Galaxies come in a lot of shapes and sizes: huge ellipticals, big spirals, weird squishy irregulars. There is a sub-class called "dwarf galaxies" which are smaller than usual. We actually think they dominate the Universe by number, but because they have fewer stars – a few billion or so tops, compared to the hundreds of billions of a big one like our Milky Way – they fade rapidly with distance. Only a handful are close enough to study well.
One of these is DDO 190, a nice little dude something like 9 million light years away. That’s close enough to resolve individual stars in the galaxy, as you can see in this really pretty Hubble image of it:
[Click to galactinate, or grab the cosmic 3700 x 2600 pixel version.]
DDO 190 is small, but not tiny: about 15,000 light years across. That’s about 1/6th the size of our galaxy. It’s also well outside our Local group of nearby galaxies (the Andromeda galaxy is less than 3 million light years distant from us, for comparison) and is thought to be part of the M94 galaxy group. But if true it’s fairly isolated even from the others on its team; the nearest neighbor appears to be another dwarf galaxy several million light years away from it.
This image is pretty nifty. For one thing, you can see lots of far more distant background galaxies, some right through DDO 190, which always gives me a kick. But the dwarf galaxy itself has some surprises. The bluish fuzzy regions are clouds of gas lit by young, hot stars. These stars don’t live long (a few million years or so), meaning there’s still some star birth going on in the little guy. That blue patch at the bottom is the brightest of them – it looks a bit like a more distant galaxy, but don’t be fooled.
Interestingly, it has two different populations of stars in it. The younger ones I mentioned (100 million years or younger) tend to be close in to the center, while older ones (4 billion years or more) are located in the outskirts. This is common in dwarf irregular galaxies. The older stars may be showing us what the primeval galaxy looked like, but now a burst of star birth has occurred near the center, making the galaxy look more condensed.
Since the vast majority of galaxies in the Universe are dwarfs like this, we think bigger ones like ours get to their size by gravitationally colliding with and absorbing dwarfs. In fact, we know the Milky Way is eating several right now!
Galaxies are cool, and pretty, and magnificent, but they’re also cannibals. DDO 190 is isolated enough that it may be safe from that fate for quite some time. But the Universe is young, and galaxies patient. In a trillion years or so, we’ll see who has whom over for dinner.
– And the cottonball galaxies shall inherit the Universe
– Hubble grills a confused galaxy
– Obese, gluttonous, and cannibalistic is no way to go through life, son
– Lonely galaxy is lonely. But it ate its friends.
You’d think that with all our fancy equipment and technology, all the nearby galaxies in the Universe would’ve been spotted by now. But it turns out that’s not the case. Some galaxies are very faint — small, with few stars — making them tough to find even when relatively speaking they’re in our neighborhood.
So say hello to our newly-discovered neighbor, UGC 4597!
[Click to galactinate.]
UGC 4597 is a dwarf galaxy. Galaxies like our Milky Way have billions or hundreds of billions of stars, but dwarf galaxies have stars numbering in the millions. That’s why it remained undiscovered until just a few years ago — it turned up in a survey taken in 2008! Astronomers were looking for dwarf companions to the splashy spiral galaxy M81 located about 12 million light years away, and dinky UGC 5497 showed up.
The image above was taken by Hubble in late 2009. Of course, in this shot it looks obvious enough, but this only shows a teeny portion of the sky. Because it’s so close to us, the entire M81 group of galaxies covers an area of the sky something like 20 times the size of the full Moon — thousands of times the size of this diminutive dwarf. That’s how it remained undiscovered for so long.
The image is a combination of two separates shots, one in visible light and one in near-infrared. The stars look very blue, with very few being red. Without a third image taken in bluer light it’s hard to be completely sure, but the color here most likely means that most of these stars are young, created in a wave of star formation a few million years ago. Just above and to the right of center of the core of the galaxy is a reddish patch; I thought initially that might be a gas cloud of some sort, but now I suspect it’s a background galaxy. In the full-res version of the picture you can see dozens of distant galaxies littering the scene, typical for a Hubble picture. They’re most likely hundreds of millions and even billions of light years away, far, far in the background.
That bright star on the right and the fainter one on the left are probably stars in the foreground, in our own galaxy. Sometimes that fact gets me even more than the rich science of the galaxies themselves: the depth of time and space we see in images like this. Nearby objects like local stars, medium-distance objects like neighborhood galaxies, and then mind-crushingly distant galaxies so far away that the light we see from them left when the newest evolutionary invention on Earth were organisms with more than one cell!
Astronomy may be all about looking out into the Universe, but it’s the perspective on ourselves that always stirs my mind.
ESA/Hubble & NASA
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).
So you’re with a friend you haven’t seen in a while, and you’re having a great time. You decide to get a picture to save the memory. The two of you pose together, you snap the picture… only to find some obnoxious person in the background is looking right in the camera and making a face, ruining the shot. You’ve been photobombed.
Now imagine the person photobombing is actually in the foreground, better lit, and eating up half the picture with their arms flung wide right in front of you.
Yeah, galaxies get photobombed too:
[Click to photobombinate.]
Man, if I wanted to study the dwarf galaxy PGC 39058, I’d be pretty ticked at the star HD 106381. What a jerk!
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.