Hidden cooling flows in clusters of galaxies – III. Accretion on to the central black hole

Recently, we have uncovered hidden cooling flows (HCFs) in the X-ray spectra of the central brightest galaxies of 11 clusters, 1 group, and 2 elliptical galaxies. Here, we report such flows in a further 15 objects, consisting of 8 clusters, 3 groups, 3 ellipticals, and 1 Red Nugget. The mass cooling rates are about $1\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$ in the ellipticals, 2 to $20\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$ in the groups, and 20 to $100\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$ in regular clusters. The Red Nugget, MRK 1216, has an HCF of $10\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$. We review the fate of the cooled gas and investigate how some of it might accrete on to the central black hole. The gas is likely to be very cold and to have fragmented into low-mass stars and smaller objects before being swallowed whole, with little luminous output. If such a scenario is correct and operates at a few $\hbox{$\hbox{$\rm \, {\rm M}_{\odot }$}{\rm \, yr}^{-1}\, $}$ then such objects may host the fastest growing black holes in the low-redshift Universe. We briefly discuss the relevance of HCF to the growth of early galaxies and black holes.