2011MNRAS.415.3473V

We present Herschel-Spectral and Photometric Imaging Receiver (SPIRE) Fourier transform spectrometer (FTS) and radio follow-up observations of two Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS)-detected strongly lensed distant galaxies. In one of the targeted galaxies H-ATLAS J090311.6+003906 (SDP.81), we detect [O III] 88 µm and [C II] 158 µm lines at a signal-to-noise ratio of ∼5. We do not have any positive line identification in the other fainter target H-ATLAS J091305.0-005343 (SDP.130). Currently, SDP.81 is the faintest submillimetre galaxy with positive line detections with the FTS, with continuum flux just below 200 mJy in the 200–600 µm wavelength range. The derived redshift of SDP.81 from the two detections is z = 3.043±0.012, in agreement with ground-based CO measurements. This is the first detection by Herschel of the [O III] 88 µm line in a galaxy at redshift higher than 0.05. Comparing the observed lines and line ratios with a grid of photodissociation region (PDR) models with different physical conditions, we derive the PDR cloud density n ≈ 2000/cm3 and the far-ultraviolet ionizing radiation field G₀ ≈ 200 (in units of the Habing field – the local Galactic interstellar radiation field of 1.6 {x} 10^–6 W/m2). Using the CO-derived molecular mass and the PDR properties, we estimate the effective radius of the emitting region to be 500–700 pc. These characteristics are typical for star-forming, high-redshift galaxies. The radio observations indicate that SDP.81 deviates significantly from the local far-infrared/radio (FIR/radio) correlation, which hints that some fraction of the radio emission is coming from an active galactic nucleus (AGN). The constraints on the source size from millimetre-wave observations put a very conservative upper limit of the possible AGN contribution to less than 33 per cent. These indications, together with the high [O III]/FIR ratio and the upper limit of [O I] 63 µm/[C II] 158 µm, suggest that some fraction of the ionizing radiation is likely to originate from the AGN.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Correction added after online publication 2011 July 12: author added.