Molecular gas in the elliptical galaxy NGC759. Interferometric CO observations

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Interferometric observations of the CO(1-0) line in the elliptical galaxy NGC759 shows that it contains 2 10⁹ Mo of molecular gas, confined to a circumnuclear ring with an average radius of only 650pc. The maximum gas surface density is 750 Mo/pc², which is comparable to the critical gas surface density for large scale gravitational instabilities. The low CO(2-1)/CO(1-0) ratio of 0.4 is consistent with a two-component molecular gas, consisting of a cold and dense phase, containing most of the mass, and a warm and diffuse component dominating the CO emission. We use a mass model for the underlying galaxy which is applicable to spherical galaxies with an r¹/4 luminosity profile. This leads to more modest estimates of the molecular gas fraction and gas surface density then would have been derived using simpler models, suggesting that many of the spectacular molecular gas properties of ultraluminous FIR galaxies, which could be described by similar mass models, may have to be revised. We discuss the possibility that NGC759 is in a late stage of a merging between two gas-rich disk galaxies.