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Title:
Dense Molecular Gas toward the Nucleus of the Seyfert Galaxy NGC 1068
Authors:
Jackson, James M.; Paglione, Timothy A. D.; Ishizuki, Sumio; Nguyen-Q-Rieu
Publication:
Astrophysical Journal Letters v.418, p.L13 (ApJL Homepage)
Publication Date:
11/1993
Origin:
APJ; KNUDSEN; NED
ApJ Keywords:
GALAXIES: INDIVIDUAL NGC NUMBER: NGC 1068, GALAXIES: ISM, GALAXIES: NUCLEI, GALAXIES: SEYFERT
DOI:
10.1086/187104
Bibliographic Code:
1993ApJ...418L..13J

Abstract

Using the Nobeyama Millimeter Array, we have imaged HCN J = 1 - 0 emission at 7" (600 pc) resolution toward the Seyfert galaxy NGC 1068. Since HCN requires large densities (n_crit_ ~ 4 x 10^6^ cm^-3^) for excitation, its emission traces high-density molecular gas. About half of the total HCN emission comes from an unresolved source coincident 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 of T_B_ over dV}) ratio is I_HCN_/I_CO = 0.4-0.9 for the nuclear region and is ~54 times smaller toward the molecular ring. The high HCN/CO intensity ratio toward the nucleus suggests that the HCN line is optically thick and that essentially all the molecular gas there is dense (n ~> 10^5^ cm^-3^). In contrast, the dense gas fraction or the opacity toward the molecular ring is significantly smaller. Optically thick HCN emission requires molecular hydrogen column densities N_H_2__ ~> 1.3 x 10^23^ cm^- 2^, or A_V_ ~> 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 ~ 180 pc. If this velocity gradient represents rotation, then the HCN rotation axis (P.A. = 33^deg^) 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 dense molecular torus which obscures the broad- line region in the Seyfert nucleus of NGC 1068.

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