Hidden cooling flows in elliptical galaxies

Abstract

The radiative cooling time of hot gas in the cool cores of many galaxy clusters and massive elliptical galaxies drops in the centre to below $10^8$ yr. The mass cooling rates inferred from simple modelling of X-ray observations of these objects are very low, indicating that either active galactic nuclei feedback is tightly balanced or that soft X-rays from cooling gas are somehow hidden from view. An intrinsic absorption model developed for application to galaxy clusters is used here to search for hidden cooling flows (HCFs) in seven nearby elliptical galaxies. Mass cooling rates of $\sim$0.5-8 $\text{M}_{\odot }$ yr^-1 are found in each galaxy. The absorbed cooling flow luminosity is in agreement with the observed far infrared (FIR) luminosity in each case, indicating absorbed emission is energetically capable of emerging in the FIR band. An observed lack of agreement between HCF rates and normal star formation rates suggests the cooled material must have an alternative fate, with low-mass star formation considered as the primary outcome.