Powerful radio pulses from the depths of the cosmos probe hidden matter around galaxies

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This artist’s concept shows distant fast radio bursts piercing the gaseous halos around galaxies in the local universe. Radio bursts are depicted traveling from the distant cosmos, through galactic halos, and eventually reaching telescopes on Earth. The bumps seen in two of the lines represent the exploded radio itself as it heads towards Earth. Credit: Courtesy of Charles Carter

According to a new study published last month in the journal natural astronomy.

So-called fast radio bursts, or FRBs, are pulses of radio waves that typically originate from millions to billions of light-years away. (Radio waves are electromagnetic radiation like the light we see with our eyes but have longer wavelengths and lower frequencies). The first FRB was discovered in 2007, and since then hundreds more have been detected. In 2020, Caltech’s STARE2 (Survey for Transient Astronomical Radio Emission 2) instrument and Canada’s CHIME (Canadian Hydrogen Intensity Mapping Experiment) detected a massive FRB that ignited in our own Milky Way galaxy. These earlier findings helped confirm the theory that energetic events most likely originate from dead, magnetized stars called magnetars.

As more FRBs arrive, scientists are now investigating how they can be used to study the gas that lies between us and the gusts. Specifically, they would like to use FRBs to probe the diffuse gas halos that surround galaxies. As the radio pulses travel towards Earth, the gas enveloping the galaxies should slow the waves and scatter the radio frequencies. In the new study, the research team examined a sample of 474 distant FRBs detected by CHIME, which has discovered the most FRBs to date. They showed that the subset of two dozen FRBs that crossed the galactic halos were indeed slowed down more than the non-crossing FRBs.

“Our study shows that FRBs can act as skewers of all matter between our radio telescopes and the source of the radio waves,” says lead author Liam Connor, a Tolman postdoctoral research associate in astronomy, who works with assistant professor d astronomer and co-author of the study, Vikram Ravi.

“We used fast radio bursts to shine a light through the halos of galaxies near the Milky Way and measure their hidden material,” Connor says.

The study also reports finding more material around galaxies than expected. Specifically, about twice as much gas was found as predicted by theoretical models.

All galaxies are surrounded and fed by huge pools of gas from which they were born. However, the gas is very thin and difficult to detect. “These gas reservoirs are huge. If the human eye could see the spherical halo that surrounds the neighboring Andromeda galaxy, the halo would appear a thousand times larger than the surface moon,” says Connor.

Researchers have developed different techniques to study these hidden halos. For example, Caltech physics professor Christopher Martin and his team have developed an instrument at the WM Keck Observatory called the Keck Cosmic Webb Imager (KCWI) that can probe gas filaments flowing into galaxies from halos.

This new FRB method allows astronomers to measure the total amount of matter in halos. This can be used to help piece together a picture of the growth and evolution of galaxies over cosmic time.

“This is just the beginning,” says Ravi. “As we discover more FRBs, our techniques can be applied to study individual halos of different sizes and in different environments, addressing the unsolved problem of the distribution of matter in the universe.”

In the future, FRB findings should continue to be disseminated. Caltech’s Deep 110-Dish Synoptic Array, or DSA-110, has already detected several FRBs and identified their host galaxies. Funded by the National Science Foundation (NSF), this project is located at Caltech’s Owen Valley Radio Observatory, near Bishop, California. In the coming years, Caltech researchers plan to build an even larger array, the DSA-2000, which will include 2,000 dishes and be the most powerful radio observatory ever built. The DSA-2000, currently being designed with funding from Schmidt Futures and the NSF, will detect and identify the source of thousands of FRBs per year.

Reference: “The Observed Impact of Galactic Halo Gas on Fast Radio Bursts” by Liam Connor and Vikram Ravi, July 4, 2022, natural astronomy.
DOI: 10.1038/s41550-022-01719-7

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