How long do high redshift massive black hole seeds remain outliers in black hole versus host galaxy relations?

Abstract

The existence of 109 M☉ supermassive black holes (SMBHs) within the first billion years of the Universe remains a puzzle in our conventional understanding of black hole formation and growth. Several suggested formation pathways for these SMBHs lead to a heavy seed, with an initial black hole mass of 104–106 M☉. This can lead to an overly massive BH galaxy (OMBG), whose nuclear black hole’s mass is comparable to or even greater than the surrounding stellar mass: the black hole to stellar mass ratio is Mbh/M∗ ≫ 10−3, well in excess of the typical values at lower redshift. We investigate how long these newborn BHs remain outliers in the Mbh − M∗ relation, by exploring the subsequent evolution of two OMBGs previously identified in the Renaissance simulations. We find that both OMBGs have Mbh/M∗ > 1 during their entire life, from their birth at z ≈ 15 until they merge with much more massive haloes at z ≈ 8. We find that the OMBGs are spatially resolvable from their more massive, 1011 M☉, neighbouring haloes until their mergers are complete at z ≈ 8. This affords a window for future observations with JWST and sensitive X-ray telescopes to diagnose the heavy-seed scenario, by detecting similar OMBGs and establishing their uniquely high black hole-to-stellar mass ratio.