PUBLISHED: 2 JUNE 2017 | VOLUME: 1 | ARTICLE NUMBER: 164 news & views GALAXY CLUSTERS Waking Perseus The Perseus cluster contains over 1,000 a Kelvin–Helmholtz instability, which galaxies packed into a region ~3,500 kpc propagates in the wave direction and in extent. It is inevitable that such close- causes the dark area indicated in the packed galaxies will interact, and the image. This dark ‘bay-like’ feature is Chandra X-ray Observatory has observed approximately the size of the Milky Way, the beautiful result of that interaction and may be the result of a cold front in (pictured). This is no snapshot — the cluster gas: an interface where the Chandra observed the galaxy cluster temperature drops dramatically on scales for over 16 days in order to capture this much smaller than the mean free path. image. Stephen Walker and colleagues An advantage of this comparison of have retrieved the archival data and observations and simulations is that processed it to enhance the edges of (unmeasurable) physical quantities the surface brightness distribution. This can be estimated. In this case, the bay analysis, reported last year (Sanders et al., 50 kpc feature only appears in this form when Mon. Not. R. Astron. Soc. 460, 1898–1911; the ratio of the thermal pressure to the ROYAL ASTRONOMICAL SOCIETY ASTRONOMICAL ROYAL 2016), highlighted two features in the magnetic pressure is ~200, and this X-ray emission that are discussed by determination in turn allows the authors Walker et al. (Mon. Not. R. Astron. Soc. pattern seen in the image could have been to obtain an order-of-magnitude estimate 468, 2506–2516; 2017): the swirling wave generated by a passing galaxy cluster about of the magnetic field. Indeed, the overall corresponding to the hot inter-galactic ten times smaller in mass than Perseus. magnetic field can be estimated for the medium, and the dark region in the lower In the simulation, gravitational interaction entire cluster volume, giving this method left of the image, indicated by arrows. between the two clusters disrupts a an advantage over Faraday rotation According to a simulation reservoir of cool gas at the centre of the measures, which are limited to small performed by Harvard astrophysicist Perseus cluster, causing it to ripple outwards sight-lines. John ZuHone (https://www.youtube.com/ and mix with hot, shocked gas. The velocity watch?v=Yu1yF1z7Ins), the spiral wave difference between the two fluids causes PAUL WOODS NATURE ASTRONOMY 1, 0164 (2017) | DOI: 10.1038/s41550-017-0164 | www.nature.com/natureastronomy 1 ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. .