Herschel observations of FIR emission lines in brightest cluster
  galaxies
  The question of how much gas cools in the cores of clusters of galaxies has
been the focus of many, multiwavelength studies in the past 30 years. In this
letter we present the first detections of the strongest atomic cooling lines,
[C II], [O I] and [N I] in two strong cooling flow clusters, A1068 and A2597,
using Herschel PACS. These spectra indicate that the substantial mass of cold
molecular gas (>10^9 Mo) known to be present in these systems is being
irradiated by intense UV radiation, most probably from young stars. The line
widths of these FIR lines indicate that they share dynamics similar but not
identical to other ionised and molecular gas traced by optical, near-infrared
and CO lines. The relative brightness of the FIR lines compared to CO and FIR
luminosity is consistent with other star-forming galaxies indicating that the
properties of the molecular gas clouds in cluster cores and the stars they form
are not unusual. These results provide additional evidence for a reservoir of
cold gas that is fed by the cooling of gas in the cores of the most compact
clusters and provide important diagnostics of the temperature and density of
the dense clouds this gas resides in.
