Program: GN-2019A-FT-204

Previous studies have found billion-solar-mass super massive black holes (SMBHs) in the first billion years of the universe. Assuming constant growth from Pop-III stellar remnant seed BHs with <1000 solar mass, the growth timescale is comparable to the age of the universe at z=6, which puts a stringent constraint on the seed BH formation models. However, this argument stands on an assumption that the accretion rates are capped at the Eddington limit. Through our recent search of z>6 quasars using the Pan-STARRS1, we identified a remarkable quasar at z=6.34, PSO J083+11, on December 2018. From our quick 6.5m-Magellan/FIRE follow-up with 1 hour integration, we found that all key emission lines (Lyman-alpha, CIV, and MgII) appear unexpectedly weak and narrow. Initial virial BH mass measurement based on MgII shows that this object has very low mass (log M_BH = 7.86 Msun) and extreme accretion rate (L_bol/L_Edd = 14.1). We propose a deep NIR follow-up of PSO J083+11 with Gemini/GNIRS to secure our BH mass measurement, because the currently available spectrum is noisy and the MgII line measurment could be affected by weak iron emission lines. The confirmation of the super-Eddington accretion will be an important stepping stone to solve the mass assembly of billion solar mass in the early universe.