Ultramassive Black Holes Formed by Triple Quasar Mergers at z ∼ 2

The origin of rare and elusive ultramassive black holes (UMBH; with MBH > 1010 M⊙) is an open question. Using the large volume cosmological hydrodynamic simulation ASTRID, we report on the formation of an extremely massive UMBH with MBH ∼ 1011 M⊙ at z ∼ 2. The UMBH is assembled as a result of two successive mergers of massive galaxies each with stellar mass M* > 3 × 1011 M⊙ that also produces a bright, rare triple quasar system powered by three ∼109 M⊙ 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 × 1012 M⊙. 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 MBH > 5 × 1010 M⊙ 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.