A submillimetre-selected, ultraluminous, active galaxy

We present a detailed study of SMM02399−0136, an ultraluminous, active galaxy selected from a sub-mm survey of the distant Universe. This object is the brightest source in the fields of four rich, lensing clusters, with a total area of ∼20 arcmin, that we have mapped with a sensitivity better than 2mJy beam at 850μm. We identify a compact optical counterpart with an apparent magnitude of B ∼ 23 and a low-surface-brightness companion about 3 arcsec away. Our spectroscopy shows that components have the same redshift; z = 2.803 ± 0.003. The narrow emission lines, FWHM≃ 1000–1500km s, and line ratios along with the compact morphology and high luminosity (MB ≃ −24.0) of the galaxy indicate that SMM02399−0136 contains a rare dust-embedded, narrow-line or type-2 active galactic nucleus (AGN). The source is gravitationally lensed by the foreground cluster, amplifying its apparent luminosity by a factor of 2.5, and our detailed lens model allows us to correct robustly for this. Taking the amplification into account we estimate that SMM02399−0136 is intrinsically a factor of five times more luminous than IRAS F10214+4724. Its farinfrared and Hα luminosities and low-surface-brightness radio emission all indicate an extremely high star-formation rate — several thousand solar masses per year — suggesting that SMM02399−0136 comprises an AGN buried in an interaction-induced starburst. A dust mass of 5–7×108M⊙ is indicated by our data for a dust temperature of 40–50k, independent of whether the dominant energy source is an AGN or a starburst. We estimate the possible space density of such luminous sub-mm sources, and find that while a very large population of these obscured sources could be detected in future wide-field sub-mm surveys, they are unlikely to dominate the faint counts in this waveband. Galaxies like SMM02399−0136 and F 10214+4724 cannot be easily detected in conventional AGN/QSO surveys, and so estimates of the prevalence of AGN in the early Universe may require significant revision.