  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.
