2022ApJ...940L..49N

The origin of rare and elusive ultramassive black holes (UMBH; with M_BH > 10¹⁰M_⊙) is an open question. Using the large volume cosmological hydrodynamic simulation ASTRID, we report on the formation of an extremely massive UMBH with M_BH ∼ 10¹¹M_⊙ at z ∼ 2. The UMBH is assembled as a result of two successive mergers of massive galaxies each with stellar mass M_* > 3 × 10¹¹M_⊙ that also produces a bright, rare triple quasar system powered by three ∼10⁹M_⊙ black holes. The second merger of supermassive black holes (SMBHs) follows the first after 150 Myr. The merger events lead to sustained Eddington accretion onto the central SMBH, forming a UMBH in the center of a massive compact stellar core with M_* > 2 × 10¹²M_⊙. The strong feedback of the UMBH quenches the surrounding star formation to <10 M_⊙ yr^–1 in the inner 50 h^–1 kpc region. There are two more UMBHs with M_BH > 5 × 10¹⁰M_⊙ at z > 2 in ASTRID that are also produced by major mergers of galaxies, and their progenitors can be observed as quasar triplets of lower luminosity. The rarely observed quasar multiples can be the cradle of UMBHs at high redshift, and likely end up in the center of the most massive clusters.