Near-infrared spectroscopy of Seyfert galaxies for examining the ionization mechanism of narrow-line regions

Koki Terao

It is widely believed that the dominant ionization mechanism of narrow-line regions (NLRs) in active galactic nuclei (AGNs) is the photoionization by ionizing photons from the central engine. However, it has been sometimes claimed that the fast shock may also contribute to the ionization of gas in NLRs. Such fast shocks in NLRs are possibly related to the so-called AGN feedback that may regulate the star formation activity in the host galaxy of AGNs, and therefore it is interesting to explore whether and how the fast shock contributes the NLR ionization in AGNs. To discriminate the ionization mechanism of gas clouds in NLRs observationally, the flux ratio of two forbidden emission lines seen in J-band, [Fe II]1.257 μm/[P II]1.188 μm, is useful (e.g., Oliva et al. 2001). The flux ratio of [Fe II]/[P II] in shock-excited regions is expected to be high (> 20) while that in purely photoionized clouds is low (< 2). We report our near-infrared spectroscopic observations of 26 nearby Seyfert galaxies with a near-infrared spectrograph (ISLE) boarded on the 188 cm telescope at Okayama Astrophysical Observatory. As a result, we measured the [Fe II]/[P II] flux ratio or its lower limit for 19 objects and 22 Seyfert galaxies from the literature. Based on the collected data, we found that the ionization mechanism of the NLR is the photoionization in most cases but the fast shock contributes in some Seyfert galaxies. The measured [Fe II]/[P II] flux ratio shows no correlation with the radio loudness, indicating that the radio jet is not the origin of fast shocks in NLRs of Seyfert galaxies. Possible candidate is AGN-driven outflow that blueshift and/or broad (FWHM > 1000 km/s) forbidden emission lines observed in low/high z AGNs.