Infrared spectroscopy of Fe II, H 2 and H line emission in galactic nuclei.

The IRSPEC infrared array spectrometer (R ~ 1500) has been used at the ESO 3.6 m telescope to search for [Fe II] (1.644 micron), H_2_(v=1-0S(1), 2.121 micron and Brγ (H7-4, 2.166 micron) line emission in 35 galaxy nuclei selected to represent the full range of optical activity from pure H II region-like to Seyfert 1. All three lines have been detected in a large fraction of those nuclei exhibiting evidence for star formation activity but not in "pure" Seyferts. The highest detection frequency, however, was obtained amongst composite nuclei in which both Seyfert and star forming regions are present. Additional measurements of [Fe II] (1.257 micron and 1.60 micron) and Brα (H5-4,4.055 micron) were made in a few cases in order to estimate the denstiy in the Fe-zone and extinction. Linewidths are mostly close to our resolution limit of a few x 10^2^ kms^-1^ with a maximum of 800 km s^-1^ FWHM for [Fe II] in the exceptional merging system NGC 6240. This galaxy also exhibits [Fe II]/Brγ and H_2_/Brγ line ratios >~ 30 compared with values typically more around unity for the rest of the sample. In the H II/starburst galaxies it appears reasonable to attribute the [Fe II] and H_2_ emission to shock excitation by supernova remnants and the Brγ to recombination in H II regions. Adopting [Fe II] luminosity estimates based on recent observations of galactic and LMC SNR, the SN to ionization rates implied by the observed [Fe II]/Brγ ratios are just consistent with the predictions of existing starburst models. Composite nuclei are characterized by larger H_2_/Brγ ratios while their [Fe II]/Brγ ratios overlap with those in the starburst nuclei. The apparent continuity from starburst to composite nuclei suggests that SNR may also be an important source of shock excitation in the latter and that the relative increase in H_2_ emission might simply reflect a higher density of molecular clouds. It is also possible that the active nucleus provides additional energy capable of exciting H_2_ in circumnuclear molecular clouds. Plausible mechanisms include UV fluorescence, X-ray heating and shock excitation associated with mass outflows or collisions involving clouds moving in non-circular orbits. Although the composite nuclei exhibit a range of properties, however, no correlations with their H_2_ emission have been found which would indicate that any of these mechanisms is dominant.