Molecular gas filaments and star-forming knots beneath an X-ray cavity
  in RXC J1504-0248
  We present recent ALMA observations of the CO(1-0) and CO(3-2) emission lines
in the brightest cluster galaxy of RXCJ1504.1$-$0248, which is one of the most
extreme cool core clusters known. The central galaxy contains $1.9\times
10^{10}~M_{\odot}$ of molecular gas. The molecular gas morphology is complex
and disturbed, showing no evidence for a rotationally-supported structure in
equilibrium. $80\%$ of the gas is situated within the central 5 kpc of the
galactic center, while the remaining gas is located in a 20 kpc long filament.
The cold gas has likely condensed out of the hot atmosphere. The filament is
oriented along the edge of a putative X-ray cavity, suggesting that AGN
activity has stimulated condensation. This is enegetically feasible, although
the morphology is not as conclusive as systems whose molecular filaments trail
directly behind buoyant radio bubbles. The velocity gradient along the filament
is smooth and shallow. It is only consistent with free-fall if it lies within
$20^{\circ}$ of the plane of the sky. The abundance of clusters with comparably
low velocities suggests that the filament is not free-falling. Both the central
and filamentary gas are coincident with bright UV emission from ongoing star
formation. Star formation near the cluster core is consistent with the
Kennicutt-Schmidt law. The filament exhibits increased star formation surface
densities, possibly resulting from either the consumption of a finite molecular
gas supply or spatial variations in the CO-to-H$_2$ conversion factor.
