A COSMIC-RAY-DOMINATED INTERSTELLAR MEDIUM IN ULTRA LUMINOUS INFRARED GALAXIES: NEW INITIAL CONDITIONS FOR STAR FORMATION

The high-density star formation typical of the merger/starburst events that power the large IR luminosities of ultraluminous infrared galaxies (ULIRGs) (L-IR(8-1000 mu m) greater than or similar to 10(12) L-circle dot) throughout the universe results in. extraordinarily high cosmic-ray (CR) energy densities of U-CR similar to few x (10(3)-10(4)) U-CR,U-Gal permeating their interstellar medium, a direct consequence of the large supernova remnant number densities in such systems. Unlike far-UV photons emanating from numerous star-forming (SF) sites, these large CR energy densities in ULIRGs will volumetrically heat and raise the ionization fraction of dense (n >10(4) cm(-3)) UV-shielded gas cores throughout their compact SF volumes. Such conditions can turn most of the large molecular gas masses found in such systems and their high redshift counterparts (similar to 10(9)-10(10) M-circle dot) into giant CR-dominated regions (CRDRs) rather than ensembles of photon-dominated regions (PDRs) which dominate in less IR-luminous systems where star formation and molecular gas distributions are much more extended. The molecular gas in CRDRs will have a minimum temperature of T-kin similar to (80-160) K, and very high ionization fractions of x(e) > 10(-6) throughout its UV-shielded dense core, which in turn will fundamentally alter the initial conditions for star formation in such systems. Observational tests of CRDRs can be provided by high-J CO and (CO)-C-13 lines or multi-J transitions of any heavy rotor molecules (e.g., HCN) and their isotopologs. Chemical signatures of very high ionization fractions in dense UV-shielded gas such as low [DCO+]/[HCO+] and high [HCO+]/[CO] abundance ratios would be good probes of CRDRs in extreme starbursts. These tests, along with direct measurements of the high CO line brightness temperatures expected over the areas of compact dense gas disks found in ULIRGs, will soon be feasible as sub-arcsecond interferometric imaging capabilities and sensitivity at millimeter/submillimeter wavelengths improve in the era of ALMA.