DENSE MOLECULAR GAS TOWARD THE NUCLEUS OF THE SEYFERT-GALAXY NGC-1068

Using the Nobeyama Millimeter Array, we have imaged HCN J = 1 --> 0 em ission at 7'' (600 pc) resolution toward the Seyfert galaxy NGC 1068. Since HCN requires large densities (n(crit) approximately 4 x 10(6) cm -3) for excitation, its emission traces high-density molecular gas. Ab out half of the total HCN emission comes from an unresolved source coi ncident with the Seyfert nucleus and half from the molecular ring at r = 1.6 kpc. The HCN J = 1 --> 0/CO J = 1 --> 0 integrated intensity (I = integral T(B)dV) ratio is I(HCN)/I(CO) = 0.4-0.9 for the nuclear re gion and is approximately 54 times smaller toward the molecular ring. The high HCN/CO intensity ratio toward the nucleus suggests that the H CN line is optically thick and that essentially all the molecular gas there is dense (n greater than or similar to 10(5) cm-3). In contrast, the dense gas fraction or the opacity toward the molecular ring is si gnificantly smaller. Optically thick HCN emission requires molecular h ydrogen column densities N(H2) greater than or similar to 1.3 x 10(23) cm-2, or A(V) greater than or similar to 70 mag toward the nucleus. A velocity gradient is found for the nuclear region which is consistent with the measured linewidth only if the source size is r approximatel y 180 pc. If this velocity gradient represents rotation, then the HCN rotation axis (P.A. = 33-degrees) is not coincident with the rotation axis of the galaxy but is instead aligned with the radio lobes and the ionization cones. Our observations support recent suggestions of a de nse molecular torus which obscures the broad-line region in the Seyfer t nucleus of NGC 1068.