UW News

September 26, 2019

Galaxy found to float in a tranquil sea of halo gas

A graphic showing a fast radio burst leaving its host galaxy and arriving at Earth.

This illustration shows the radio signal from the fast radio burst FRB 181112 passing through the halo of a foreground galaxy on its way toward the telescopes that detected it on Earth.J. Josephides/Swinburne University of Technology

Using one cosmic mystery to probe another, an international team of astronomers has analyzed the signal from a fast radio burst — an enigmatic blast of cosmic radio waves lasting less than a millisecond — to characterize the diffuse gas in the halo of a massive galaxy. Their findings, published online  Sept. 26 in Science, reveal that the massive galaxy sports an unexpectedly quiescent halo, with a very low density and weak magnetic field.

This discovery gave scientists a rare glimpse of galactic halos, which contain clues to how gas feeds onto and is expelled from galaxies. Astronomers think these galactic halos could be more massive than the galaxies themselves, and that studying them could reveal how galaxies form.

“The signal from this fast radio burst went just by the edge of a galaxy, giving us our first glimpse into the structure of the most diffuse halo gas that surrounds galaxies,” said co-author Matthew McQuinn, an assistant professor of astronomy at the University of Washington. “This event demonstrates a technique that is going to transform our understanding of this diffuse gas.”

McQuinn proposed this technique in a 2014 paper published in the Astrophysical Journal Letters.

Vast halos of low-density gas extend far beyond the luminous parts of galaxies where the stars are concentrated. Although this hot, diffuse gas makes up more of a galaxy’s mass than stars, and may make up as much as half of the gas in the universe, it is nearly impossible to see. In November 2018, astronomers detected a fast radio burst that passed through the halo of a massive galaxy on its way toward Earth, allowing them to learn about the nature of the halo gas from this elusive radio signal.

Image of the position of a galaxy that emitted a fast radio burst, and the position of a second galaxy that the fast radio burst passed through on its way to Earth.

Imaging with the Very Large Telescope in Chile shows the host galaxy of the fast radio burst, with the position of the burst depicted by the red ellipses. The brighter galaxy located nearby is in the foreground, and the sight-line to the burst passes through the halo of this foreground galaxy.Prochaska et al., Science, 2019

Astronomers still don’t know what produces fast radio bursts, and only recently could trace some of these very short, very bright radio signals back to the galaxies in which they originated. The November 2018 burst, named FRB 181112, was detected and localized by the instrument that pioneered this technique, Australian Square Kilometre Array Pathfinder, a radio telescope in Western Australia operated by the Commonwealth Scientific and Industrial Research Organisation. Follow-up observations with European Southern Observatory’s Very Large Telescope in Chile identified not only its host galaxy but also a bright galaxy in front of it.

“When we overlaid the radio and optical images, we could see straight away that the fast radio burst pierced the halo of this coincident foreground galaxy and, for the first time, we had a direct way of investigating this otherwise invisible matter surrounding this galaxy,” said co-author Cherie Day at Swinburne University of Technology.

A galactic halo contains both dark matter and baryonic matter — or ordinary matter — which is expected to be mostly hot ionized gas. While the luminous part of a massive galaxy might be around 30,000 light-years across, its roughly spherical halo is ten times larger. Halo gas fuels star formation as it falls in toward the center of the galaxy, while other processes, such as supernova explosions, can eject material out of the star-forming regions and into the galactic halo. One reason astronomers want to study the halo gas is to better understand these ejection processes, which can shut down star formation.

“The halo gas is a fossil record of these ejection processes, so our observations can inform theories about how matter is ejected and how magnetic fields are threaded through galaxies,” said J. Xavier Prochaska, a professor of astronomy and astrophysics at the University of California, Santa Cruz and lead author on the paper.

Contrary to expectations, the results of the new study indicate a very low density and a relatively feeble magnetic field in the halo of this intervening galaxy.

“The radio signal was largely unperturbed by the galaxy, which is in stark contrast to what previous models predict would have happened to the burst,” said Prochaska.

Image of The Australian Square Kilometre Array Pathfinder radio telescope in Western Australia.

The Australian Square Kilometre Array Pathfinder radio telescope in Western Australia.Commonwealth Scientific and Industrial Research Organisation/Alex Cherney

FRB 181112 consisted of several pulses, each lasting less than 40 microseconds — ten thousand times shorter than the blink of an eye. McQuinn helped lead the interpretation of this signal. The short duration of the pulses puts an upper limit on the density of the halo gas, because passage through a denser medium would lengthen the radio signals. The researchers calculated that the density of the halo gas must be less than a tenth of an atom per cubic centimeter, which is equivalent to several hundred atoms in a volume the size of a balloon.

“Like the shimmering air on a hot summer’s day, the tenuous atmosphere in this massive galaxy should warp the signal of the fast radio burst,” said co-author Jean-Pierre Macquart, an astronomer at the International Center for Radio Astronomy Research and associate professor at Curtin University. “Instead we received a pulse so pristine and sharp that there is no signature of this gas at all.”

The density constraints also limit the possibility of turbulence or clouds of relatively cool gas within the halo, which astronomers have theorized might be present in halos.

The FRB signal also yields information about the magnetic field in the halo, which affects the polarization of the radio waves. Analyzing the polarization as a function of frequency gives a “rotation measure” for the halo, which the researchers found to be about a billion times weaker than an ordinary refrigerator magnet, said Prochaska.

Since these results come from only one galactic halo, the team cannot say whether the low density and magnetic field strength are unusual or if previous studies of galactic halos have overestimated these properties. But radio telescopes can use fast radio bursts to study many more galactic halos and resolve their properties.

Additional co-authors are Sunil Simha at UC Santa Cruz; Ryan Shannon, Adam Deller and Chris Flynn at the Swinburne University of Technology; Lachlan Marnoch and Stuart Ryder of Macquarie University; Keith Bannister, Shivani Bhandari, John Bunton, Elizabeth Mahony and Chris Phillips of the Commonwealth Science and Industrial Research Organisation; Rongmon Bordoloi of the North Carolina State University; Hyerin Cho of the Gwangju Institute of Science and Technology; Hao Qiu of the University of Sydney; and Nicolas Tejos of the Pontificia Universidad Católica de Valparaíso. The work was funded by the National Science Foundation, the Australian Research Council and the Pontificia Universidad Católica de Valparaíso.


For more information, contact McQuinn at mcquinn@uw.edu.

Adapted from a release by the University of California, Santa Cruz.