Astronomers got PHANGS

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Astronomy News

It’s a big Universe. Here’s a thing about it.

Astronomers sometimes liken themselves to vampires. The metaphor is obvious enough, since we stay up all night, and like to suck the blood of innocent victims.

Haha, no, not that last part. Probably. But in fact some astronomers do have PHANGS.

As in, Physics at High Angular Resolution in Nearby GalaxieS, an ambitious project to map out the interstellar gas in dozens of nearby galaxies. The “high angular resolution” part means being able to see detail in stuff that’s very small on the sky.

A lot of this gas is forming stars, and emits light in very long wavelengths compared to what our eyes can see: Around a millimeter, which is over a thousand times longer than visible light. To detect that kind of light the astronomers are using the fantabulous Atacama Large Millimeter/submillimeter Array, or ALMA, a collection of dishes high in the Atacama Desert in Chile. They’ve been mapping star-forming gas in galaxies for years.

The science has been great, and is helping astronomers understand details in the process of star birth, something that’s hard to do at other wavelengths. ALMA has exceptional vision and can see how this gas behaves in different galactic environments, especially since this gas is utterly invisible in the kind of light we see. It’s found in cold, dense, and dark molecular clouds, and these can be immense, hundreds of light years in size.

The European Southern Observatory released that lovely image above of the gorgeous spiral galaxy M99, a beauty about 50 million light-years away. It’s a combination of visible light using the Very Large Telescope (shown in blue, green, and red) combined with the ALMA observations (in orange), and you can see the spiral pattern traced in the gas. A lot of that material may eventually form stars, given enough time.

If you look carefully you’ll notice the gas follows the overall shape of the spiral arms in visible light, but not necessarily their location. In general, the cold gas seems to be between the bright blue spiral arms, where the galaxy looks darker. That surprises me: Similar images of other galaxies shows the gas tracing the arms more closely.

Spiral arms look blue because that’s where gas is actively making stars, and many of these stars are massive, luminous, and blue. They don’t live long, so they don’t have time to move all around the galaxy, instead staying near where they’re born. Since that’s in the arms, the arms look blue. That’s also where’d I’d expect to see a lot of the huge, cold molecular clouds as well, and that’s true for lots of galaxies.

Why? I’m not sure. M99 is a little weird; it’s falling into the huge Virgo Cluster of galaxies, and as it does so it experiences drag from the gas between galaxies. This can strip the gas out of a galaxy and affect how it forms stars (this is called ram pressure stripping). One arm of M99 is distorted, possibly from this effect.

I may be misinterpreting this image, but no paper has come out yet that I’ve seen about this galaxy. I’ll keep my eyes open though.

PHANGS is actually mapping these galaxies across the EM spectrum with different telescopes. Hubble observed M99 as part of PHANGS, for example. JWST has been used for PHANGS as well, and those infrared images are beautiful if a little eerie:

That’s NGC 7496, another spiral galaxy about 24 million light years away. In this case you’re seeing the infrared light from warm dust, grains of rocky/sooty material expelled from stars as they die. It looks like a web or skeleton — creepy, either way. You can read more about this image on my old blog at SYFY.

Vampires or no, astronomers can use some PHANGS. Because these galaxies are close we can see them more clearly, and that in turn means seeing details that will help us figure out how stars are born and out that into a galactic context, which is hard to do in our own galaxy. We’re inside it, so getting an overall picture is tough. Seeing these other galaxies like this will hopefully take a bite out of their mystery.

Et alia

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