blog qualifies you everyday as a genius, since you often say something brand new.

    Genius, like cool, is not an appellation we can claim for ourselves or, in this particular instance, disclaim.

    Gary 7

  13. jimmy Kay

    Is that a bulge in your galaxy or are you just happy to see me? To paraphrase Mae West on 1st meeting Albert Einstein.

  14. Aleina

    The New York Times on March 15th 2009 brought out another sensation. The above noted Chinese official, Mao Kan mentioned that he has obtained more than 1000 secret photographs which reveal not only human footprints but a human dead body on the surface of the moon. It was also stated by the said official that some bones from that dead body was missing. It is believed that the human dead body have been dropped on the moon from alien spaceship and extraterrestrials kept few tissues for research.

  15. breadbox

    “I picked this galaxy out of billions. I didn’t like the other galaxies; they were all too flat.”

  16. Joseph G

    So galaxies bulge more in the middle the more they consume? Huh, they’re just like us 😛

    @#10 Acronym Jim: Thank you!!! I knew there was a joke in there somewhere, but all mine sounded a little “off” :)

  17. RwFlynn

    So I suppose one would call this a spiral disc galaxy?

  18. Messier Tidy Upper

    Great picture, fascinating galaxy, thanks. :-)

  19. Jess Tauber

    Actually this is an artificially created galaxy. Fear the builders.

  20. Keith Bowden

    Breadbox: LOLs!

    “Some have huge bulges, some tiny.”

    Obviously this is a girl galaxy!

  21. For a view of many different shapes and types of galaxy, you may be interested in visiting http://www.galaxyzoo.org

    If you register with the site, you can help classify galaxies in pictures taken by the Hubble telescope, some of which are without a central bulge. Others have a central bar, rather like a simple catherine wheel firework.

  22. Because it’s SCIENCE, scientific method is backed by experimental and theoretical research using hypothesis including reason and mathematics read DEFINITION: a branch of knowledge or study dealing with a body of facts or truths systematically arranged and showing the operation of general laws: the mathematical sciences, systematic knowledge of the physical or material world gained through observation and experimentation.

  23. Did we both go to NED and find the SAME article about it having an AGN? 😉

  24. Dutch

    Off topic. Unknown metal object crashed through roof. Dutch police made some pictures. Any idea? http://www.mobypicture.com/user/politieglm/view/8874951


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