Using the PACS spectroscopic range mode, we propose to follow-up one of the most exciting discoveries of the Herschel mission: strong negative feedback in massive molecular outflows as traced by the OH molecule. Here we propose, for the first time in the far-Universe, to perform a sensitive observation in the ground-state OH 34 micron doublet in APM 08279+5255, a gravitationally lensed, radio quiet, broad absorption line (BAL) quasar at z=3.91. There is reason to believe that this source may be a scaled up version similar to local ULIRGs such as Mrk 231, with an apparent bolometric luminosity of L virgul 7 x 10^15 Lo: it is over three orders of magnitude more luminous than Mrk 231-, and with highly luminous radiatively pumped far-IR emission lines of H2O. The OH 34 micron line will be red-shifted to 170 microns in a sensitive PACS spectral region and will be observed in absorption against the extraordinarily strong rest-frame mid-IR continuum emission close to the peak of the quasar SED. It will provide the opportunity to study the extreme luminosity end of the feedback phenomenon in ULIRGs. By scaling the OH 34 micron absorption derived from our outflow models of the Herschel observations of Mrk 231 to the continuum level of APM 08279+5255, we estimate that 11.0 hours of observing time will be enough to detect the line with 5-sigma confidence; nevertheless, a stronger signal may be detected given the extraordinary properties of the source. If so, this will allow for velocity characterization of the flow. To our knowledge, this will be the first attempt to observe molecular outflows in hyper-ULIRGs in the early Universe and to test the current paradigm of negative feedback at the high end of the luminosity scale.
Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.