# An extended cold gas absorber in a central cluster galaxy

## TL;DR

This paper reports the discovery of an extended cold gas structure near a galaxy in a cluster, observed through absorption lines, suggesting infall and providing insights into gas dynamics in galaxy clusters.

## Contribution

It presents the first detection of a large-scale cold gas absorber in a cluster galaxy using integral field spectroscopy, revealing its properties and possible origins.

## Key findings

- Gas extends for ~25 kpc with a width of 2-4 kpc.
- Gas shows infall velocity offset of ~100 km/s.
- Weak Hα emission and dust features suggest a complex origin.

## Abstract

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.