An extended cold gas absorber in a central cluster galaxy
  We present the serendipitous discovery of an extended cold gas structure
projected close to the brightest cluster galaxy (BCG) of the z=0.045 cluster
Abell 3716, from archival integral field spectroscopy. The gas is revealed
through narrow Na D line absorption, seen against the stellar light of the BCG,
which can be traced for $\sim$25 kpc, with a width of 2-4 kpc. The gas is
offset to higher velocity than the BCG (by $\sim$100 km/s), showing that it is
infalling rather than outflowing; the intrinsic linewidth is $\sim$80 km/s
(FWHM). Very weak H$\alpha$ line emission is detected from the structure, and a
weak dust absorption feature is suggested from optical imaging, but no stellar
counterpart has been identified. We discuss some possible interpretations for
the absorber: as a projected low-surface-brightness galaxy, as a stream of gas
that was stripped from an infalling cluster galaxy, or as a "retired" cool-core
nebula filament.
