The state of the molecular gas in a luminous starburst/Seyfert 2 galaxy: NGC 1068 revisited
We present fully sampled ^{12}$CO, ^{13}CO J=2-1, 3-2 maps of the inner $\sim 1'\times 1'$ region of NGC 1068. We combine these measurements with an existing interferometric map of ^{12}CO J=1-0 that includes single dish data and thus contains all the flux present. This allows a reliable estimate of the ^{12}CO (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 ^{18}O J=2--1 emission which confirm earlier measurements of a high $C ^{18}O/ ^{13}CO$ intensity ratio in this galaxy. A simple two-phase model for the gas can account for all the observed line ratios if C ^{18}O emission and part of the ^{13}CO emission arise in a dense spatially concentrated component. The ^{12}CO emission originates from a warmer, diffuse gas phase.The dense gas phase contains the bulk of the molecular gas mass while the diffuse phase may not be virialized leading to an overestimate of molecular gas mass when deduced from the luminosity of the ^{12}CO J=1--0 line and a standard galactic conversion factor.