Hidden cooling flows in clusters of galaxies

The radiative cooling time of the hot gas at the centres of cool cores in clusters of galaxies drops down to 10 Myr and below. The observed mass cooling rate of such gas is very low, suggesting that active galactic nucleus feedback is very tightly balanced or that the soft X-ray emission from cooling is somehow hidden from view. We use an intrinsic absorption model in which the cooling and coolest gas are closely interleaved to search for hidden cooling flows in the Centaurus, Perseus, and A1835 clusters of galaxies. We find hidden mass cooling rates of between 10 and $500\,{{\rm M_{\odot }}\,{\rm yr}^{-1}}$ as the cluster mass increases, with the absorbed emission emerging in the far-infrared (FIR) band. Good agreement is found between the hidden cooling rate and observed FIR luminosity in the Centaurus Cluster. The limits on the other two clusters allow for considerable hidden cooling. The implied total mass of cooled gas is much larger than the observed molecular masses. We discuss its fate including possible further cooling and collapse into undetected very cold clouds, low-mass stars, and substellar objects.