Investigating the residuals in the $M_\bullet-M_*$ relation using the SIMBA cosmological simulation

TL;DR

This study uses the SIMBA simulation to analyze the black hole-stellar mass relation, revealing how residuals correlate with galaxy properties and evolve through different accretion modes, shedding light on galaxy and black hole coevolution.

Contribution

It demonstrates that SIMBA accurately reproduces the $M_ullet-M_*$ relation and elucidates the roles of torque and Bondi accretion in shaping residuals and galaxy evolution.

Findings

Residuals correlate with gas content, star formation, and black hole accretion.

Torque accretion dominates at high redshift and low mass, Bondi at low redshift and high mass.

Early-formed galaxies have larger black holes at fixed stellar mass due to different accretion histories.

Abstract

We study the scaling relation between the black hole and stellar mass ($M_\bullet-M_*$), diagnosing the residual $\Delta \log(M_\bullet/M_\odot)$ ($\Delta$) in this relation to understand the coevolution of the galaxy and black hole (BH) in the cosmological hydrodynamic simulation SIMBA. We showed that SIMBA can reproduce the observed $M_\bullet-M_*$ relation well with little difference between central and satellite galaxies. By using the median value to determine the residuals, we found that the residual is correlated with galaxy cold gas content, star formation rate, colour and black hole accretion properties. Both torque and Bondi models implemented in SIMBA, contribute to this residual, with torque accretion playing a major role at high redshift and low-mass galaxies, while Bondi (also BH merge) takes over at low redshift and massive galaxies. By dividing the sample into two populations: $\Delta>0$ and $\Delta <0$, we compared their evolution paths following the main progenitors. With evolution tracking, we proposed a simple picture for the BH-galaxy coevolution: Early-formed galaxies seeded black holes earlier, with stellar mass increasing rapidly to quickly reach the point of triggering `jet mode' feedback. This process reduced the cold gas content and stopped the growth of $M_*$, effectively quenching galaxies. Meanwhile, during the initial phase of torque accretion growth, the BH mass is comparable between galaxies formed early and those formed later. However, those galaxies that formed earlier appear to attain a marginally greater BH mass when shifting to Bondi accretion, aligning with the galaxy transition time. As the early-formed galaxies reach this point earlier -- leaving a longer time for them to have Bondi accretion as well as merging, their residuals become positive, i.e., having more massive BHs at $z=0$ compared to these late-formed galaxies at the same $M_*$. This picture is further supported by the strong positive correlation between the residuals and the galaxy age, which we are proposing as a verification with observation data on this story suggested by SIMBA.