A plain-language summary of Searching for the Sources of Excess Extragalactic Dispersion of FRBs (Simha et al. 2023, The Astrophysical Journal; arXiv).
In one sentence: four bursts had passed through far more gas than their distance could explain, and we found that for some the extra gas was in galaxies along the way, while for others it we did not find much along the way, so it must have been in the burst’s home galaxy or immediate surroundings.
What was the puzzle?
This is along the same lines as my work on FRB 190608, which I wrote about in this post. In that case, we mapped the galaxies along the burst’s path and found that most of the gas it passed through was in the space between galaxies, not around them. That was a first step toward using bursts to weigh the universe’s gas.
On average, the farther a burst has travelled, the more gas it has crossed. While this is true on average,
There are two suspects. Either the burst happened to pass through the gas around galaxies or groups of galaxies on its way here, or it was born inside an unusually thick patch of gas.
What did we do?
We chose four of these rule-breaking bursts. For each one, we used telescopes in Australia and Hawaii to measure the distances to the galaxies in that part of the sky, which gave us a 3D map of everything in front of the burst.
Then we worked out which galaxies and groups of galaxies the burst passed near, and how much gas each one would have added.
What did we find?
It was a split verdict.
- One burst: galaxies and groups along the way explain about two-thirds of the extra gas. This burst simply took a crowded route.
- One burst: things along the way explain part of the extra gas, but not all of it.
- Two bursts: there is little in the way. The extra gas must be at the source, in the burst’s home galaxy or in the immediate surroundings of whatever produced it.
Why does it matter?
If you want to use bursts to weigh the universe’s gas, you need to know which ones give a fair reading and which ones are skewed by where they were born.
The two “at home” cases are interesting in their own right. They tell us that some bursts come from places wrapped in a lot of dense gas, which might be a clue to what makes them.
Words you’ll see in the paper
- Dispersion measure (DM): the count of how much gas a burst passed through.
- Macquart relation: the rule that more distant bursts have crossed more gas.
- Halo: the large, invisible cloud of gas and dark matter that surrounds every galaxy or group of galaxies.
- Progenitor: whatever object produced the burst.