Studying the redshift evolution of narrow-line regions in the active galactic nucleus

Yusuke Nitta

It is important to reveal how the interstellar mater (ISM) changed its properties across the cosmic time for understanding the total picture of the galaxy evolution. Theoretically, Kewley et al. (2013) showed that such redshift evolution of the ISM properties can be examined using emission-line diagnostics such as the so-called BPT diagram, for both star-forming galaxies and active galactic nuclei (AGNs). In star-forming galaxies, recent observational results show a clear redshift evolution of emission-line flux ratios, that is consistent to the expectation from the ISM evolution (e.g.,Steidel et al. 2014; Hayashi et al. 2015). However, the redshift evolution of the ISM in AGN host galaxies has not yet been investigated very well. Araki et al. (2012) performed near-infrared spectroscopic observation of a type 1 quasar at z~3 and measured its emission-line flux ratios of [NeIII]λ3869/[OIII]λ5007 and [OII]λ3727/[OIII]λ5007. They showed that some flux ratios of emission lines arising at the narrow-line region (NLR) in this object are systematically different from those seen in nearby Universe. The comparison between the obtained observational results with photoionization models suggests that the gas density in the NLR is significantly higher than that in low-z quasars. However, it is completely unclear whether or not such a high density is a typical NLR property at z~3. In this work, we performed near-infrared spectroscopy observations of five z~3 quasars with MOIRCS on the Subaru Telescope. We successfully detect three key diagnostic lines, [OII], [OIII], and [NeIII] in two of our targets, and measured their flux ratios. As a result, the emission-line flux ratios of [NeIII]/[OIII] and [OII]/[OIII] are different from those seen at z~1 but consistent to the result of Araki et al. (2012). These results suggest that the ISM in AGNs changed its properties across the cosmic time, as seen also in star-forming galaxies.