Infrared Emission-Line Signatures of Tori in the Nuclei of Seyferts

Abstract

AbstractWe present infrared (IR) long-slit spectroscopy of two galaxies with Seyfert 2 nuclei, NGC 2110 and the Circinus galaxy, both known from previous studies to exhibit anisotropic high-excitation gaseous emission. In the “unified models” for Seyferts, this anisotropy is due to collimation of the ionizing nuclear radiation by an optically thick molecular torus which surrounds the nucleus. We have looked for signatures of this torus and its collimating effect in the IR (J and K bands), taking advantage of the reduced relative attenuation at these wavelengths. The two galaxies are found to show extended emission in the lines [Fe II] λl.257 µm, Paβ, H2λ2.12 °m, and Brγ, both along the collimation axis as well as perpendicular to it. The [Fe II] line is found to trace the high-excitation emission, and, in the case of NGC 2110, is more extended along the collimation axis. The H2 emission, although consistent with the presence of a molecular torus, extends for hundreds of parsecs and the derived kinematics is consistent with circular rotation in the plane of the galaxies. The emission-line ratios [Fe II]/Paβ and H2/Brγ are much smaller in Circinus (respectively ~ 0.5 and 1) than in NGC 2110 (~ 7 and ≥ 4, respectively), and indicate the presence of a nuclear starburst in the former galaxy. IR rotation curves were determined for both galaxies. In the case of NGC 2110, the IR rotation curve shows different behavior than the rotation curve determined from optical lines, which shows the importance of obscuration effects in the study of the gas kinematics.