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Session 83 - Starbursts and Mergers.
Oral session, Thursday, June 13
Union Theater,
We have undertaken a 2\arcsec\ resolution imaging survey of the CO (1\to0) and HCN (1\to0) line in six luminous infrared galaxies using the Owens Valley millimeter array. We have also imaged three of the most compact of these objects in the CO (2\to1) transition at 1\arcsec\ resolution. All six are from strongly interacting systems; five are classified as mergers by their optical morphology, while the sixth is a member of an interacting pair.
\looseness=-1 The aperture synthesis CO maps strongly suggest that the morphology of the molecular gas is related to the interaction class of the object. The single nucleus mergers contain bright, sub-kpc radius CO cores coincident with the near-infrared nuclei. The double nucleus mergers are also dominated by bright CO cores, but their gas peaks between the near-infrared nuclei and has an extent nearly equal to the nuclear separation. The only non-merger in the sample is not dominated by a compact CO core, but instead by a 12 kpc CO bar, roughly coextensive with the near-infrared stellar bar. These morphological variations are consistent with simulations of the gas behavior in merging galaxies, where a large fraction of the total gas mass of the galaxies is quickly funneled into the central few hundred parsecs, and also suggest that the gas nuclei merge prior to the stellar nuclei.
Our measurements of the 86 GHz continuum provide an upper limit to the thermal bremsstrahlung contribution and hence to the rate of production of ionizing photons, which is used to constrain starburst parameters. With published Br\thinspace \gamma line fluxes, the thermal radio continuum limit also provides upper limits to the near-infrared extinction if the molecular and ionized gas are distributed similarly. These extinction limits imply gas column densities 1.2--5 times smaller than those derived using the Galactic CO mass conversion factor.
In an attempt to improve the understanding of the uncertainties in deriving gas masses from CO luminosities, we have analyzed the behavior of the CO mass conversion factor relative to the Galactic value for a variety of gas geometrical configurations, environments, and conditions.
