We present three-dimensional hydrodynamical simulations showing the effect of kinetic and radiative AGN feedback on a model galaxy representing a massive quiescent low-redshift early-type galaxy of M _ { * } = 8.41 \times 10 ^ { 10 } M _ { \odot } , harbouring a M _ { \mathrm { BH } } = 4 \times 10 ^ { 8 } M _ { \odot } black hole surrounded by a cooling gaseous halo .
We show that , for a total baryon fraction of \sim 20 \% of the cosmological value , feedback from the AGN can keep the galaxy quiescent for about 4.35 Gyr and with properties consistent with black hole mass and X-ray luminosity scaling relations .
However , this can only be achieved if the AGN feedback model includes both kinetic and radiative feedback modes .
The simulation with only kinetic feedback fails to keep the model galaxy fully quiescent , while one with only radiative feedback leads to excessive black-hole growth .
For higher baryon fractions ( e.g .
50 % of the cosmological value ) , the X-ray luminosities exceed observed values by at least one order of magnitude , and rapid cooling results in a star-forming galaxy .
The AGN plays a major role in keeping the circumgalactic gas at observed metallicities of Z / Z _ { \odot } \gtrsim 0.3 within the central \sim 30 kpc by venting nuclear gas enriched with metals from residual star formation activity .
As indicated by previous cosmological simulations , our results are consistent with a model for which the black hole mass and the total baryon fraction are set at higher redshifts z > 1 and the AGN alone can keep the model galaxy on observed scaling relations .
Models without AGN feedback violate both the quiescence criterion as well as CGM metallicity constraints .