Molecular hydrogen excitation around active galactic nuclei

We report R similar to 3000 Very Large Telescope ISAAC K-band spectroscopy of the nuclei (i.e., central 100 - 300 pc) of nine galaxies hosting an active galactic nucleus. For five of these we also present spectra of the circumnuclear region out to 1 kpc. We have measured a number of molecular hydrogen lines in the nu = 1 - 0, 2 - 1, and 3 - 2 vibrational transitions, as well as the Br gamma and He I recombination lines and the Na I stellar absorption feature. Although only three of the galaxies are classified as type 1 Seyfert galaxies in the literature, broad Br gamma (FWHM greater than or similar to 1000 km s(-1)) is seen in seven of the objects. The nu = 1-0 emission appears thermalized at temperatures T similar to 1000 K. However, the nu = 2 - 1 and nu = 3 - 2 emission show evidence of being radiatively excited by far-ultraviolet photons. The photodis-sociation region models that fit the data best are, as for the ultraluminous infrared galaxies in Davies et al., those for which the H-2 emission arises in dense clouds illuminated by intense far-ultraviolet radiation. The Na I stellar absorption line is clearly seen in six of the nuclear spectra of these AGNs, indicating the presence of a significant population of late-type stars. It is possible that these stars are a result of the same episode of star formation that gave rise to the stars heating the photodissociation regions. It seems unlikely that the AGN is the dominant source of excitation for the near-infrared H-2 emission: in two of the nuclear spectra H-2 was not detected at all, and in general we find no evidence of suppression of the 2 - 1 S(3) line, which may occur in X-ray-irradiated gas. Our data do not reveal any significant difference between the nuclear and circumnuclear line ratios, suggesting that the physical conditions of the dominant excitation mechanism are similar both near the AGN and in the larger scale environment around it, and that star formation is an important process even in the central 100 pc around AGNs.