The State of the Molecular Gas in a Luminous Starburst/Seyfert 2 Galaxy: NGC 1068 Revisited

We present fully sampled 12CO, 13CO J=2-1, 3-2 maps of the inner ~1′ × 1′ region of NGC 1068. We combine these measurements with an existing interferometric map of 12CO J=1-0 that includes single dish data and thus contains all the flux present. This allows a reliable estimate of the 12CO (J=3-2)/(J=1-0) ratio at the highest angular resolution currently possible and the use of this sensitive line ratio to probe the physical condition of the molecular gas. We also present two measurements of the faint C18O J=2-1 emission that confirm earlier measurements of a high C18O/13CO intensity ratio in this galaxy. The ratios of the 12CO, 13CO isotopes can only be reproduced for small to moderate optical depths of 12CO J=1-0 (τ ~1-2), which is incompatible with the high C18O/13CO ratios observed. A simple two-phase model for the gas can account for all the observed line ratios if the C18O emission and part of the 13CO emission arise in a dense spatially concentrated component, where C18O J=1-0 has optical depths of τ ≳ 1. The 12CO emission originates from a warmer, diffuse gas phase with τ ~1-2 for J=1-0. The dense gas phase contains the bulk of the molecular gas mass, whereas the diffuse phase may not be virialized, leading to an overestimate of molecular gas mass when deduced from the luminosity of the 12CO J=1-0 line and a standard galactic conversion factor. This suggests that, since type 2 Seyferts harbor a central starburst more often than type 1, the higher average 12CO J=1-0 luminosity of type 2 hinted by earlier studies may simply reflect a difference in molecular gas excitation rather than in gas mass.