Galaxy forms raptured stars: Left Behind

Contributed by
Sep 20, 2007

Astronomers have found a galaxy forming stars in a long tail behind it. The stars are being left behind as the galaxy plows through a galaxy cluster.

Coooool. The image is a composite of an X-ray observation (purple, from Chandra), an image that detects hydrogen gas (red) and an image of plain ol' optical light (white; red and white from the Southern Astrophysical Research telescope in Chile). The galaxy is a member of a cluster of galaxies called Abell 3627. The galaxies in the cluster all orbit the cluster center of mass, usually at pretty high speed. The cluster has lots of hot gas in between the galaxies, and as they move through it they ram into it. Any gas in the galaxy itself can get stripped off by this ram pressure, where it will merge with the gas between the galaxies.

Any given galaxy plowing through all that crap will leave behind a tail of gas as does. The galaxy in question -- poetically named ESO 137-001 -- is doing just that. The purple in the image shows the gas being heated to millions of degrees by shock waves as the galaxy does the Superman thing through the cluster (in fact, I see what looks a little like a bow front off the galaxy in white light, which is interesting, but may not be real). The tail is about 200,000 light years long-- twice the size of our own Milky Way Galaxy.

But what happens to the gas as it's stripped off? The pressure from the collision with the intergalactic gas will compress it, and when you compress gas, you get stars forming. The galaxy is too far away (220 million light years) to detect individual stars directly. However, as massive stars form from the cooling gas they ionize it, and that can easily be detected: the red color shows that. There are 29 detected areas where the cooler gas is ionized, and these indicate sites of star formation.

We know that these regions are not just hot gas because they are clumped; the superhot X-ray emitting gas is more evenly distributed, and you wouldn't expect the kind of emission seen in red by that kind of gas.

This means that the galaxy is actively forming stars, but they're orphans! They aren't gravitationally bound to the galaxy. Over time, the stars formed will move apart, and are doomed to wander the space between galaxies in the cluster. It's pretty lonely out there, with gaps of hundreds of thousands of light years between galaxies (the nearest star to the Sun is just over 4 light years away).

If planets form around those stars -- and there's no reason I can think of that that wouldn't happen -- then the view would be amazing: hundreds, maybe thousands of galaxies in the sky, glowing like ghosts, apparitions in an otherwise smooth black vista, unbroken by any other stars. If there are a few other stars nearby, perhaps siblings, then that would only punctuate the emptiness of the region.

That would be pretty cool to see, but I think I like our sky better. We get to see galaxies easily enough (like Andromeda, or the Magellanic Clouds, all of which are relatively easy to spot with the unaided eye) and we get stars too. Things are more crowded here, but there's still plenty of room to breathe.

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