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That’s NGC 4921, a face-on spiral galaxy in the Coma cluster of galaxies, over 300 million light years away. This Hubble image is a mosaic of 80 separate images, and has stunning clarity and depth. Look at all those background galaxies! You really want to click that and get the bigger version. Trust me here. Wow.
The spiral arms of this galaxy are a bit poorly defined, but that adds to the beauty of this in my opinion. That struck me instantly, and the reason struck me as well: spiral galaxies in clusters are odd ducks. Coma is a rich, dense cluster, with thousands of citizens. That makes collisions common, and spiral disks are relatively fragile. When a spiral collides with another galaxy of comparable mass the disk is disrupted. Whether the two galaxies pass through each other or eventually merge, the disk is usually collaterally damaged or destroyed.
So NGC 4921 is a bit of a freak in that it exists at all. But the cluster itself is filled with tenuous gas, like smog in a city. As galaxies pass through it, the pressure from this gas pushes on the gas inside the galaxy, sweeping it out (like driving with your windows open removes bad odors lingering in your car, possibly also due to gas). In general (though not always!) spirals in rich clusters don’t have lots of gas, and so they can’t form stars. In “field” spirals – galaxies that are on their own in deep space, something like ours – images show them to have strings of intensely reddish concentrated gas clouds vigorously forming new stars, but NGC 4921 is devoid of those. Most of its gas has been swept away by the ram pressure of its travels across the cluster. That makes the spiral arms smooth. But since no stars have been formed in this galaxy for a long, long time [Edited to add: whoa, hold on there. Maybe there has been recent star formation; see this comment and my follow-up below], the galaxy is also predominantly yellow-red, because all the hot young blue stars are long gone, exploded as supernovae shortly after they were born millions of years ago.
Wait, yellow-red? But the galaxy looks blue! Yes, in real life this galaxy should look reddish, but this image is false-color: it was taken using a yellow filter and a near-infrared one. So what you see as bluish in the image is actually yellow or red to the eye. What’s seen as red is actually infrared. Weird, isn’t it?
You can also see a ragged ring of dust circling the galaxy’s core. That’s interesting to me; the gas is gone, but dust remains. Either it’s harder to remove than gas, or it’s a relatively new feature created in stars and blown out into the galaxy.
Image Credit: NASA, ESA and K. Cook (Lawrence Livermore National Laboratory, USA)