Starbursting nuclear CO disks of early-type spiral galaxies

We have initiated the first CO interferometer survey of early-type spiral galaxies (S0-Sab). We observed five early-type spiral galaxies with HII nuclei (indicating circumnuclear starburst activities). These observations indicate gas masses for the central kiloparsec of ~$1{-}5\%$ of the dynamical masses. Such low gas mass fractions suggest that large-scale gravitational instability in the gas is unlikely to be the driving cause for the starburst activities. We estimated Toomre  Q  values and found that these galaxies have $Q>1$ (mostly >3) within the central kiloparsec, indicating that the gas disks are globally gravitationally stable. From the brightness temperatures of the CO emission we estimated the area filling factor of the gas disks within the central kiloparsec to be about 0.05. This small value indicates the existence of lumpy structure, i.e. molecular clouds, in the globally-gravitationally stable disks. The typical surface density of the molecular clouds is as high as ~$3000\,{M_{\odot}}\,{\rm \, pc}^{-2}$. In the light of these new observations, we reconsider the nature of the Toomre Q criterion, and conclude that the Toomre Q  parameter from CO observations indicates neither star formation nor molecular cloud formation. This argument should be valid not only for the circumnuclear disks but also for any region in galactic disks. We tentatively explore an alternative model as an initiating mechanism of star formation. Cloud-cloud collisions might account for the active star formation.