Molecular hydrogen line emission in Seyfert galactic nuclei

Abstract

Abstract : We report on 2 micrometers spectroscopy of three Seyfert and two star burst galactic nuclei. We have detected line emission from vibrationally excited H2 in the Seyfert galactic nuclei NGC 1275, NGC 3227, and NGC 4151. For NGC 1275 and NGC 4151, these detections are the first reported detections of molecular line emission. We have also measured the Br(gamma) line flux in NGC 4151 and obtained an upper limit on the Br(gamma) line flux in NGC 1275. There is large range in the observed S(1) to Br(gamma) line ratio for both Seyfert and starburst galaxies (measured in this work and by others). We rule out UV fluorescence based on the S(1) to Br(gamma) line ration and the H2 line ratios in the Seyfert galaxy NGC 1275. Shocks probably excite the H2 emission in this galaxy. UV fluorescence may be the excitation mechanism in the Seyfert 1 galaxies NGC 4151 and NGC 3227. The H2 lines are not formed in the broad-line regions these Seyfert 1 galaxies based on our measured upper limits on the S(1) line widths. Simple starburst models cannot account for the highest of the measured ratios of S(1) to Br(gamma) line flux, most notably in the starburst galaxy NGC 6240 and in the peculiar Seyfert NGC 1275. Since the galaxies with the largest values of this ratio also have strong morphological evidence of galaxy-galaxy interactions, global shocks rather than shocks within young stellar outflows and remnants may be responsible for the excitation of the molecular hydrogen in these galaxies.