A cosmic-ray dominated ISM in Ultra Luminous Infrared Galaxies: new initial conditions for star formation

Abstract

The high-density star formation typical of the merger/starburst events that power the large IR luminosities of Ultra Luminous Infrared Galaxies (ULIRGs) (L_{IR}>10^{12}Lsol) throughout the Universe results to extraordinarily high cosmic ray (CR) energy densities of U_CR~(few)x(10^3--10^4)U_{CR,Gal} permeating their interstellar medium (ISM), a direct consequence of the large supernovae remnants (SNRs) number densities in such systems. Unlike far-UV photons emanating from their numerous star forming 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 star-forming volumes. Such conditions can turn most of the large molecular gas masses found in such systems and their high redshift counterparts (M(H2)~10^9-10^10 M_{sol}) 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 {\it minimum} temperature of T_{kin}~(80--160)K, and very high ionization fractions of x(e)>10^{-6} throughout its UV-shielded dense cores, which in turn will {\it fundamentally alter the initial conditions for star formation in such systems.}. Observational tests of CRDRs can be provided by ......