Abstract
Observations of multiple CO transitions provide a unique tool for constraining the excitation conditions of the molecular gas in high-redshift objects. We present the first results of our ongoing multi-CO-transition study in six QSO host galaxies and four submm galaxies at, redshifts between z = 1.4 and 4.7 obtained at the IRAM 30 m telescope. Observed CO lines typically include transitions between CO(3-2) and CO(8-7) but range up to the CO(11-10) line in the BAL QSO APM08279 (z = 3.9). This unique set of rest-frame mm/submm transitions provides information on the full CO ladder in ultra-luminous infrared galaxies for the first time at any redshift. Using large velocity gradient (LVG) models we conclude that, surprisingly, all CO line SEDs (CO flux density vs. rotational quantum number) can be explained by single temperature/density models. We discuss variations of the CO line SEDs and of the underlying physical conditions within the massive molecular gas reservoirs found in these high-z objects.