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 similar to 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-COSMOS20115 as a MQG at z = 3.717 with an estimated stellar mass of similar to 10(11) M circle dot, 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 similar to 10(13)M circle dot, 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 similar to 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.