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What Fast Radio Bursts Tell Us About Galaxy Halos

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Illustration showing a distant galaxy emitting a pulse of light that passes through the halo of an intervening galaxy and arrives at the Milky Way.
This artist’s impression represents the path of the fast radio burst FRB 181112 traveling from a distant host galaxy to reach the Earth. Along its way, the burst passes through the halo of an intervening galaxy. [ESO/M. Kornmesser]

In recent years, we’ve recorded hundreds of brief, powerful flashes of radio light originating from outside of our galaxy. In new work, scientists are now leveraging these enigmatic fast radio bursts to learn about the hot gas around galaxies.

An Epic Voyage

FRB

Artist’s conception of a fast radio burst originating in a distant galaxy. [Danielle Futselaar]

Fast radio bursts (FRBs) are intense bursts of radio emission that last only milliseconds. At their source, the light of these powerful eruptions contain as much energy in a single millisecond as the Sun emits across 3 days. But FRBs primarily arise from distant sources that can lie billions of light-years away — so that light has a long journey ahead of it.

To reach us, this emission first passes through the source’s local environment, then through the interstellar medium (ISM) of its host galaxy, and then through that galaxy’s halo. Once free of the galaxy, the light must traverse the intergalactic medium (IGM) — potentially passing through intervening galactic halos — before it eventually enters the circumgalactic environment around the Milky Way. There, it travels through the halo and ISM of the Milky Way and finally arrives at our detectors here on Earth.

This epic voyage is fraught with obstructions: the burst emission encounters clumps of hot, ionized, and turbulent gas that slows its passage and leaves distinct imprints on the signal we eventually see. In a new study led by Stella Ocker (Cornell University), scientists have used these signatures to probe the ionized gas that lies between us and distant FRBs.

FRB dispersion

Schematic illustrating how transient radio signals travel to us. Pulsars (marked by sun symbols) lie in the galaxy, interior to the halo; their signals are affected only by the Milky Way’s interstellar matter. Fast radio bursts (marked by lightning symbol) lie in other galaxies; their signals travel through multiple different regions. [Platts et al. 2020]

Constraints from Bursts and Pulses

Ocker and collaborators combine multiple different diagnostics: 

  • dispersion of bursts, which occurs when different frequencies of light travel at different speeds through intervening gas
  • pulse and angular broadening, or smearing in time and space due to scatter as light travels along multiple different paths through the gas
  • scintillation, or twinkling of a compact source caused by turbulence in the intervening medium

To disentangle the relative contributions of the ionized gas in the different regions of the FRB’s journey, the authors took advantage of data from multiple FRBs along various lines of sight passing through different sections of our galaxy. They combined this information with further constraints from pulsars — pulsing, magnetized neutron stars — that lie within our galaxy, to better understand the density fluctuations along these varying lines of sight.

Downplaying Halo Contributions

Plot showing the relative scattering contributions of different regions

A representation of the scattering contributions estimated by the authors for different regions of ionized gas. The scattering that occurs due to intervening galactic halos (M33 and FG–181112) is consistent with the upper limits found for the Milky Way’s halo (MW Halo), and substantially smaller than the scattering caused within the inner disk of our galaxy (MW Inner Galaxy). [Ocker et al. 2021]

From their work, Ocker and collaborators were able to place an upper limit on the amount of scattering contributed by the Milky Way’s halo to the FRBs that they explored. The authors then compared these results to data from FRB signals that passed through additional, intervening galaxy halos on their way to us. They found that the scattering contributions from other halos are consistent with the upper limits set on the Milky Way’s halo.

Ocker and collaborators’ study suggests that galaxy halos have only a very small impact on the scattering of light in FRBs. While additional FRB and pulsar data will be helpful in constraining more lines of sight, this work provides a valuable step in disentangling different reservoirs of ionized gas to ultimately probe the density fluctuations across our universe.

Citation

“Constraining Galaxy Halos from the Dispersion and Scattering of Fast Radio Bursts and Pulsars,” Stella Koch Ocker et al 2021 ApJ 911 102. doi:10.3847/1538-4357/abeb6e