Sen—I love a good coincidence. Here on Sen, I’ve been writing for the past few weeks about features of galaxies. For this week I was thinking of talking about the different kinds of galaxies themselves, and then the European Space Agency goes and releases the lovely and odd photo shown above: That’s M33, also known as the Triangulum Galaxy (after the constellation in which we see it).
M33 is part of a small collection of galaxies called the Local Group; the Milky Way and the Andromeda Galaxy (M31) are the two biggest members. M33 takes the bronze for third largest, and—like our home galaxy and the magnificent M31—is a spiral. But unlike its big brothers, M33 is not as tidy. In fact, it’s downright messy; the spiral arms are a wreck, discernible but hardly as well-organized as the “grand design” spirals of the bigger galaxies.
Because the arms are shorter and patchy, we say that M33 is a flocculent galaxy, literally like clumps of cotton or wool. It’s not clear why some galaxies are shaped this way, but it’s likely related to the way star formation is occurring. As I wrote in last week’s article, stars form from huge clouds of gas and dust in galaxies. Spiral “density waves”, like cosmic traffic jams, can cause these clouds to collide and collapse, creating huge bursts of star birth. The brightest stars formed are very luminous, and their locations help define the shape of the spiral arms.
In flocculent galaxies, it seems that these stellar nurseries get stretched apart by the rotation of the galaxy, suppressing the long, sweeping arms we’re used to seeing. But the exact mechanism isn’t well understood, and flocculent galaxies are an area of active study. About a third of all spiral galaxies fall into this category, so whatever is going on, it happens a lot.
The Herschel image of M33 is fascinating. Herschel sees in the far-infrared, well outside what the human eye can detect. The image is a combination of observations taken at wavelengths of 70 microns (blue), 100 microns (green), and 160 microns (red)—bear in mind that the reddest light the human eye can see is only about 0.7 microns, and you’ll get an appreciation for just how far out in the infrared this image is.
At those wavelengths, what you see is what astronomers call warm dust. But to normal humans it’s incredibly cold, just 20-40° above absolute zero! Dust is created when stars are born and when they die, and therefore tends to be near star formation regions, which themselves fall along the spiral arms. That means the dust traces the arms, so images like this are important to understanding the structure of the arms, how they form, and how they evolve over time.
You can see numerous bright knots in the dust, where stars are being born in higher numbers and warm up the dust around them. To the upper left you can see a particularly large clump; that’s NGC 604, a huge site of star birth, one of the largest known and certainly M33’s biggest. It’s 1500 light years across! The famous Orion Nebula in our own galaxy is only about 25 light years in size. NGC 604 is a monster.
I’ve seen this galaxy myself a few times; it rises around sunset in the fall. Even though it’s relatively close by—a bit under three million light years away—it’s a small galaxy, and face-on to us. Its light is spread out, so you need pretty dark skies to spot it. I’ve never seen it naked eye, but in binoculars and small telescopes it presents a faint, fuzzy appearance. The arms are hard to discern, largely because they don’t have that overall structure so many other spirals have. But that hardly makes it of lesser worth! Every galaxy we see is a piece to a grand puzzle, even (perhaps especially) the oddly-shaped examples. Every galaxy in the Universe dances to the tune of physics, and the more we watch the dance, the more we understand the music.