Molecular hydrogen and [Fe II] in active galactic nuclei

Abstract

Near-infrared spectroscopy is used to study the kinematics and excitation mechanisms of H2 and [Fe II] lines in a sample of mostly Seyfert 1 galaxies. The spectral coverage allows simultaneous observation of the JHK bands, thus eliminating the aperture and seeing effects that have usually plagued previous works. The H2 lines are unresolved in all objects in which they were detected while the [Fe II] lines have widths implying gas velocities of up to 650 km s-ˡ. This suggests that, very likely, the H2 and [Fe II] emission does not originate from the same parcel of gas. Molecular H2 lines were detected in 90% of the sample, including PG objects, indicating detectable amounts of molecular material even in objects with low levels of circumnuclear starburst activity. Analysis of the observations favors thermal excitation mechanisms for the H2 lines. Indeed, in NGC3227, Mrk 766, NGC 4051 and NGC 4151, the molecular emission is found to be purely thermal but with heating processes that vary between the objects. Thermal excitation is also confirmed by the rather similar vibrational and rotational temperatures in the objects for which data were available. [Fe II] lines are detected in all of the sample AGN. The [Fe II] 1.254 μm/Paβ ratio is compatible with excitation of the [Fe II] lines by the active nucleus in most Seyfert 1 galaxies, but in Mrk 766 the ratio implies a stellar origin. A correlation between H2/Brγ and [Fe II]/Paβ is found for our sample objects supplemented by data from the literature. The correlation of these line ratios is a useful diagnostic tool in the NIR to separate emitting line objects by their level of nuclear activity. X-ray excitation models are able to explain the observed H2 and part of the [Fe II] emission but fail to explain the observations in Seyfert 2 galaxies. Most likely, a combination of X-ray heating, shocks driven by the radio jet and circumnuclear star formation contributes, in different proportions, to the H2 and [Fe II] lines observed. In most of our sample objects, the [Fe II] 1.257 μm/1.644 μm ratio is found to be 30% lower than the intrinsic value based on current atomic data. This implies either that the extinction towards the [Fe II]-emitting clouds is very similar in most objects or there are possible inaccuracies in the A-values in the Fe II transitions.