Probing Galactic Halo Gas with Fast Radio Bursts

Mysterious blasts of radio waves lasting a tiny fraction of the blink of an eye have provided researchers with a unique insight into the invisible gas of galactic haloes surrounding every galaxy.

An artist’s impression of Fast Radio Burst 181112 travelling through the halo of a galaxy 4 billion light-years from Earth. ( © J. Josephides, Centre for Astrophysics and Supercomputing, Swinburne University of Technology.)

The origins of fast radio bursts (FRBs) — bursts of radio waves from deep space lasting less than a millisecond— may still be a mystery. But, that doesn’t prevent researchers from using them to answer other lingering questions about the Universe.

A fantastic example of the utilization of FRBs is a study recently conducted by a team of researchers at the University of California Sant Cruz. The team used FRBs to probe the diffuse gas that surrounds galaxies within a larger galactic halo — also comprised of dark matter.

Despite being vast in size — extending way beyond the concentration of stars that comprise the luminous part of a galaxy and accounting for more of the galaxy’s mass— this halo gas has proved difficult to study as it is nearly impossible to see.

“The primary challenge is the gas is too diffuse to emit light brightly,” J. Xavier Prochaska, professor of astronomy and astrophysics at UC Santa Cruz tells me. “With a few exceptions, halo gas is invisible to even our largest telescopes.”

“The study of halo gas is critical to learning how galaxies like our own form. Full stop.”

Enter the FRB discovered in November 2018 — designated FRB 181112. Like other FRBs, FRB 181112 has been traced back to its origin galaxy, but the phenomenon that produces it is still unknown. After initially being observed and localized by the Australian Square Array Pathfinder (ASKAP) radio telescope, further investigations were able to identify not just the galaxy from which it was emitted, but another galaxy in-front of it along our line of sight. It was this intercepting galaxy the team were able to investigate.

“The signal from the fast radio burst exposed the nature of the magnetic field around the galaxy and the structure of the halo gas,” Professor Prochaska, lead author of the paper published in the journal Science, continues.

When the team overlaid the radio and optical images, they were able to determine that FRB 181112 had penetrated the galactic halo of the foreground galaxy providing — for the first time — a direct way to investigate matter, that is for all-purposes, invisible, adds Cherie Day of Swinburne University, coauthor of the study.

This investigation provided the team with a few surprises.

Did the team’s results match their expectations, I ask Prochaska. He answers with a smile: “Nope!”

The image of a tranquil galaxy

Contrary to expectations, the results of the study indicate that the gas in galactic halo of the intervening galaxy possesses a very low density and an almost feeble magnetic field. This runs contrary to the team’s previous work which had revealed almost every galaxy surrounded by a diffuse, relatively cool (around 10⁴K) gas, Prochaska tells me.

“This data can also be used to infer gas density, with most estimates on the high side,” Prochaska continues. “Our new measurements on a single galaxy with this FRB observation give a density and magnetic field strength which are both lower than previous estimates.”

A tranquil scene: CSIRO’S ASKAP Radio telescope detects a FRB (Swinburne University of Technology)

The team was able to determine this relatively tranquil state from the fact that the radio signal was largely unperturbed by its passage through the galaxy. “This is in stark contrast to what previous models predict would happen to the FRB,” the astrophysicist clarifies.

As the signal of FRB 181112 consists of several pulses, each of which lasts less than 40 microseconds — ten thousand times shorter than the blink of an eye — an upper limit is placed on the density of the gas it passes through. This is because passage through a denser medium would lengthen the signal’s arrival time — in turn, lengthening its duration.

This is analogous to the air shimmering on a hot summer’s day, points out co-author Jean-Pierre Macquart, an astronomer at the International Centre for Radio Astronomy Research at Curtin University, Australia.

This lack of distortion in the radio signal indicates that the halo gas must have a density of less than a tenth of an atom per cubic cm. That’s roughly equivalent to a party balloon containing only a few hundred atoms. To grasp just how diffuse this is, consider that a normal, fully inflated balloon can contain roughly 4 x 10²⁵ helium atoms — that’s 4 followed by 25 zeroes.

Another consequence of the team’s results is the implication that turbulence or clouds and clumps of cool gas within the halo are severely constrained. “One favoured model is that halos are pervaded by clouds of clumpy gas,” says Prochaska.

“But, we found no evidence of these clouds whatsoever.”

The use of FRB signals also enables the team to study the magnetic field within the halo as magnetic fields should polarize the radio wave. The team found that the polarization was extremely low too, indicating the magnetic field in the galactic halo is approximately a billion times weaker than that of a refrigerator magnet.

Prochaska points out that further study of halo gas in other galaxies is required before solid conclusions about galaxies in general, can be drawn: “This galaxy may be special. We will need to use FRBs to study tens or hundreds of galaxies over a range of masses and ages to assess the full population.”

Gas haloes and galactic evolution

The halo that surrounds the stars and luminous matter of a galaxy is comprised of a halo of dark matter and a smaller halo of hot ionized gas. Whilst the luminous part of a massive galaxy maybe 3 x 10⁴ light-years across, the surrounding halo is ten times larger and is where the majority of a galaxy’s mass resides.

Galactic structure with the galactic disc surrounded by a halo of gas and dark matter extending ten times beyond the reach of its luminous components (©Pearson Education)

It is these gas haloes that are the fuel for star formation as they fall toward the galactic centre, whilst simultaneously acting as a receptacle for matter ejected from star-forming regions by powerful supernova explosions — which can, in turn, are believed to halt star-formation.

As such, understanding this halo, and the gas it is comprised of is of the utmost importance if we are to unlock the secrets of galactic evolution.

“When you look at an image of a lovely galaxy, you have to imagine that system was ten times more to it in both mass and extent than you can see,” Prochaska posits.

“And that’s before we even talk about dark matter.”

The team hope that these measurements, and similar ones yet to be developed, will help astrophysicists build more accurate physical models of how gas haloes are supported, distributed and formed.

This is especially important when we consider that star formation has ceased in many galaxies — they are “red and dead” in Prochaska’s words. Thus understanding how this star formation was quenched is vital, and gas haloes are key to this investigation.