Co in Ultraluminous and High Z Galaxies

Abstract

Dense molecular clouds are critical to the activity occurring in galactic nuclei. They are the active component from which starbursts arise, and this dense interstellar gas may be the fuel for AGNs. In the ultraluminous IRAS galaxies, high resolution millimeter line mapping has shown extremely high gas surface densities in the central kpc, and often a significant fraction (> 25%) of the total molecular line emission from the galaxy arises from these central regions. New maps at resolutions down to 0.8” in the ultraluminous IRAS galaxies reveal kinematic gradients parallel to the major axis of the CO intensity distribution, suggesting that the gas is situated in a central rotating disk. The most extreme central concentrations are seen in Arp 220 and Mrk 231 (Sey 1) which have now been mapped in both the 2.6 and 1.3 mm CO transitions. In both galaxies, the high observed CO brightness temperatures indicate large area filling factors with mean H₂ densities exceeding 10⁴cm⁻³. To produce the observed luminosities, the star formation rates must be ~100 M⨀ yr⁻¹ within the central 500 pc radius. Estimated time scales for both the dynamical evoution and the exhaustion of the observed central ISM are typically 2x10⁸ years. At higher redshift, CO emission has been unambiguously detected in two objects, FSC 10214+4724 (z = 2.3) and H1413+117 (z = 2.5), both of which are probably gravitationally lensed. High resolution mapping of FSC 10214+4724 reveals two components: an unresolved core with 2/3 of the emission and an extended “disk” (9x24 kpc). In H1413+117 our CO(7–6) maps at 0.8" show a morphology similar to the cloverleaf pattern seen in the optical and the relative fluxes of the four components varies with velocity. Models for the CO lensing are consistent with the molecular emission arising from a disk within about 1 kpc in radius of the quasar. These high redshift systems may therefore be early universe counterparts of the ultraluminous IRAS galaxies.