Free Astronomy Magazine November-December 2019

50 NOVEMBER-DECEMBER 2019 SPACE CHRONICLES burst. Instead we re- ceived a pulse so pris- tine and sharp that there is no signature of this gas at all,” said coauthor Jean-Pierre Macquart, an as- tronomer at the Inter- national Center for Radio Astronomy Re- search at Curtin Uni- versity, Australia. The study found no ev- idence of cold turbu- lent clouds or small dense clumps of cool halo gas. The fast ra- dio burst signal also yielded information about the magnetic field in the halo, which is very weak—a billion times weaker than that of a refrigerator mag- net. At this point, with re- sults from only one galactic halo, the re- searchers cannot say whether the low den- sity and low magnetic field strength they measured are unusual or if previous studies of galactic halos have over- estimated these properties. Prochaska said he expects that ASKAP and other radio telescopes will use fast radio bursts to study many more galactic halos and re- solve their properties. “This galaxy may be special,” he said. “We will need to use fast radio bursts to study tens or hundreds of galaxies over a range of masses and ages to assess the full population.” Optical telescopes like ESO’s VLT play an important role by revealing how far away the galaxy that played host to each burst is, as well as whether the burst would have passed through the halo of any galaxy in the foreground. S oon after the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope pinpointed a fast radio burst, named FRB 181112, ESO’s Very Large Telescope (VLT) took this image and other data to determine the distance to its host galaxy (FRB 181112 location indicated by the white ellipses). The analysis of these data revealed that the radio pulses have passed through the halo of a massive galaxy (at the top of the image) on their way toward Earth. [ESO/X. Prochaska et al.] that is primarily in the form of a hot ionised 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 in diameter. Halo gas fuels star forma- tion as it falls towards the centre of the galaxy, while other processes, such as supernova explosions, can eject material out of the star-form- ing regions and into the galactic halo. One reason astronomers want to study the halo gas is to better un- derstand these ejection processes which can shut down star formation. “This galaxy’s halo is surprisingly tranquil,” Prochaska said. “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.” The signal of FRB 181112 was com- prised of a few pulses, each lasting less than 40 microseconds (10000 times shorter than the blink of an eye). The short duration of the pulses puts an upper limit on the density of the halo gas because pas- sage through a denser medium would broaden the duration of the radio signal. The researchers calcu- lated that the density of the halo gas must be less than 0.1 atoms per cubic centimeter (equivalent to sev- eral hundred atoms in a volume the size of a child’s balloon). “Like the shimmering air on a hot summer’s day, the tenuous atmos- phere in this massive galaxy should warp the signal of the fast radio !