While studying a different galaxy, NC astrophysicist Patrick Treuthardt noticed a ringed galaxy in the corner of the image. His chance discovery turned out to be a rare, double-ringed galaxy 359 million light years from Earth. Read Treuthardt's interview with UNC-TV Science's Frank Graff below.
Patrick Treuthardt, astrophysicist at the NC Museum of Natural Sciences: So the galaxy on the right, this was discovered by Art Hogue, an astronomer in the 1950s. And what makes it unique is there is an orange-ish, central body that looks like what we call an elliptical galaxy. This galaxy is very striking, very circular, very spherical and surrounding it is a bright blue ring that is also very circular, and between the two there is nothing connecting them.
And it’s really not clear how a galaxy like this forms, but Art Hogue was the first to identify this type of galaxy. Since then, astronomers have a few other objects like this and so we call them Hogue type galaxies.
But only a handful, so we estimate only 0.1 percent or less of the observed galaxies we have seen are Hogue type galaxies, so they are extremely rare to find. So my collaborators and I found a Hogue type galaxy, which is the one on the left, which has a catalogue number of PGC1000714.
Frank Graff: How do you go about finding something like this, and the light from this is 359 million light years away? So it is old.
Treuthardt: It is old. The distance is pretty far, the light we are seeing now left the galaxy 359 million years ago.
Graff: So how do you find something like this?
Treuthardt: So, years ago I was a post-doc, and I was working on a project, looking at a galaxy and the surrounding galaxies in a field of sky, and I looked to the side and saw what I thought was an elliptical galaxy. And I thought, “Oh that’s interesting." I wondered if that influenced the galaxy I was looking at. And I looked closer and I noticed what I thought was this blue ring, and I thought, “This is unusual, that might be a Hogue type galaxy.” So I started looking at the literature and I looked online and it seemed nobody had noticed this. And the images we were using were basically long exposure images, so we were able to see more deeply into the sky, which is why the blue ring pops out, it’s really faint, but because of the deep image it pops out.
Graff: So tell me a little more about this.
Treuthardt: So the galaxy on the left, you can see the core, the bulge area that looks circular, and there’s this blue ring surrounding it. So it doesn’t look as fantastic as Hogue’s object. That’s because the image of Hogue’s object was taken by the Hubble Space Telescope. The image on the left, the galaxy we found, was taken by a ground-based telescope, so it’s not nearly as good as the Hubble image.
The image on the right we call a color index map; we just look at the difference in color. We add artificial color to highlight things. But you can see the blue circle around the center, that’s that outer ring.
But what we were able to do is subtract the bulge light, we were able to model the bulge light and subtract it out, and what we found when we took that out, under the light of the bulge, is another, faint, diffuse, red ring. And that’s what makes this galaxy so unique. Not only does it have one ring, it has two rings; two seemingly circular rings surrounding this bulge.
Graff: And there are different ages in the ring. One of the rings is older and one is younger?
Treuthardt: Right, we’re not entirely sure; we’re not quantitatively sure what the age of the inner ring is. We’re getting more data to figure that out.
But qualitatively, we know that the inner ring has to be older, and the reason we know that is the way stars form. When stars form you get blue stars and red stars, And blue stars shine very brightly and burn out very quickly. Red stars don’t shine as bright and last a lot longer. And the outer ring is mostly blue, which means these are mostly blue stars and must have formed very recently. But the secondary inner red is mostly red stars, which means the blue stars have burned out already, only red stars are left, so the inner ring is older than the outer ring.
Frank: When you look at this, the frustrating thing is probably that you don’t know much about it, but the wonderful part is probably—Wow!
Truethardt: Yes, it’s exciting, because we don’t know much about it. So it’s still a puzzle, and it’s an interesting puzzle. So that’s what makes it really exciting, because we still have lot of work to do to know what is going on with this galaxy.
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