Fast radio bursts penetrate gaseous halos around galaxies

Powerful radio pulses from deep universe detect hidden matter around galaxies

This artist’s concept shows distant FRBs penetrating gaseous halos around galaxies in the local universe. Radio bursts are pictured traveling from the distant universe, through the galactic halo, and finally to telescopes on Earth. The bumps seen in the two lines represent the radio blasts themselves as they travel towards Earth.Credit: Courtesy of Charles Carter

Powerful cosmic radio pulses originating from the depths of the universe can be used to study gas cocoons hidden in nearby galaxies, 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 millions to billions of light-years away. (Radio waves are electromagnetic radiation, like light we see with our eyes, but with longer wavelengths and lower frequencies). The first FRB was discovered in 2007, and since then, hundreds more have been discovered. In 2020, Caltech’s STARE2 instrument (Transient Astronomical Radio Emissions Survey 2) and Canada’s CHIME (Canadian Hydrogen Intensity Mapping Experiment) detected a giant FRB occurring in our own galaxy. These early findings help confirm the theory that energetic events most likely originate from dead, magnetized stars called magnetars.

As more and more fast radio bursts appear, scientists are now investigating how to use them to study the gas that lies between us and the burst. Specifically, they hope to use FRBs to detect halos of diffuse gas around galaxies. As radio pulses travel toward Earth, the gas enveloping galaxies is expected to slow the waves and scatter radio frequencies. In the new study, the team looked at a sample of 474 distant FRBs detected by CHIME, which has found the most FRBs to date. They show that a subset of the two dozen FRBs passing through the halo is indeed much slower than the disjoint FRBs.

“Our study shows that FRBs can act as a skewer for everything between our radio telescopes and the source of radio waves,” said lead author Liam Conner, a Tolman postdoctoral researcher in astronomy who is collaborating with an assistant professor of astronomy. Study co-author Vikram Ravi.

“We used fast radio bursts, illuminated by halos of nearby galaxies

Milky Way
The Milky Way is the galaxy that contains Earth, so named because it emerged from Earth. It is a barred spiral galaxy containing an estimated 10-400 billion stars and is between 150,000 and 200,000 light-years in diameter.

“data-gt-translation-attributes=”[{” attribute=””>Milky Way and measure their hidden material,” Connor says.

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

All galaxies are surrounded and fed by massive pools of gas out of which they were born. However, the gas is very thin and hard to detect. “These gaseous reservoirs are enormous. If the human eye could see the spherical halo that surrounds the nearby Andromeda galaxy, the halo would appear one thousand times larger than the moon in area,” Connor says.

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

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

“This is just the start,” 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 how matter is distributed in the universe.”

In the future, the FRB discoveries are expected to continue streaming in. Caltech’s 110-dish Deep 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 have plans to build an even bigger 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 galaxy halo gas on fast radio bursts” by Liam Connor and Vikram Ravi, 4 July 2022, Nature Astronomy.
DOI: 10.1038/s41550-022-01719-7

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