Molecular Gas in Galactic Nuclei

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 both the nearby infrared bright galaxies (e.g., M82, NGC 253, and IC342) and the ultraluminous IRAS galaxies, high resolution millimeter line mapping has shown extremely high gas surface densities in the central kpc. 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″ of 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 evolution and the exhaustion of the observed central ISM are typically 2 × 10⁸ years.

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