A plain-language summary of FLIMFLAM DR1: The First Constraints on the Cosmic Baryon Distribution from Eight Fast Radio Burst Sight Lines (Khrykin et al. 2024, The Astrophysical Journal; arXiv).
TL;DR: using eight radio flashes and maps of the galaxies in front of them, we made the first measurement of how the universe’s ordinary matter is split between the space between galaxies and the gas around them.
Context
Ordinary matter is the stuff that makes up stars, planets and people. We know how much of it the universe contains. But stars and the gas we can easily see add up to only a small part of it. The rest is gas so thin and hot that it is almost invisible.
Fast radio bursts, which are flashes of radio waves from distant galaxies, can detect that gas. As a burst travels, the gas it passes through slows its low-pitched radio waves a little more than its high-pitched ones. The size of that lag counts all the gas along the way, visible or not.
So the fast radio bursts told astronomers that the gas is there. But where is it? Some of it floats in the vast spaces between galaxies. Some of it surrounds galaxies in huge clouds. How much is in each?
What did we do?
A burst gives one number for its whole journey, so on its own it cannot answer that. You also need to know what the burst passed through.
Our survey measured the distances to thousands of galaxies in front of eight bursts, using telescopes in Australia, Hawaii and Chile. From those we built a 3D map of the matter around each galaxy in the: where the galaxies are, and where the emptier space between them is. We then asked what split of gas, between galaxies’ clouds and the space between them, best explains the gas counts of all eight bursts at once. This is a logical extension of the work I led on FRB 190608, which I wrote about in this post. Instead of a single sight line, we now have eight, which allows us to measure the average properties of the universe’s gas.
For this work, I led the deeper observations of galaxies close the the FRB path in hopes of catching any faint galaxy whose gas might intersect the path and add a measurable signal.
What did we find?
- About 60% of the universe’s ordinary matter is in the thin gas between galaxies.
- The clouds around galaxies hold roughly half of the gas they would have if every galaxy had kept its full share. The rest was never pulled in, or has been pushed out.
- The home galaxy of a burst, and whatever surrounds the burst’s source, adds a modest amount of gas of its own. We were able to estimate that separately too.
With only eight bursts, all of these numbers still come with wide margins of error.
Why does it matter?
Galaxies do not just collect gas. Exploding stars and giant black holes blow it back out, and how hard they blow is one of the big open questions about how galaxies grow. The amount of gas left around galaxies is a direct record of that tug of war.
This was the first measurement of its kind. It is not yet precise enough to pick between competing theories, but it shows the method works, and more bursts will sharpen it.
Words you’ll see in the paper
- Baryons: ordinary matter.
- Intergalactic medium (IGM): the gas in the space between galaxies.
- Circumgalactic medium (CGM): the cloud of gas around a galaxy.
- Feedback: exploding stars and black holes pushing gas out of galaxies.
- FLIMFLAM: the name of our survey.