Molecular gas in the elliptical galaxy NGC 759 - Interferometric CO observations

We present interferometric observations of CO(1-0) emission in the elliptical galaxy NGC 759 with an angular resolution of 3.''1 x 2.''3 (990 x 735 pc at a distance of 66 Mpc). NGC 759 contains 2.4 x 10(9) M. of molecular gas. Most of the gas is confined to a small circumnuclear ring with a radius of 650 pc and with an inclination of 40 degrees. The maximum gas surface density in the ring is 750 M. pc(-2). Although this value is very high, it is always less than or comparable to the critical gas surface density for large scale gravitational instabilities. The CO J=2 - 1/J=1 - 0 line ratio is low (0.4), implying sub-thermal excitation. This low ratio 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 gas component, dominating the CO emission. Compared to galaxies of similar gas surface densities, NGC 759 is underluminous in L-FIR with respect to its molecular gas mass, suggesting that the star forming efficiency in NGC 759 is low. We discuss the possibility that the molecular gas and current star formation activity in NGC 759 could be signatures of a late stage of a merging between two gas-rich disk galaxies. We use a mass model of the underlying galaxy which is applicable to spherical galaxies with an r(1/4) - luminosity profile when interpreting our CO data. This leads to more modest estimates of the molecular gas mass fraction and the surface gas density than 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 distributions, may have to be revised.