Detection of neutral gas in the central galaxy of the Centaurus cluster

We describe the detection of neutral gas within the central galaxy of the Centaurus cluster of galaxies, NGC 4696. The detection is in the form of sodium D absorption, against the background stellar emission. The gas is clearly associated with the compact emission filament system and dust lane in the central 20 '' of the galaxy; it shares the same velocity field as the emission-line gas, the absorption line strength correlates with the amount of extinction due to dust, and the equivalent width of narrow absorption features is related to the emission-line strength. The quantitative relationship between absorption line strength and extinction is compared with Galactic ratios. The strength of sodium absorption is large for the typical E(B-V) observed, although similar to what is found in external spiral galaxies. Several possibilities may account for this, including larger internal random motions than are present in the Galactic ISM, higher gas-to-dust ratio than Galactic, or a higher neutral fraction of sodium in NGC 4696. The observations imply a common origin for the warm and neutral components, which may be a merger.