High redshift radio galaxies (HzRGs) are the most dramatic examples of the formation of massive galaxies in the early Universe, as evidenced by their extreme (>1000Msun-yr) star formation rates. At the same time they coincide with a phase of rapid coeval black hole (BH) growth, as revealed by their powerful active galactic nuclei (AGN). This makes HzRGs unique laboratories in which to study the coevolution of massive galaxies and their central BHs. Our team has successfully carried out comprehensive imaging surveys of 62 powerful HzRGs at z > 1 using Spitzer and Herschel. These surveys have demonstrated that HzRGs have high stellar masses (>1e11Msun), and mid-IR AGN luminosities comparable to the most powerful QSOs known. Also, they have allowed us to separate the AGN and starburst contributions to the total IR luminosity. In parallel to this, we are undertaking a large CO(1-0) line survey towards a subset of 10 HzRGs using ATCA, which will provide us with excitation-unbiased mass estimates of the total molecular gas available for star formation. With this proposal we will constrain the bulk physical conditions of the star forming gas in HzRGs using the far-infrared OI fine-structure line at 63micron, which is one of the brightest diagnostic lines of the interstellar medium (ISM) in galaxies. The [OI] line, which can contain as much as >0.1% of the total IR luminosity of a galaxy, is one of the major coolants of the ISM, and is expected to be particular bright in the type of dense, hot, star forming gas expected to dominate the ISM in HzRGs. Utilizing our accurate IR luminosity estimates and molecular gas masses in combination with [OI] we will employ photo-dissociation models to constrain the gas density and the impinging FUV radiation field. We request a total of 13.1hrs of PACS spectroscopy (~2.2hrs per source, ensuring a S-N > 5) in order to detect the [OI] line towards six carefully chosen HzRG, representative... of the luminous 1 < z < 2.3 radio galaxy population.
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.