Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

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-1. 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 NGC 3227, 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 transitions.

We provide a comprehensive census of the near-infrared (NIR, 0.8–2.4 μm) spectroscopic properties of 102 nearby (z < 0.075) active galactic nuclei (AGN), selected in the hard X-ray band (14–195 keV) from the Swift-Burst Alert Telescope survey. With the launch of the James Webb Space Telescope, this regime is of increasing importance for dusty and obscured AGN surveys. We measure black hole masses in 68 per cent (69/102) of the sample using broad emission lines (34/102) and/or the velocity dispersion of the Ca ii triplet or the CO band-heads (46/102). We find that emission-line diagnostics in the NIR are ineffective at identifying bright, nearby AGN galaxies because [Fe ii] 1.257 μm/Paβ and H₂ 2.12 μm/Brγ identify only 25 per cent (25/102) as AGN with significant overlap with star-forming galaxies and only 20 per cent of Seyfert 2 have detected coronal lines (6/30). We measure the coronal line emission in Seyfert 2 to be weaker than in Seyfert 1 of the same bolometric luminosity suggesting obscuration by the nuclear torus. We find that the correlation between the hard X-ray and the [Si vi] coronal line luminosity is significantly better than with the [O iii] λ5007 luminosity. Finally, we find 3/29 galaxies (10 per cent) that are optically classified as Seyfert 2 show broad emission lines in the NIR. These AGN have the lowest levels of obscuration among the Seyfert 2s in our sample (log N_H < 22.43 cm⁻²), and all show signs of galaxy-scale interactions or mergers suggesting that the optical broad emission lines are obscured by host galaxy dust.

Near-infrared spectroscopy is used to study the kinematics and excitation mechanisms of H₂ and [Fe ii] lines in a sample dominated by Seyfert 2 galaxies. The spectra simultaneously cover the JHK bands, allowing us to compare line fluxes emitted in the interval 0.8–2.4 μm and avoiding aperture and seeing effects. The H₂ lines are systematically narrower than the narrow-line region lines, suggesting that, very likely, the H₂ does not originate from the same parcel of gas that forms the narrow-line region. Emission-line ratios between H₂ lines favour thermal excitation mechanisms for the molecular gas in active galactic nuclei. It was found that non-thermal excitation contributes, at most, 30 per cent of the observed H₂. Thermal excitation is also confirmed by the rather similar vibrational and rotational temperatures in the objects (∼2000 K). The mass of hot H₂ ranges from 10² to 10³M_⊙, with nearly half of objects showing values of <500 M_⊙. It shows that the fraction of molecular mass present in the nuclear region and emitting in the near-infrared is a very small fraction of the warm molecular mass present in the centre. A diagnostic diagram composed of the line ratios H₂/Brγ and [Fe ii]/Paβ proves to be a useful tool in the near-infrared for separating emission-line objects by their degree of nuclear activity. We found that active galactic nuclei are characterized by H₂ 2.121 μm/Brγ and [Fe ii] 1.257 μm/Paβ flux ratios between 0.6 and 2. Starburst/H ii galaxies display line ratios <0.6 while low-ionization nuclear emission-line regions are characterized by values larger than 2 in either ratio.

We study the kinematics and excitation mechanisms of H₂ and [Fe ii] lines in a sample of 67 emission-line galaxies with Infrared Telescope Facility SpeX near-infrared (0.8–2.4 μm) spectroscopy together with new photoionization models. H₂ emission lines are systematically narrower than narrow-line region lines, suggesting that the two are, very likely, kinematically disconnected. The new models and emission-line ratios show that the thermal excitation plays an important role not only in active galactic nuclei but also in star-forming galaxies. The importance of the thermal excitation in star-forming galaxies may be associated with the presence of supernova remnants close to the region emitting H₂ lines. This hypothesis is further supported by the similarity between vibrational and rotational temperatures of H₂. We confirm that the diagram involving the line ratios H₂ 2.121 μm/Brγ and [Fe ii] 1.257 μm/Paβ is an efficient tool for separating emission-line objects according to their dominant types of activities. We suggest new limits to the line ratios in order to discriminate between the different types of nuclear activities.

We employ Infrared Telescope Facility SpeX near-infrared (NIR; 0.8–2.4 μm) spectra to investigate the stellar population (SP), active galactic nuclei