Dense Molecular Gas toward the Nucleus of the Seyfert Galaxy NGC 1068

Details Dense Molecular Gas toward the Nucleus of the Seyfert Galaxy NGC 1068 Dense Molecular Gas toward the Nucleus of the Seyfert Galaxy NGC 1068

Nov 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...418l..13j&link_type=abstract

Astrophysical Journal Letters v.418, p.L13

Astronomy and Astrophysics

Astronomy

69

Galaxies: Individual Ngc Number: Ngc 1068, Galaxies: Ism, Galaxies: Nuclei, Galaxies: Seyfert

Scientific paper

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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