Molecular Hydrogen Excitation around Active Galactic Nuclei

Abstract

We report R~3000 VLT ISAAC K-band spectroscopy of the nuclei (i.e. central 100-300pc) of 9 galaxies hosting an active galactic nucleus. For 5 of these we also present spectra of the circumnuclear region out to 1kpc. We have measured a number of H_2 lines in the v=1-0, 2-1, and 3-2 vibrational transitions, as well as the Br_gamma and HeI recombination lines, and the NaI stellar absorption. Although only 3 of the galaxies are classified as Seyfert 1s in the literature, broad Br_gamma (FWHM 1000kms) is seen in 7 of the objects. The v=1-0 emission appears thermalised at T~1000K. However, the v=2-1 and 3-2 emission show evidence of being radiatively excited by far-UV photons. The PDR models that fit the data best are, as for the ultraluminous infrared galaxies in Davies et al. (2003), those for which the H_2 emission arises in dense clouds illuminated by intense FUV radiation. The NaI stellar absorption is clearly seen in 6 of the nuclear spectra, 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 PDRs. It seems unlikely that the AGN is the dominant source of excitation for the near infrared H_2 emission: in two nuclei H_2 was not detected at all, and in general we find no evidence of suppression of the 2-1S(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 100pc acround AGN.