Pp. Papadopoulos et Er. Seaquist, The state of the molecular gas in a luminous starburst Seyfert 2 galaxy: NGC 1068 revisited, ASTROPHYS J, 516(1), 1999, pp. 114-126
We present fully sampled (CO)-C-12, (CO)-C-13 J =2-1, 3-2 maps of the inner
similar to 1' x 1' region of NGC 1068. We combine these measurements with
an existing interferometric map of (CO)-C-12 J=1-0 that includes single dis
h data and thus contains all the flux present. This allows a reliable estim
ate of the (CO)-C-12 (J =-3-2)/(J =1-0) ratio at the highest angular resolu
tion currently possible and the use of this sensitive line ratio to probe t
he physical condition of the molecular gas. We also present, two measuremen
ts of the faint (CO)-O-18 J =2-1 emission that confirm earlier measurements
of a high (CO)-O-18/(CO)-C-13 intensity ratio in this galaxy. The ratios o
f the (CO)-C-12,(CO)-C-13 isotopes can only be reproduced for small to mode
rate optical depths of (CO)-C-12 J = 1-0 (tau similar to 1-2), which is inc
ompatible with the high (CO)-O-18/(CO)-C-13 ratios observed. A simple two-p
hase model for the gas can account for all the observed line ratios if the
(CO)-O-18 emission and part of the (CO)-C-13 emission arise in a dense spat
ially concentrated component, where (CO)-O-18 J = 1-0 has optical depths of
tau greater than or similar to 1. The (CO)-C-12 emission originates from a
warmer, diffuse gas phase with tau similar to 1-2 for J =1-0. The dense ga
s phase contains the bulk of the molecular gas mass, whereas the diffuse ph
ase may not be virialized, leading to an overestimate of molecular gas mass
when deduced from the luminosity of the (CO)-C-12 J = 1-0 line and a stand
ard galactic conversion factor. This suggests that, since type 2 Seyferts h
arbor a central starburst more often than type 1, the higher average (CO)-C
-12 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.