Co-evolution of Galaxies and Central Black Holes: Observational Evidence on the Trigger of AGN Feedback

A comprehensive analysis of the extended emission-line region (EELR) around quasars is presented. A new Subaru/Suprime-Cam observation is combined with a literature search, resulting in a compilation of 81 EELR measurements for type-1 and type-2 quasars with an associated active galactic nucleus (AGN) and host galaxy properties. It is found that the EELR phenomenon shows clear correlation with the Eddington ratio, which links EELR to the constituents of principal component 1, or eigenvector 1, of the AGN emission correlations. We also find that EELR is preferentially associated with gas-rich, massive blue galaxies. This supports the idea that the primary determinant of EELR creation is gas availability and that the gas may be brought in by galaxy merger, triggering the current star formation as well as AGN activity, and also gives an explanation for the fact that most luminous EELRs are found around radio-loud sources with low Eddington ratio. By combining all the observations, it is suggested that EELR quasars occupy the massive blue corner of the green valley, the AGN realm, on the galaxy color-stellar mass diagram. Once a galaxy is pushed to this corner, an activated AGN would create an EELR by energy injection into the interstellar gas and eventually blow it away, leading to star formation quenching. The results presented here provide a piece of evidence for the presence of such an AGN feedback process, which may play a leading role in the co-evolution of galaxies and central super-massive black holes.