Detection of optical coronal emission from 106-K gas in the core of the Centaurus cluster

We report a detection (3.5 × 1037± 5.6 × 1036 erg s−1) of the optical coronal emission line [Fe x] λ6374 and upper limits of four other coronal lines using high-resolution VIMOS spectra centred on NGC 4696, the brightest cluster galaxy in the Centaurus cluster. Emission from these lines is indicative of gas at temperatures between 1 × 106−5 × 106 K, so traces the interstellar gas in NGC 4696. The rate of cooling derived from the upper limits is consistent with the cooling rate from X-ray observations (∼10 M⊙ yr−1); however, we detect twice the luminosity expected for [Fe x] λ6374 emission, at 106 K, our lowest temperature probe. We suggest this emission is due to the gas being heated rather than cooling out of the intracluster medium. We detect no coronal lines from [Ca xv], which are expected from the 5 × 106 K gas seen near the centre in X-rays with Chandra. Calcium is, however, likely to be depleted from the gas phase on to dust grains in the central regions of NGC 4696.