The Redshift Evolution of the <i>M</i> _BH–<i>M</i> _* Scaling Relation: New Insights from Cosmological Simulations and Semianalytic Models

Abstract We study the coevolution of black holes (BHs) and their host galaxies in the A STRID and TNG300 cosmological simulations and the D ARK S AGE semianalytic model (SAM), focusing on the evolution of the BH mass–stellar mass ( M BH – M * ) relation. Due to differences in the adopted subgrid modeling of BH seeding, dynamics, and feedback, the models differ in their predicted redshift evolution of the M BH – M * relation. We find that it is the interplay between the star formation rate (SFR) and the black hole accretion rate (BHAR) that drives the evolution of the mean relation. We define a quantity <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">R</mml:mi> </mml:math> , the ratio between the specific BHAR and SFR (i.e., <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">R</mml:mi> <mml:mspace width="0.25em"/> <mml:mo>≡</mml:mo> <mml:mspace width="1em"/> </mml:math> sBHAR/sSFR), and demonstrate that it is <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">R</mml:mi> </mml:math> that governs the evolution of individual sources in the M BH – M * plane. The efficiency of BH growth versus stellar mass growth in the sSFR–sBHAR plane reflects the partitioning of gas between fueling star formation versus BH accretion. This partitioning depends on the implementation of BH dynamics and the nature of how black hole feedback quenches galaxies. In the cosmological simulations (A STRID and TNG300 ), the BHAR and SFR are intrinsically linked, resulting in a tight M BH – M * correlation, while the D ARK S AGE SAM produces a significantly larger scatter. We discuss these results in the context of recently discovered overmassive BHs and massive quenched galaxies at high redshift by the James Webb Space Telescope.