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 ergs^-1) of the optical coronal emission line [Fex]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 × 10⁶K, 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 [Fex]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 [Caxv], which are expected from the 5 × 10⁶K 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.