Extended gas in Seyfert galaxies: near-infrared observations of NGC 2110 and Circinus

Abstract

We present results of near-IR long-slit spectroscopy in the J and K bands of the Seyfert 2 galaxies NGC2110 and Circinus. Our goal is to investigate the gaseous distribution, excitation, reddening and kinematics, looking for signatures of the molecular torus hypothesized in unified models to both obscure and collimate the nuclear radiation. The two galaxies show extended emission in the IR emission lines [Fe II]λ1.257 μm, Paβ and H2ѵ ˆ 1-0 S(1), both along the major axis of the galaxy disc and perpendicular to it. In NGC 2110, the emission-line ratio [Fe II]/Paβ increases towards the nucleus, where its value is ~~ 7. Further, the nuclear [Fe II] and Paβ lines are broader (FWHM -~ 500 km s-ˡ) than the H2 line (FWHM ≤ 300 km s-ˡ). Both these results suggest that shocks, driven by the radio jet, are an important source of excitation of [Fe II], while the H2 excitation is dominated by X-rays from the nucleus. Br У is only observed at the nucleus, where H2/Br У ~~ 3. In the case of Circinus, both [Fe II]/ Pa β and H2/Br У decrease from ~~ 2 at 4 arcsec from the nucleus to nuclear values of ~~ 0.6 and ~~ 1, respectively, suggesting that the starburst dominates the nuclear excitation, while the active galactic nucleus (AGN) dominates the excitation further out (r ≥ 2 arcsec). For both galaxies, the gaseous kinematics are consistent with circular rotation in the plane of the disc. Our rotation curves suggest that the nucleus (identified with the peak of the IR continuum) is displaced from the kinematic centre of the galaxies. This effect has been observed previously in NGC2110 based on the kinematics of optical emission lines, but the displacement is smaller in the infrared, suggesting the effect is related to obscuration. The continuum J - K colours indicate a red stellar population in NGC 2110 and a reddened young stellar population in Circinus, outside the nucleus. Right at the nucleus of both galaxies, the colours are redder, apparently a result of hot dust emission, perhaps from the inner edge of a circumnuclear torus. In NGC2110, the signature of the hot dust emission is particularly clear in the K band, being seen as an additional component superimposed on the continuum observed in the J band.