Why do spiral galaxies have central bulges?
Some are bigger, some smaller, but pretty much every spiral galaxy we see has a roughly spherical puffy bulge of stars in its core (like in the edge-on spiral NGC 4565, shown here to the right). This downtown region of a galactic city is a bit mysterious. It contains old stars, very little gas, lots of dust… and we’re not sure how they form.
But a new observation of a cluster of stars in our Milky Way’s bulging center may have the key we’ve been looking for. Behold Terzan 5:
[Click to galactify, or grab the higher-res 1350 x 1370 version.]
Pretty, isn’t it? My first glance at this image made me think, "Oooh, sweet." My second glance made me think "Hey, wait a sec…" and my third, after reading the scientific paper, made me smile. Terzan 5 is a pretty interesting place.
It’s just over 19,000 light years away, toward the galactic center. That area is lousy with thick patches of dust, making it very difficult to see anything, like trying to see a forest through a thick fog. These images were taken with the Very Large Telescope (srsly), an 8-meter goliath in Chile. The observations were done in the infrared, which can travel more easily through the thick dust — specifically at 1.2 and 2.2 microns (our eyes can see out to about 0.8 microns; anything longer than that is infrared). Amazingly, this image is a total of only four minutes of observations, two minutes in each filter! And while the size of the image is comfortably larger than the full Moon on the sky, the resolution is about 0.1 arcseconds, about that of Hubble! That’s why the second time I glanced at the image I was amazed; the star images are sharp and clear.
Also, see how the stars appear to be redder on the left in the picture of the cluster above, and bluer on the right? That’s not because the stars themselves are different; it’s because the dust between us and the cluster is thicker on the left, making stars appear redder. The astronomers studying Terzan 5 had to account for that when they investigated the stars. The wide-field picture to the right shows you just how hard this can be; the center of our galaxy is a frakkin’ mess. You can see Terzan 5 in the center of this very wide image as the blue glow; everything else is stars and dust obscuring the view. I’ll add that this picture is pretty darn cool all by its lonesome, especially if you download the grossly embiggenated 240 Mb version.
Still, astronomers are clever, and were able to tease solid data out of the observations. And when they did get their results, they were surprised to see two different kinds of stars in the cluster. In the image, Terzan 5 appears to be a globular cluster: a spheroidal ball of stars held together by its own gravity. The Milky Way has well over a hundred globulars orbiting it. But usually, stars in globular clusters are all about the same age, indicating they were all formed at the same time. Terzan 5, however, appears to have two different populations of stars, one older than the other. Moreover, the younger ones appear to be more centrally concentrated in the cluster, with older stars farther out from the center.
That’s pretty weird. One possible way this could happen is if Terzan 5 isn’t really a globular cluster, which form from collapsing clouds of gas around the same time the Milky Way itself did. Maybe instead Terzan 5 is the remnant of a galaxy in and of itself, a small dwarf galaxy that got torn apart by our Milky Way’s gravity. Terzan 5 may have been a snack for our galaxy!
That would explain the two kinds of stars — galaxies, especially dwarfs, typically undergo different epochs of star formation — as well as their different positions inside the cluster. But the weird thing here is that the amount of iron in the stars matches the amount of iron in stars in the bulge of our galaxy. That’s an odd coincidence, if Terzan 5 formed separately from the Milky Way’s bulge.
But maybe, all together, this makes sense. Perhaps Terzan 5 really was once a dwarf galaxy. It fell in to the galactic center and got torn apart (we know that happens, and may explain how galaxies like the Milky Way grow to such large size). If that happens, the stars from the shredded dwarf 5 get tossed out, and become part of the Milky Way’s bulge, which puffs up due to the adding of these stars to its population. The remaining stars form the small ragtag spherical clump which we now see as Terzan 5.
If this is true, then it may be the key we need to understanding why spirals have bulges. They don’t necessarily form at the same time the galaxy itself does, but instead grow over time as the galaxy feeds on smaller, weaker galaxies. [Note that this is related to, but different from, a post I put up last week about globular clusters and galactic bulges.]
This type of galactic archaeology is amazing to me. We see what looks like one kind of object — a globular cluster — which upon closer examination (which itself was pretty tough to do) turns out to be perhaps a totally different kind of object — a half-digested galactic corpse — that itself became part of our own Milky Way, and by the way may also be the missing link we’ve been looking for between how spiral galaxies are born and how they form their central bulges.
All in all, not a bad piece of detective investigating! But of course, for astronomers, it’s all in a day’s night’s work.
Image credits: NGC 4565: Bruce Hugo and Leslie Gaul/Adam Block/NOAO/AURA/NSF; Terzan 5: ESO/F. Ferraro; Region around Terzan 5: ESO/Digitized Sky Survey 2.
November 25th, 2009 at 11:09 am
So if I sit on the couch reading a scientific paper instead of watching the Lion’s lose, am I less of a guy? I honestly think reading the paper will be a lot more fun!
Great post, I enjoyed reading it.
Science kicks @$$!
November 25th, 2009 at 11:16 am
Nice pix!
Anywhere I can get the full sized image tho, the link provided requests a username and password
November 25th, 2009 at 11:18 am
The image links require an ESO login and password. Care to share, Phil?
The message on the login window reads “ESO page for embargoed PRs” or something to that effect.
November 25th, 2009 at 11:48 am
So our galaxy has a bulge in the middle because it sits around eating Dwarf Galaxy brand chips?
November 25th, 2009 at 11:55 am
The “embiggened” version (1355 x 1373; 3234 kB TIFF) of Terzan 5 is available here (Hint: Right click; “Save Link As…” or “Save Target As…”).
November 25th, 2009 at 12:33 pm
It would be interesting to add an approximate count of how many stars Terzan 5 contains. I haven’t been able to find that information elsewhere.
November 25th, 2009 at 12:47 pm
@ TechyDad:
That’s MY excuse!
Very nice article, Dr. BA.
November 25th, 2009 at 12:47 pm
Absolutely fascinating. If I understand this right, we would then expect to find a relative preponderance of stars from “eaten” dwarf galaxies in the central lens of ours? And the galactic arms would have mostly “native” stars, kindled from local gas and dust?
I wonder then: might it be possible to reconstruct the paths some of the eaten dwarfs took as they were pulled towards our galactic center? Are there traces like gas clouds, or regions of stars that might have belonged to the assimilated dwarfs that can be identified and examined to establish possible vectors?
November 25th, 2009 at 2:04 pm
That’s a fascinating hypothesis. I have read that the composition and structure of spiral bulges resembles that of elliptical galaxies – ellipticals tend to be dominated by older population stars as well. Is it possible that the formation of both of these things is similar? Perhaps both are formed by the same processes. Taking this a bit further, most of the largest galaxies tend to be ellipticals – perhaps when large spiral galaxies consume galaxies much smaller then themselves, a central bulge is formed – the total gravitational disruption is insufficient to disrupt the spiral structure. But when galaxies of similar size collide, the gravity is sufficient to completely disrupt the structures of the constituent galaxies, and you get a galaxy that is, essentially, all bulge: an elliptical.
I’m just thinking off the top of my head, of course, but it seems like it might fit. This would also fit the observation that ellipticals are preferentially found in galaxy clusters.
November 25th, 2009 at 3:37 pm
[...] The Milky Way galaxy bulges with cannibalized corpses… [...]
November 25th, 2009 at 5:26 pm
[...] http://blogs.discovermagazine.com/badastronomy/2009/11/25/the-milky-way-bulges-with-cannibalized-cor... [...]
November 25th, 2009 at 6:01 pm
Is that a bulge in your galaxy, or are you just happy to see me?
November 25th, 2009 at 7:20 pm
Good article! As I understand it, galaxy clusters are gravitationally bound. It would be expected that big galaxies eat small ones. The milky way may one day be a part of the bulge in the Andromeda Galaxy, since it is nearly twice our size. The resulting galaxy will be even larger. I am suddenly curious exactly how large the largest galaxy we have observed and calculated is. I am also curious about how Dark energy effects this, in the long term. Hmm, now I have something to think about while digesting turkey.
November 25th, 2009 at 7:45 pm
worlebird: yes, that’s a fairly accurate description of how we think galaxies of different morphologies form. the only other piece is that, once you have a big bulge galaxy, it can accrete further gas and grow more disk (if the conditions are right… if it’s in a cluster, all of the gas is too hot to accrete and so you don’t get any new disks – hence why clusters are full of ellipticals).
wright: there are lots of people looking at detailed kinematics and chemical abundances of stars to try to find “streams” that look like they were torn off of dwarf galaxies. there are some streams that are fairly well established (the most prominent is the one that’s currently coming off of the Sgr dSph, but there are others such as the Arcturus stream – yes, a naked eye star probably came in from one of these dwarf galaxies), but we’d really like to know how many there are and what kinds of stars they have.
[TMB]
November 26th, 2009 at 1:13 am
That is why I’ve been coming here for the past 5 years.
thank you sir.
November 26th, 2009 at 4:24 am
Thanks Phil, a great read, and insightful. You should write a book!
November 26th, 2009 at 6:31 am
Would it be possible that some of these stars have planets orbiting them, and some of these planets harbor life? From an other galaxy? That would be called what: extra extra terrestrials?
Even Voyager (from star trek) never even met aliens from another galaxy…
November 26th, 2009 at 7:17 am
I know why they bulge: too much Thanksgiving turkey.
November 26th, 2009 at 2:08 pm
@Mikel:
I was about to say, I know how the Milky Way feels. I just cannibalized the corpse of a dwarf vertebrate (albeit avian), and now I have a central bulge.
@philippec: Go watch the pilot again.
November 26th, 2009 at 5:44 pm
@Ken_g6: oops. forgot about that caretaker thingy that started the whole thing…
November 26th, 2009 at 9:17 pm
“But usually, stars in globular clusters are all about the same age, indicating they were all formed at the same time.”‘
Well of course? But, surely I’ve missed something.
November 27th, 2009 at 1:50 am
Great scientific detective work and very cool. Thanks for the post!
Dan
November 27th, 2009 at 4:58 pm
I sometimes cannibalize a dwarf galaxy. I particularly like the ones with the soft, gaseous exteriors and the hard, gravity well interiors. It’s a singularity experience I tell you!
Personally I stick to the dwarfs because the big ones leave too many leftovers. You end up eating galactic sandwiches for a week and after a while you can’t even look at a globular cluster anymore.
November 28th, 2009 at 3:32 am
Big size, small resolution!
I’ve seen a 18,000 x 18,000 resolution picture of the Orion nebula and it’s only 27 MB in size!
You can find it at the HST website!
November 28th, 2009 at 9:14 am
Thanks BA – great article & awesome images.
Has anyone else yet mentioned that Omega Centauri like Terzen 5 is believed to perhaps be the stripped down corpse of a former dwarf elliptical satellite now incorporated into the Milky Way? Or that Omega Cen too has been found to have had multiple starbirth episodes and boast populations of differently aged stars? No then I will or, uh, just have!
There may be other globular clusters in this class too …
As for :
But of course, for astronomers, it’s all in a
day’snight’s work.You don’t need the strike-through there – after all there are those solar observers as well y’know!
(The Sun seen in Hydrogen alpha -stunningly red & with prominences bigger than Earth – there’s a must see at least once in your life to add to everyone’s list if they haven’t already!)
Plus we can also observe via other wavelengths than the visible that are not so restricted even incl. neutrinos eg. for SN 1987 a, use satellites incl. from different time zones … & correct me if I’m wrong please but aren’t a lot of the data crunching, image processing and theoretical studies done during daylight hours too?
November 28th, 2009 at 11:36 pm
[...] Zombie Galaxies say (Dust) Graaaains Hello there! If you are new here, you might want to subscribe to the RSS feed for updates on this topic.Powered by WP Greet BoxPhil Plait has a really neat post on the evolution of spiral galactic bulges by means of dwarf cannibalization here. [...]
November 30th, 2009 at 8:45 am
Alright… Now who can explain why there are so many “strings” of stars in that picture? It looks like there are many groups of stars arranged in linear fashion… For example, at the top of the cluster there are several strings of 10 or more stars. Or is it just me?
November 30th, 2009 at 10:31 am
[...] Bad Astronomy, Phil, dives deep, with alliteration, into the origin of bulgy galactic middles. You [...]
December 2nd, 2009 at 7:52 pm
To Jason Nyberg:
The apparent strings of stars need to be analyzed for their actual 3D positions to even know if they are actually in a line. We see a flat representation of a 3D distribution of stars from a single point of reference. Sort of like a Japanese Zen sand garden which are purposely arranged so that you cannot see all of the objects in it from any one point of view.
December 6th, 2009 at 10:41 am
[...] week you can read about the origin of bulgy galactic middles, how the remaining space shuttle missions will finish building the ISS, black hole powered space [...]
May 30th, 2011 at 2:38 pm
[...] galaxy not a disk galaxy also? If he is referring to the Galactic Bulge, this could be formed by cannibalism of other galaxies, and there are spiral galaxies without them. And the winding problem? There are answers, but the [...]
June 27th, 2011 at 2:40 am
[...] it’s creating a (beautiful) mess – while there is good evidence that the milky way is swallowing small galaxies now, without major [...]