Probing the Physical Conditions of the Interstellar Medium in High Redshift Radio Galaxies

High redshift radio galaxies (HzRGs) are the most dramatic examples of theformation of massive galaxies in the early Universe, as evidenced by theirextreme (>1000Msun/yr) star formation rates. At the same time they coincidewith a phase of rapid coeval black hole (BH) growth, as revealed by their powerfulactive galactic nuclei (AGN). This makes HzRGs unique laboratories in which tostudy the coevolution of massive galaxies and their central BHs. Ourteam has successfully carried out comprehensive imaging surveys of 62 powerfulHzRGs at z > 1 using Spitzer and Herschel. These surveys have demonstrated thatHzRGs have high stellar masses (>1e11Msun), and midIR AGN luminositiescomparable to the most powerful QSOs known. Also, they have allowed usto separate the AGN and starburst contributions to the total IR luminosity. Inparallel to this, we are undertaking a large CO(10) line survey towards asubset of 10 HzRGs using ATCA, which will provide us with excitationunbiased mass estimates of the total molecular gasavailable for star formation. With this proposal we will constrain the bulkphysical conditions of the star forming gas in HzRGs using the farinfrared OIfinestructure line at 63micron, which is one of the brightest diagnostic linesof the interstellar medium (ISM) in galaxies. The OI line, which can containas much as >0.1% of the total IR luminosity of a galaxy, is one of the majorcoolants of the ISM, and is expected to be particular bright in the type ofdense, hot, star forming gas expected to dominate the ISM in HzRGs. Utilizingour accurate IR luminosity estimates and molecular gas masses in combinationwith OI we will employ photodissociation models to constrain the gas densityand the impinging FUV radiation field. We request a total of 13.1hrs of PACSspectroscopy (virgul2.2hrs per source, ensuring a S/N > 5) in order to detect theOI line trunc [truncated!, Please see actual data for full text]