Ping @stockholm-uni.bsky.social, you might find this relevant 🙂

Paper day!

The latest article by Yours Truly is live on Astronomy & Astrophysics' web page today.

Buckle up, it's 🧵 time! #Astronomy #Astrophysics #Science

Looney Tunes "That's all Folks" end title card.

That's it! Thanks for staying with me.

This work was done in collaboration with, among others, @alereste.bsky.social, @janerigby.bsky.social, and @jensmelinder.bsky.social. 25/25

And if tidal stripping + star formation in tidal bridges is as important as we think, it would have another consequence: When we look for these LyC leaking galaxies in the early Universe, we shouldn't only look for galaxies that look like leakers do today - we could miss a lot of them that way! 24/

If galaxy interactions are really as important as we think for this ionizing escape, could explain how so much more LyC light escaped in the early times of the Universe - galaxy mergers and interactions were much more common in those times than they are today. 23/

Annotated version of the starlight image of the LACES104037(-S) galaxy pair from previous posts, pointing out where ionizing photons can and cannot escape.

What can we learn from this? We can learn that in this galaxy,
most of the ionized light is produced in the bright center of the galaxy, but it cannot get out from there. The smaller amount which is produced in the tidal bridge has a much easier time getting out.

Panels from the previous figures showing the interacting galaxy pair LACES104037 and LACES104037-S, shown side-by-side. The left panel shows the emission from ionized gas, the right panel shows the emission from star light. A strand of gas is visible between the two galaxies, and a strand of starlight juts out of the upper galaxy towards the lower one.

We can also see a bridge of gas between the two galaxies (a vague green strand between them). There is also a strand of starlight, where the ionizing light escapes, but it is not at the same place as the gas! That is normal for galaxy interactions and why the ionizing light can escape. 21/

JWST observations of the distant interacting galaxy pair LACES104037 and LACES104037-S. One panel shows the redshift of the interstellar gas, while the right panel shows an image of the emission strength from ionized Oxygen. Figure from Rivera-Thorsen et al, 2026.

We used publicly availabe data from JWST to study how warm ionized is moving. In the picture below, the left panel shows how the gas moves - the difference in redshift means the two galaxies move about 500 km/s relative to each other. The circle is where the ionizing light escapes. 20/

What we found was that the smaller galaxy in the bottom of the green square is not just a foreground galaxy, but is actually interacting gravitationally with the main one, and the two are probably going to merge later.
(Which means they already merged long ago, because the light is 11bn years old!)

This suspicion was fully reasonable, by the way!

Interlopers are masquerading as escaping LyC all the times. But we found that in that case, the escape was legit, and more interesting than most cases. 18/

To be clear, what we found wasn't the escaping ionizing light - others had already found that. But they had flagged it as "shaky", because it didn't come from the central part of the galaxy as expected. That made them suspicious it might be a foreground object masquerading as ionizing light. 17/

Side by side hubble telescope images of the distant galaxy LACES-104037. Left panel shows normal visual star light, the right panel shows ionizing LyC light. While most ionizing light is produced in the center of the galaxy, it gets absorbed by neutral hydrogen in there. In the tidal arm, less ionizing light is produced, but there is so little gas that it can easily escape from there without being absorbed. Small zoom-ins are inset in the upper right corner to better show small details.

But if a particularly large clump of young stars form in such a bridge, they are already removed from most of the neutral gas in the galaxy, and can easily blow the rest away and their ionizing light get out. And that is exactly what we found! 16/

Hubble telescope image of the merging galaxy pair Arp 242, with a strand of gas and stars strung out between them, a so called tidal bridge. Image credit: ESA/Hubble & NASA, Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA, J. Dalcanton

Our paper, which just was published yesterday, is about a *third* effect. When galaxies interact, they can draw out long strands of gas between them, called a "tidal bridge". And sometimes, new stars can form in such a tidal bridge - this is all
something that has been observed before.... 15/

Astronomical image of the galaxy Haro 11, showing how its neutral gas has been moved away from where the most stars are by a gravitational interaction. Figure from Le Reste et al, 2025, with annotations by E. Rivera-Thorsen.

This effect was observed for the first time a year ago by my collaborator @alereste.bsky.social. The contours on the image below show where the neutral hydrogen is, the red stuff is hydrogen that is already mostly ionized, and the bright star clusters are where the ionizing light escapes from. 14/

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A *second* way that galaxy interactions can cause LyC escape is by peeling the gas away from where the stars are with the pull of gravity. The stars produce the ionizing light, and if (most of) the absorbing gas is removed from the stars, the ionizing light has a much easier time getting out. 13/

But these are exactly the internal properties that we just found out cannot be the whole story, so this effect is less interesting in this context. We need something that doesn't look like your typical strong-starburst galaxy in the local Universe. 12/

Galaxy interactions can cause ionizing light to leak in a few different ways.

The first one is by sparking very stong star formation, which in turn leads to all the stuff I menationed above: strong stellar emission, stellar winds, and so on. 11/

Hubble Space Telescope image of the interacting galaxy pair Arp 282 with slightly silly annotations by E. Rivera-Thorsen. Image credit: ESA/Hubble & NASA, J. Dalcanton, Dark Energy Survey, Department of Energy (DOE), Cerro Tololo Inter-American Observatory/NoirLab/National Science Foundation/Association of Universities for Research in Astronomy (AURA), Sloan Digital Sky Survey (SDSS); Acknowledgment: J. Schmidt

So what can come from the outside and help ionizing light to escape? Something that was more common in the early Universe? We don't know for sure, but one thing that could be the explanation is galaxy when galaxies either collide or fly by each other and tug at each other with their gravity. 10/

But there's a hitch: Further away, farther back in time (remember that light takes time to travel!), this escape of ionizing light seems to depend less and less on the specifics of the galaxy that it escapes from. This suggests that something else, from the outside, helps it along. 9/

Diagram showing how ionizing light called Lyman Continuum sometimes can escape from a galaxy, and sometimes not. The diagram is from Bingjie Wang's 2022 paper about the same topic, with slight and slightly silly alterations by E. Rivera-Thorsen.

And that can certainly work! In the not-so-distant Universe, most galaxies that leak ionizing light do show signs of this.
Here is a nice little illustration that I have stolen and modified from Bingjie Wang's 2022 article about this. 8/

Most research has focused on how the galaxies can punch holes in the surrounding hydrogen from within. Either by having a lot of very bright young stars that can pour out more ionizing light than the gas can absorb; or if stellar winds or supernovae can push the gas outward and makes cracks in it.

Because this LyC light has been so important for the Universe, of course astronomers are very eager to find out what it is
that regulates whether it can escape or not. Why can it get out of some galaxies, but not others? And why does it seem to get more and more difficult with time?

Annotated Hubble image of Dwarf starburst galaxy Henize 2-10. NASA, ESA, Zachary Schutte (XGI), Amy Reines (XGI); Image Processing: Alyssa Pagan (STScI)

But sometimes, something makes holes in the surrounding hydrogen gas so the ionizing LyC-light can escape. 5/

Annotated Hubble image of Dwarf starburst galaxy Henize 2-10. NASA, ESA, Zachary Schutte (XGI), Amy Reines (XGI); Image Processing: Alyssa Pagan (STScI)

Usually, this ionizing light does not escape easily. Most galaxies are wrapped in a thick blanket of hydrogen gas which is completely transparent to visible light, but catches and absorbs this ionizing light with very high efficiency. 4/

This LyC light is important, because it at somehow it once changed the fundamental properties of ALMOST THE ENTIRE UNIVERSE, and we aren't even sure how it can get out of a galaxy.

#Astronomy

In short: We have used JWST to spy a young cluster of stars that has formed between two galaxies, in a large strand of gas drawn out between them by their mutual gravitational pull.

From there, the star cluster pours out high-energy UV
light called "Lyman continuum" (LyC) into intergalactic Space.

Paper day!

The latest article by Yours Truly is live on Astronomy & Astrophysics' web page today.

Buckle up, it's 🧵 time! #Astronomy #Astrophysics #Science

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