S22A0025

The early evolution of supermassive black holes (SMBHs) is a central issue of astrophysics today. Past surveys revealed a number of massive SMBHs at z ≥ 6, while the lower-mass (< 10⁸ M_⊙) population remains poorly explored. Recently, we found two unusual z ∼ 6 quasars reminiscent of narrow-line Seyfert 1s – analogs to “the youngest AGNs" at lower z – from the HSC survey data. One of them hosts a fast-accreting SMBH with 4 × 10⁷ M_⊙, by far the smallest mass known at z ≥ 6. These objects may represent the long-sought, lowest-mass SMBH population prevalent in the early universe. We propose a systematic exploration of this new and intriguing population, through FOCAS spectroscopy of the bluest high-z quasar candidates that were previously overlooked. Our immediate objective is to establish a statistical sample and answer the following fundamental questions; (i) Is this population a rare outlier or a prevalent (but overlooked to date) class of objects at high redshifts? (ii) How does the number density compare to those of normal quasars? (iii) What fraction of galaxies host such fastest phase of SMBH growth? The established sample will trace the black hole mass function down to an unprecedentedly low-mass end (∼10⁶ M_⊙), providing a key to disentangling SMBH seeding models.

This document was translated from L^AT_EX by H^EV^EA.