The discovery of a multiphase orphan core in a nearby galaxy cluster

The star formation rate density in the Universe increases with redshift and peaks at z~2. Clusters of galaxies have experienced significant evolution over the same time span. Unlike still assembling protoclusters at high redshift, nearby galaxy clusters rarely harbor reservoirs of cold gas, despite that their intracluster medium (ICM) radiates vigorously in X-rays at their centers with a short cooling time (cool cores). Mechanical feedback from the supermassive black hole in the brightest cluster galaxy (BCG) is likely preventing the hot ICM from cooling, as revealed by the ubiquitous X-ray cavities at cluster cool cores. Here we report the discovery of a multiphase cool core not appearing to be associated with any galaxy, in a nearby cluster, Abell 1142. Its X-ray surface brightness peak, which is cooler and more metal rich than the ambient ICM, is offset from its BCG by 100 kpc. Our recent IRAM 30 meter observation has detected CO1-0 emission at this X-ray peak, corresponding to more than 10¹⁰ solar mass of H2 molecular gas and extending over at least 50 kpc without decreasing intensity. The redshift of this emission line is >1800 km/s higher than the BCG. Other nearby galaxies are small elliptical galaxies. The cold gas is therefore unlikely to directly originate from ram pressure stripping. The cold gas may be formed from the long sought runaway cooling of the X-ray emitting hot gas, precisely due to the absence of supermassive blackhole feedback in this orphan cool core.