Growth of massive black holes by super-Eddington accretion

Abstract

Narrow-Line Seyfert 1 galaxies (NLS1s) and Narrow-Line quasars (NLQs) seem to amount to ∼10-30% of active galactic nuclei (AGNs) in the local universe. Together with their average accretion rate, we argue that a black hole (BH) growth by factor of 8-800 happens in these super-Eddington accretion phase of AGNs. Moreover, there is a possible, systematic underestimation of accretion rates (in the Eddington unit) due to an overestimation of BH mass by massive accretion discs for super-Eddington objects. If it is true, the factor of BH growth above may be larger by order(s) of magnitude. In contrast, the growth factor expected in sub-Eddington phase is only ∼2. Therefore, the cosmic BH growth by accretion is likely dominated by super-Eddington phase, rather than sub-Eddington phase which is the majority among AGNs. This analysis is based on the fraction and the average accretion rate of NLs1s and NLQs obtained for z ≲ 0.5. If those numbers are larger at higher redshift (where BHs were probably less grown), super-Eddington accretion would be even more important in the context of cosmic BH growth history.