Astrophysics and opinions.
Opinions are often someone else's, but most likely not my employers'.
Toots in 🇬🇧 🇩🇰 🇸🇪
Paper day!
The latest article led by Yours Truly is live on Astronomy & Astrophysics @aanda@social.oalm.gub.uy today.
Buckle up, it's 🧵 time! 1/
#astronomy #astrophysics #science
https://www.aanda.org/articles/aa/full_html/2026/03/aa58750-25/aa58750-25.html
That's it! Thank you for seeing me through to the end of this journey.
This work was done together with a crew of brilliant and talented people, including but not limited to,
@Kerens and @janerigby
plus a number of folks not present in the Fediverse.
Bye for now! 👋
23/23
@dennix Yes, exactly. The light is affected by the expansion, but the hydrogen absorbs at the same old wavelengths. To our luck!
@dennix
The reason why Alex Le Reste's radio observations of her galaxy, Haro 11, were so cool is that it is at the very far end of what we can observe with radio telescopes, and at the same time the closest known ionizing emitter.
A bit further away, and we couldn't see it with present-day radio telescopes; a bit closer, and its ionizing light would have been all blocked inside the Milky Way. So her work is truly unique!
@dennix
Galaxies that are too close are indeed invisible in ionizing light.
A bit further out, a narrow window of LyC light becomes visible, and that window widens as we move further away.
LACES104037 from this paper is at a redshift of 3, meaning that its light has been stretched by a factor of 4, so it has no problem with the Milky Way interstellar gas.
Ah, of course! Good point! And you are right, the Milky Way neutral ISM does indeed block ionizing radiation from outside!
But because of cosmological redshift - the effect that light has its wavelength stretched because of the expansion of the Universe - if we look far enough away, some of the ionizing light gets redshifted to longer, non-ionizing wavelengths, that *can* penetrate the neutral ISM in the Milky Way.
Btw, here is the link to Le Reste's paper with the so far only mapping of neutral hydrogen in a LyC emitting galaxy:
@dennix In the left panel of #7, the neutral gas is perforated and the LyC gets out. In the right panel, it is blocked because the neutral gas completely covers it.
Yes, that blocks it both way, for sure! But the ionizing light is produced inside the galaxy, so usually, what little comes from other, neighboring galaxies, doesn't add up to much in comparison.
JWST on the other hand is a dead ringer for looking at this kind of object -
I myself and a band of collaborators (strong overlap with the authors of this paper!) have a proposed program under review at the moment (Please pick it! Please pick it!), aiming to look at a couple handfuls of candidate objects like this.
🤞
@zrb Not sure, but if I should give an educated guesstimate I would say "barely". Roman has the same light gathering area as Hubble so also comparable sensitivity. Hubble could certainly see this galaxy, as you can see in the thread; but it took a long time to get the images we see here!
I'm not sure Roman will dedicate that kind of exposure times to a single patch of the sky. Maybe? But Roman is more for industry scale observations of slightly brighter objects.
And if tidal stripping + star formation in tidal bridges is as important as we think for this escape of ionizing light, 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 ionizing leakers do today - we could miss a lot of them that way! 22/
If galaxy interactions are really as important as we think for this ionizing escape, that could help explain how so much more LyC light escaped in the early times of the Universe then does today - galaxy mergers and interactions were much more common in those times than they are now. 21/
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. 20/
If we look at gas and stars side by side, we can see that there isn't a lot of gas by the "spike" of stars where the ionizing gas escapes.
We can also see a bridge of gas as a vague green strand between the two galaxies. The strand of starlight, from which the ionizing light escapes, is not at the same place as the gas! That is normal for galaxy interactions and is exactly **why** the ionizing light can escape so easily from there. 19/
We used publicly available data from the JWST archives to study how warm ionized is arranged and moves around in these galaxies.
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, which is very typical for galaxies in the early stages of merging.
The right panel shows the brightness of a spectral line shining from warm ionized gas. The circle shows where the ionizing light escapes. 18/
What we found was that the smaller galaxy in the bottom of the green square is not just a chance aligned galaxy, but is physically very close to and 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 we look at here is 11 **billion** years old!) 17/
This suspicion was fully reasonable, by the way!
Foreground galaxies do masquerade as escaping LyC all the time.
But we found that in that case, the escape was legit, and more interesting than usual. 16/
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.
A link to the paper that discovered the ionizing escape is here (free access):
https://iopscience.iop.org/article/10.3847/1538-4357/ab2045
15/
...but if a large clump of young and powerful stars happen to form in such a bridge, they are already removed from most of the neutral gas in the galaxy, and can easily blow or burn the rest away, and their ionizing light get out!
That is exactly what we found!
Here's two Hubble telescope images of the galaxy, the left shows normal starlight, the right shows the ionizing light. Notice how the core of the galaxy is much brighter, but its hydrogen gas completely blocks the ionizing light! 14/
But! What we found in the newly published paper was neither of those, but instead 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 many times before.
(Here's a nearby galaxy pair with a tidal bridge for illustration) 13/
