The First Astrophysical Result of Hisaki: A Search for the EUV He Lines in a Massive Cool Core Cluster at z = 0.7

Molecular cold gas and star formation have been observed at centers of cool core clusters, albeit at a level much smaller than expected from the classic cooling model. Feedback from the supermassive black hole is likely to have prevented hot gas from cooling. However, the exact cooling and heating processes are poorly understood. The missing key piece is the link between the hot gas (10⁷ K) and cold gas (10³ K). Using the extreme ultraviolet spectrometer on board Hisaki, we explore a distant galaxy cluster, RCS2 J232727.6-020437, one of the most massive cool core clusters with a cooling rate of 400 M _o yr^-1. We aim to detect gas at intermediate temperatures (3×10⁴ K) emitting He i α and He i β at rest wavelengths of 58.4 nm and 53.7 nm, respectively. Our target resides at z = 0.6986, for which these He i lines shift away from the absorption of the Galaxy. Our findings show that the amount of 10^4-5 K gas at the center of this cluster is smaller than expected if cooling there was uninhibited, which demonstrates that feedback both operates and is efficient for massive clusters at these epochs.