  Massive quiescent galaxies (MQGs) are thought to have formed stars rapidly at
early times followed by a long period of quiescence. The recent discovery of a
MQG, ZF-COSMOS-20115 at $z\sim4$, only 1.5 Gyr after the big bang, places new
constraints on galaxy growth and the role of feedback in early star formation.
Spectroscopic follow-up confirmed ZF-COSMOS-20115 as a MQG at $z=3.717$ with an
estimated stellar mass of ${\sim}10^{11}\mathrm{M}_\odot$, showing no evidence
of recent star formation. We use the Meraxes semi-analytic model to investigate
how ZF-COSMOS-20115 analogues build stellar mass, and why they become
quiescent. We identify three analogue galaxies with similar properties to
ZF-COSMOS-20115. We find that ZF-COSMOS-20115 is likely hosted by a massive
halo with virial mass of ${\sim}10^{13}\mathrm{M}_\odot$, having been through
significant mergers at early times. These merger events drove intense growth of
the nucleus, which later prevented cooling and quenched star formation.
ZF-COSMOS-20115 likely remained quiescent at $z<3.7$. We find that the
analogues host the most massive black holes in our simulation and were luminous
quasars at $z\sim5$, indicating that ZF-COSMOS-20115 and other MQGs may be the
descendants of high-redshift quasars. In addition, the model suggests that
ZF-COSMOS-20115 formed in a region of intergalactic medium that was reionized
early.
