2010A&A...518L..36S

We present Herschel-PACS observations of rest-frame mid-infrared and far-infrared spectral line emissions from two lensed, ultra-luminous infrared galaxies at high redshift: MIPS J142824.0+352619 (MIPS J1428), a starburst-dominated system at z=1.3, and IRAS F10214+4724 (F10214), a source at z=2.3 hosting both star-formation and a luminous AGN. We have detected [OI]63µm and [OIII]52µm in MIPS J1428, and tentatively [OIII]52µm in F10214. Together with the recent ZEUS-CSO [CII]158µm detection in MIPS J1428 we can for the first time combine [OI], [CII] and far-IR (FIR) continuum measurements for PDR modeling of an ultra-luminous (L_IR≥10¹²L_☉) star forming galaxy at the peak epoch of cosmic star formation. We find that MIPS J1428, contrary to average local ULIRGs, does not show a deficit in [OI] relative to FIR. The combination of far-UV flux G₀ and gas density n (derived from the PDR models), as well as the star formation efficiency (derived from CO and FIR) is similar to normal or starburst galaxies, despite the high infrared luminosity of this system. In contrast, F10214 has stringent upper limits on [OIV] and [SIII], and an [OIII]/FIR ratio at least an order of magnitude lower than local starbursts or AGN, similar to local ULIRGs.