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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.

 

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. The so-called direct-collapse scenario suggests that the formation of supermassive stars (SMSs) can yield the massive seeds of early SMBHs. This scenario leads to an overly massive BH galaxy (OMBG), whose nuclear black hole’s mass is comparable to or even greater than the surrounding stellar mass: a 104 − 106 M⊙ seed black hole is born in a dark matter halo with a mass as low as 107 − 108 M⊙. 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 have𝑀bh/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⊙, neighboring 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 direct-collapse scenario, by detecting similar OMBGs and establishing their uniquely high black hole-to-stellar mass ratio.