Dark-ages reionization and galaxy formation simulation - XIII. AGN quenching of high-redshift star formation in ZF-COSMOS-20115

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 aMQG, ZF-COSMOS-20115 at z ~ 4, 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 ~1011 M⊙, 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 ~1013M⊙, 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. Therefore, ZFCOSMOS- 20115 is unlikely to have experienced strong or extended star formation events at z < 3.7.We find that the analogues host the most massive black holes in our simulation and were luminous quasars at z ~ 5, indicating that ZF-COSMOS-20115 and other MQGs may be the descendants of high-redshift quasars. In addition, themodel suggests that ZF-COSMOS-20115 formed in a region of intergalactic medium that was reionized early.