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

Abstract

We present fully sampled ^12CO, ^13CO J=2-1, 3-2 maps of the inner ~1^'x1^' 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 C^18O J=2-1 emission that confirm earlier measurements of a high C^18O/^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 (tau~1-2), which is incompatible with the high C^18O/^13CO ratios observed. A simple two-phase model for the gas can account for all the observed line ratios if the C^18O emission and part of the ^13CO emission arise in a dense spatially concentrated component, where C^18O J=1-0 has optical depths of tau>~1. The ^12CO emission originates from a warmer, diffuse gas phase with tau~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.