BAN #429: Heels over head for the Cartwheel Galaxy

[Hubble image of NGC 3603. Credit: NASA, ESA, R. O'Connell (UVa), F. Paresce (NIA, Bologna, Italy), E. Young (USRA/Ames Research Center), the WFC3 Science Oversight Committee, and the Hubble Heritage Team (STScI/AURA)]

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Pic o’ the Letter

A cool or lovely or mind-bending astronomical image/video with a short description so you can grok it

I do love to post the lovely image or two of spiral galaxies here, as you may know. But there are other types of galaxies, too. Ellipticals are pretty common, and smaller galaxies tend to be irregulars, kinda sorta messy-looking without a defined shape.

And then there are peculiars. They’re, well, peculiar; they have an obvious shape to them, but it’s weird.

Perhaps the mother of all of them is the Cartwheel galaxy, made famous(er) in a Hubble Space Telescope image:

[The weird Cartwheel Galaxy. Credit: ESA/Hubble & NASA]

I know, right? So odd. The idea is that it was once a spiral galaxy much like the Milky Way, but a smaller galaxy plunged right through the middle of it, generating a gravitational wake behind it that created the ring structure as well as the reddish spiral in the center (and note the inner spiral is connected to the outer ring by gossamer spokes that may be the spiral arms reinstating themselves).

The galaxy to the upper left is called G1, which is also a small spiral, and the redder one is G2. Galaxies look blue due to star formation; the most massive stars are blue and dominate the light we see; they die young so if a galaxy is blue it must still be making stars. Clearly the Cartwheel and G1 are doing so, but G2 is not, though it still has a spiral structure to it.

So which one of these is the culprit, the bullet that shot the Cartwheel through the heart?

Haha, trick question: Neither! There is a third galaxy, called G3, that’s not in this image. It can be seen in wider images, like this one from the Digitized Sky Survey:

[Wide field view of the Cartwheel. G3 is indicated. Credit: Aladin/DSS]

G3 is the more nearly edge-on spiral above and to the left.

I was playing around with different images of this on Aladin, a database that lets you enter coordinates or object names and see different survey images of it, and saw the Galex one and laughed:

[Galex image of the Cartwheel. Credit: Aladin/DSS]

Galex was an ultraviolet imager, so it sees only the bluest objects; note that in the Galex image G2 and the Cartwheel’s inner spiral are gone! Those are red, so they don’t show up. It’s neat what you can and can’t see when you change telescopes and wavelengths. Galex was also a wide-field imager, so the resolution isn’t nearly as high as other telescopes, but the UV sruveys it did of nearby galaxies really helped astronomers understand a lot of phenomena, like, say, starbirth.

Anyway, the reason I bring this up is that the ESO posted a new image of the Cartwheel that also made me chuckle. It’s a before and after; the left image was taken in 2014 using the Very Large Telescope, and the right image was taken in December 2021 with the New Technology Telescope. Can you see the difference?

[The Cartwheel galaxy, seen in 2014 (left) and 2021 (right). Credit: ESO/Inserra et al., Amram et al.]

The resolutions of the two telescopes are different, so it looks a little fuzzier on the right, but if you look closely at about the 7:00 position on the ring you’ll see a new bright star on the later image: a supernova!

If you don’t see it, here’s a short animation fading in between the two images:

The reason I chuckled when I saw this is because I knew right away what kind of supernova it was: a core collapse from a massive star. How did I know? Because massive stars don’t live long before they explode, so they tend to be right where stars form, which is where the galaxy appears blue. The other kind of supernova, where a white dwarf steals matter from a companion star until it gets massive enough to explode, takes billions of years to set up, so they can be anywhere in a galaxy, and are unlikely to be in the relatively small regions where star birth is occurring.

And I was right! SN2021 afdx is a core collapse supernova. So, that was reassuring. I still understand some things.

If you want to read more about the Cartwheel, Wikipedia has a decent if short entry. The website for the Chandra X-Ray Observatory has more, too.

Et alia

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