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Session 66 - Galaxies.
Display session, Thursday, June 11
Atlas Ballroom,
We report the results of our recently completed CO(J=1--0) survey of 20 nearby spiral galaxies using the NRO and OVRO millimeter arrays. We have found in the majority of galaxies a strong concentration of CO emission toward the galactic center. The radial scale lengths of the gas concentrations, \lesssim 0.5 kpc, are much smaller than those of the disk molecular gas. Within a radius of 0.5 kpc, the estimated gas and dynamical masses range over 0.3--8\times10^8 M_ødot and 1--10\times10^9 M_ødot,respectively, and the gas-to-dynamical mass ratios are around 0.1 (0.3 at most). It is statistically significant that the degree of gas concentration is larger in barred galaxies than in unbarred systems, a fact which strongly supports the theoretical prediction that a bar funnels gas to the nucleus. Despite the large concentrations of molecular gas toward the nuclei, no clear correlation is found between gas mass and the type of nuclear spectrum (HII, LINER, or Seyfert), suggesting that the amount of gas at this scale does not determine the nature of the nuclear activity. There is, however, a clear correlation that galaxies with larger gas-to-dynamical mass ratios have HII nuclei, while galaxies with smaller ratios show Seyfert or LINER spectra. This trend may well be related to the gravitational stability of the circumnuclear gas disk, which is generally lower for larger gas mass fractions. It is possible that all galaxies have active nuclei, but that the nuclei are obscured by the surrounding star formation when the nuclear molecular gas disk is unstable.
