The Formation of the Most Massive Supermassive Black Hole at z=7.5

The highest redshift quasars provide direct probes of the evolution ofsupermassive black holes (SMBHs) and the intergalactic medium at the earliestcosmic times. The discovery of a 7.8E8 Msun SMBH at z7.54 (J1342+0928) put thestrongest constrains on the seed BH and BH accretion history (Banados et al.2018). Very recently, our team has found an even more extreme object, J1007 atz7.5. J1007 is 1.8 times more luminous than J1342+0928 and hosts a SMBH withmass 1.6E^9 Msun, two times more massive than J1342+0928. The existence of J1007posts the most significant challenge to our understanding of the formation ofthe earliest SMBHs. Here, we propose deep XMMNewton observations tocharacterize the accretion properties of J1007 and investigate its formation and growth mechanisms. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]