  A comprehensive analysis of extended emission line region (EELR) around
quasars is presented. New Subaru/Suprime-Cam observation is combined with
literature search, resulting in a compilation of 81 EELR measurements for
type-1 and type-2 quasars with associated active galactic nucleus (AGN) and
host galaxy properties. It is found that EELR phenomenon shows clear
correlation with Eddington ratio, which links EELR to the constituents of the
principal component 1 (PC 1), or eigenvector 1, of the AGN emission
correlations. We also find that EELR is preferentially associated with
gas-rich, massive blue galaxies. It supports the idea that the primary
determinant of EELR creation is the 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
EELR is 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, activated AGN would create
EELR by the 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 AGN feedback process, which may be
playing a leading role in the co-evolution of galaxies and central
super-massive black holes.
