Physical conditions of the interstellar medium of high-redshift, strongly lensed submillimetre galaxies from the Herschel-ATLAS
Altieri, B.; Rodighiero, G.; Valtchanov, I.; Ivison, R. J.; de Zotti, G.; Scott, D.; Smail, Ian; Fritz, J.; Cooray, A.; Ibar, E.; Rigopoulou, D.; Vaccari, M.; Negrello, M.; Auld, R.; Baes, M.; Cava, A.; Dannerbauer, H.; Dariush, A.; Dunne, L.; Dye, S.; Eales, S.; Frayer, D.; Gurwell, M.; Hopwood, R.; Leeuw, L.; Lupu, R.; Maddox, S.; Michałowski, M. J.; Serjeant, S.; van der Werf, P.; Conversi, L.; Pohlen, M.; Vieira, J. D.; da Cunha, E.; Riechers, D.; Birkinshaw, M.; Pearson, C.; Marston, A.; Pascale, E.; Thompson, M. A.; Buttiglione, S.; Smith, D. J. B.; Temi, P.; Clements, D.; Bolatto, A.; Carpenter, J.; Benford, D. J.; Swinyard, B.; Jarvis, M.; Gomez, H.; Rigby, E.; Coia, D.; Portal, M. Sanchez; Virdee, J.
Spain, United Kingdom, Netherlands, Greece, Germany, United States, Italy, South Africa, Canada, Belgium, France, Iran
Abstract
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 cm-3 and the far-ultraviolet ionizing radiation field G0≈ 200 (in units of the Habing field - the local Galactic interstellar radiation field of 1.6 × 10-6 W m-2). 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.