| Description |
The key diagnostic of the warm star-forming molecular clouds in galaxies is provided by their cooling radiation: CII and OI lines, butalso lines of CO, the main tracer of molecular gas. While in our Galaxycooling by CO lines is unimportant, this situation is totally different inlocal ULIRGs, where the CO is an important coolant, and the thermalbalance is totally different from that in our Galaxy. These resultsdemonstrate the enormous, as yet almost unexplored, diagnosticpower of these lines for probing physical conditions. Since these linesare very luminous, they can be used to probe galaxies out to veryhigh redshifts.Here we propose a systematic study with the SPIRE-FTS and PACS ofthe principal neutral gas cooling lines (CII, OI and the CO ladderfrom 5-4 to 13-12) in a 60 micron flux-limited sample of 32 galaxies:24 LIRGs and 8 ULIRGs.Aims of this key project are:- analysis of the neutral gas cooling budget, in particular the relativeimportance of CO and CII emission;- analysis of the CO rotational ladder to derive the mass of moleculargas as a function of temperature and density;- modeling of the data using advanced PDR/XDR models with the goalof separating UV-excited (starburst) and X-ray excited (AGN)components;- analysis of the implications for molecular mass measurements basedon CO lines, both at low and high redshift;- achieving statistical robustness, so that inferences can be made onthe infrared galaxy population in general, and trends with other galaxycharacteristics (IR luminosity, type of power source,.) can be traced.Only Herschel can provide the necessary high-J CO and cooling linedata. Combined with our extensive ground-based data (low-J CO, HCN lines) this dataset will elucidate the energy source and physicalconditions in local ULIRGs, and establish the critical link necessary forthe interpretation of observations of high-z galaxies in mid-J COlines and other luminous lines, which will become routine in theALMA era. |
