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 } ( 8–1000 \mu m ) \gtrsim 10 ^ { 12 } L _ { \odot } ) throughout the Universe results to extraordinarily high cosmic ray ( CR ) energy densities of U _ { CR } \sim few \times ( 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 ( \sim 10 ^ { 9 } –10 ^ { 10 } M _ { \odot } ) 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 } \sim ( 80–160 ) K , and very high ionization fractions of x ( e ) > 10 ^ { -6 } throughout its UV-shielded dense cores , 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 ^ { 13 } CO lines or multi-J transitions of any heavy rotor molecules ( e.g .
HCN ) and their isotopologues .
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 mm/submm wavelengths improve in the era of ALMA .