BAN #449: JWST watches galaxies collide and light up fireworks

1 August 2022 Issue #449

August 01, 2022

Blog Jam

[The galaxy GLASS-z13, a candidate for one of the most distant galaxies ever seen. From Yuesday’s article. Credit: Naidu et al. 2022; image composite: Gabriel Brammer (Cosmic Dawn Center, Niels Bohr Institute, University of Copenhagen); raw data: T. Treu (UCLA) and GLASS-JWST ]

Pic o’ the Letter

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

If the name Roberto Colombari sounds familiar, it’s because I’ve written about astronomical images he’s processed many, many times, both here in the BAN and on the blog. His usual modus operandi is combining wide-field ground-based images from big telescopes like Subaru with higher-resolution images from Hubble. That way, for example, you can see the wide splashy arms of a spiral galaxy but also incredible detail in the center of the galaxy as well.

I got an email from him the other day with a new image he’s put together, and I’m always happy to see that! But I was surprised when it wasn’t a slick combo of two different observatories: Instead, he processed only JWST images of a deep-space object. Not that I’m disappointed, because the image is very cool.

[Colliding galaxies IC 1623, seen by JWST’s NIRCAM. Credit: NASA / ESA; Processing: R. Colombari]

Whoa. That’s IC 1623 (also called VV 114), a pair of galaxies something less than 300 million light-years from us. And I know, in today’s BA blog article I compare Hubble and JWST images of spiral galaxies, so this may seem like just more of that, but this is different: These galaxies are colliding!

It’s not common in the life of a big galaxy like the Milky Way, but sometimes it will collide with another big galaxy. Collisions with much smaller ones happen all the time, and in fact the Milky Way is in the process of eating several small galaxies right now! But big ones are more rare, since big galaxies themselves are less common.

We do see lots of galaxy collisions in the sky, because there are so many galaxies out there — even a relatively uncommon event can be seen many times if you have enough objects to study. IC 1623 is an example of that.

These observations were made by astronomer Lee Armus, who is looking to understand better what happens when big galaxies collide and set off a burst of star formation, as well as dumping a lot of gas into the centers of the galaxies where their supermassive black holes can gobble it down. This can create huge winds of subatomic particles that can blow gas and dust out of the galaxy, suppressing star formation. It’s a complicated feedback mechanism that isn’t well understood, so these JWST observations can help.

Most of the JWST observations being made right now are archived and immediately released so that anyone can get them if they want to play with processing them. Colombari grabbed observations from the archive in four filters to make this image: 1.5, 2, 3.56, and 4.44 microns. These are called “general purpose” filters because they let in a wide range of infrared light. Some filters only let through a very narrow slice of wavelengths because that is where some interesting element or molecule emits light. For example, hydrogen strongly emits light at 0.653 microns, so a filter that only lets through that wavelength selects strongly for hydrogen gas.

But really wide filters are good for seeing things like stars, which emit across the electromagnetic spectrum, and dust that can do the same.  Still, these filters do include strong emission from certain things — for example, cold molecular hydrogen gas (H2) emits at 2.1 microns, so the 2-micron filter will see that as well as starlight (generally from cooler, redder stars). Interpreting what you’re seeing can be difficult.

However, what you see as red, from the 4.44-micron filter, is probably from what astronomers call PAHs — polycyclic aromatic hydrocarbons, which are essentially soot: long molecular chains of carbon atoms combined with other elements like hydrogen and oxygen. These are created when stars start to die and turn into red giants or supergiants; this material blows off them in prodigious quantities and litters the galaxy. It’s one form of dust; others include tiny grains of material that are high in silicates (rock) and iron (metal).

Galaxy collisions can draw out long streamers from both galaxies as the gravity of each galaxy affects the other. This can cause gas clouds to collide and collapse, forming stars. Many of these stars are massive, live short lives, turn into red supergiants, and explode all over the course of just a few million years. All those brilliant red clouds are probably marking where this occurred, and you can see they form long strings as expected.

I wondered if Hubble had observed this pair, so I could understand better what we’re seeing here — I’m still not nearly as familiar with infrared views as I am in visible light. Sure enough it did, and I immediately recognized the pair. I searched my own blog, and AHA! I wrote about them last year.

[Hubble image of the colliding galaxies IC 1623. Credit: ESA/Hubble & NASA, R. Chandar; CC BY 4.0]

This image is way easier for me to interpret (I rotated it to match the orientation of the JWST image). The two galaxies are clearly seen, even though they overlap a lot. The pink blobs on the left galaxy are where hydrogen clouds are busily making stars, and you can see the dust lanes in both galaxies — in visible light dust is opaque, so you see it in silhouette. If you compare the two images you can see the long dark dust lanes in the Hubble image correspond to the bright dust in the JWST shots. The dust is warm, so it glows in infrared even as it blocks the light from stars behind it in visible light.

And this is exactly why we observe the same objects with different telescopes that see different kinds of light. The Hubble image is dominated by warm hydrogen gas and massive, luminous blue stars, but the JWST image sees cooler stars like red giants and dust. Together they provide a more complete picture because they show different physical phenomena, and astronomers can use that to understand how these events are connected. How much dust is dumped in the galactic centers? How efficiently are the black holes there eating that material? How strong are the winds that follow? How does this affect star formation across the individual galaxies?

Plenty of answers can be found in images like these, and I’ll be honest: I’m excited to see the papers that start rolling out from JWST observations, just as I was in the 90s to see them coming from Hubble observations. We’re about to learn a lot of very nifty stuff about the Universe.

And we’ll get amazing images like these to go with them.

Et alia

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