An AGN with an ionized gas outflow in a massive quiescent galaxy in a protocluster at $\bf z=3.09$

We report the detection of an ionized gas outflow from an $X$-ray active galactic nucleus (AGN) hosted in a massive quiescent galaxy in a protocluster at $z=3.09$ (J221737.29+001823.4). It is a type-2 QSO with broad ($W_{80}>1000$ km s$^{-1}$) and strong ($\log (L_{\rm [OIII]}$ / erg s$^{-1})\approx43.4$) [O {\footnotesize III}]$\lambda\lambda$4959,5007 emission lines detected by slit spectroscopy in three-position angles using Multi-Object Infra-Red Camera and Spectrograph (MOIRCS) on the Subaru telescope and the Multi-Object Spectrometer For Infra-Red Exploration (MOSFIRE) on the Keck-I telescope. In the all slit directions, [O {\footnotesize III}] emission is extended to $\sim15$ physical kpc and indicates a powerful outflow spreading over the host galaxy. The inferred ionized gas mass outflow rate is $\rm 22\pm3~M_{\odot}~yr^{-1}$. Although it is a radio source, according to the line diagnostics using H$\beta$, [O {\footnotesize II}], and [O {\footnotesize III}], photoionization by the central QSO is likely the dominant ionization mechanism rather than shocks caused by radio jets. On the other hand, the spectral energy distribution of the host galaxy is well characterized as a quiescent galaxy that has shut down star formation by several hundred Myr ago. Our results suggest a scenario that QSOs are powered after the shut-down of the star formation and help to complete the quenching of massive quiescent galaxies at high redshift.