A COSMIC-RAY-DOMINATED INTERSTELLAR MEDIUM IN ULTRA LUMINOUS INFRARED GALAXIES: NEW INITIAL CONDITIONS FOR STAR FORMATION
- Argelander-Institut fuer Astronomie, Auf dem Huegel 71, D-53121 Bonn (Germany)
The high-density star formation typical of the merger/starburst events that power the large IR luminosities of ultraluminous infrared galaxies (ULIRGs) (L{sub IR}(8-1000 {mu}m) {approx}>10{sup 12} L{sub sun}) throughout the universe results in extraordinarily high cosmic-ray (CR) energy densities of U{sub CR} {approx} few x(10{sup 3}-10{sup 4}) U{sub CR,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{sup 4} cm{sup -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 ({approx}10{sup 9}-10{sup 10} M {sub sun}) 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{sub kin} {approx} (80-160) K, and very high ionization fractions of x(e) > 10{sup -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 {sup 13}CO 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{sup +}]/[HCO{sup +}] and high [HCO{sup +}]/[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.
- OSTI ID:
- 21460134
- Journal Information:
- Astrophysical Journal, Vol. 720, Issue 1; Other Information: DOI: 10.1088/0004-637X/720/1/226; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
COSMOLOGY AND ASTRONOMY
ABUNDANCE
CARBON MONOXIDE
COSMIC RADIATION
ENERGY DENSITY
GALAXIES
HYDROCYANIC ACID
IONIZATION
LUMINOSITY
PHOTONS
RED SHIFT
STARS
SUPERNOVA REMNANTS
UNIVERSE
WAVELENGTHS
BOSONS
CARBON COMPOUNDS
CARBON OXIDES
CHALCOGENIDES
COSMIC RADIO SOURCES
ELEMENTARY PARTICLES
HYDROGEN COMPOUNDS
INORGANIC ACIDS
INORGANIC COMPOUNDS
IONIZING RADIATIONS
MASSLESS PARTICLES
OPTICAL PROPERTIES
OXIDES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
RADIATIONS