MOLECULAR GAS IN THE ELLIPTIC GALAXY NGC-759 - INTERFEROMETRIC CO OBSERVATIONS

We present interferometric observations of CO(1-0) emission in the ell iptical galaxy NGC 759 with an angular resolution of 3.''1 x 2.''3 (99 0 x 735 pc at a distance of 66 Mpc). NGC 759 contains 2.4 x 10(9) M. o f 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. Th e maximum gas surface density in the ring is 750 M. pc(-2). Although t his value is very high, it is always less than or comparable to the cr itical 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 molecul ar gas, consisting of a cold and dense phase, containing most of the m ass, and a warm and diffuse gas component, dominating the CO emission. Compared to galaxies of similar gas surface densities, NGC 759 is und erluminous in L-FIR with respect to its molecular gas mass, suggesting that the star forming efficiency in NGC 759 is low. We discuss the po ssibility that the molecular gas and current star formation activity i n 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 estim ates of the molecular gas mass fraction and the surface gas density th an would have been derived using simpler models, suggesting that many of the spectacular molecular gas properties of ultraluminous FIR galax ies, which could be described by similar mass distributions, may have to be revised.