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 similar to 25 kpc, with a width of 2-4 kpc. The gas is offset to higher velocity than the BCG (by similar to 100 km s(-1)), showing that it is infalling rather than outflowing; the intrinsic linewidth is similar to 80 km s(-1) (FWHM). Very weak Ha 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.