Herschel observations of Cen A: stellar heating of two extragalactic dust clouds

We present the first results of a multiwavelength survey, incorporating Herschel-inline image, Spitzer, GALEX and Australian Telescope Compact Array (ATCA) observations, of a 1°× 1° field centred on Centaurus A. As well as detecting the inner lobes of the active galactic nucleus (AGN) jet and counterjet, we have found two clouds, bright at submillimetre wavelengths, ∼15 kpc from the centre of Cen A that are co-aligned with the jets. Flux measurements at Herschel wavelengths have proved vital in constraining fits to the spectral energy distributions (SEDs). The clouds are well fitted by a single-temperature, modified blackbody spectrum (β = 2) indicating that we are looking at two cold dust clouds on the outskirts of Cen A. The temperature and masses of the clouds are T_(north) = 12.6^(+1.1)_(-1.2) K, T_(south) = 15.1^(+1.7)_(-1.6) K; log (M_(north)/M_☉) = 5.8^(+0.2)_(-0.2), log(M_(south)/M_☉) = 5.6^(+0.2)_(-0.2) and the gas-dust ratio for both clouds is ∼100. The measured values for the northern dust cloud are consistent with previous measurements from ISO while the southern cloud is a new submillimetre detection. The two dust clouds are located at the termini of the partial H i ring that surrounds Cen A which is also where the gas column density peaks. The Herschel survey encompasses the partial H i ring yet we find no evidence of dust emission in any other part of the ring. Assuming that the gas–dust ratio is the same in the rest of the ring, dust mass upper limits in the H i ring are consistent with low column density dust being present but falling below the SPIRE detection limit. We have discussed the origin of these clouds and various possible heating mechanisms. The observations favour a scenario in which the gas and dust were once part of a late-type galaxy, which has since merged with Cen A. The dominant heating mechanism which adequately explains the observed temperatures in both clouds is heating from the evolved stellar population within Cen A.