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27th of September 2019
Using an FRB to study a galactic halo
by Prochaska et al.
Using one cosmic mystery to probe another, Prochaska et al. have used a fast radio burst to characterize the diffuse gas in the halo of a massive galaxy. A vast halo of low-density gas extends far beyond the luminous part of a galaxy where the stars are concentrated. Although this hot, diffuse gas makes up more of a galaxy's mass than stars do, it is nearly impossible to see. A fast radio burst, named FRB 181112, was detected and localized by ASKAP (the Australian Square Kilometre Array Pathfinder). Follow-up observations with ESO’s Very Large Telescope identified not only its host galaxy but also a bright galaxy in front of it. As the FRB passed through the halo of this massive galaxy on its way toward Earth, it provides clues to the nature of the halo gas.

Contrary to expectations, the FRB was relatively unaffected by its passage through the intervening galaxy, indicating a very low density and a feeble magnetic field in the halo. The signal of FRB 181112 consisted of several pulses, each lasting less than 40 microseconds. 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 magnetic field in the halo would affects the polarization of the FRB. Analyzing the polarization as a function of frequency gives a "rotation measure" for the halo, which was found to be very low. At this point, with results from only one galactic halo, the researchers cannot say whether the unexpectedly low density and magnetic field strength are unusual or if previous studies of galactic halos have overestimated these properties. Further observations of fast radio bursts will enable the study of many more galactic halos to help resolve their properties. Full details are given in the paper, published in this week's issue of the journal Science.

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