Gas Accretion versus BH Merger driven Growth Modes of Supermassive Black Holes and Implications for the Little Red Dots

Abstract

We investigate the growth of central supermassive black holes in galaxies, aiming to distinguish between gas accretion versus BH merger-driven growth modes. By performing and analysing cosmological hydrodynamical simulations of $(50 ~ {\rm Mpc})^3$ comoving boxes, we also study how the BH feedback parameters affect the coevolution between SMBHs and their host galaxies. Starting as $10^5 M_{\odot}$ seeds, we find that the BHs grow initially via BH mergers to $\sim 10^7 M_{\odot}$. Gas accretion onto the BHs is initially low, then increases with time, and reaches the Eddington rate after $7-9$ Gyrs. The BHs then undergo very fast growth via efficient gas accretion over a period of $600 - 700$ Myr, when the BH mass increases $10^2 - 10^3$ times, causing their predominant growth from $10^7 M_{\odot}$ to $(10^9 - 10^{10}) M_{\odot}$. Taking into account the cosmological gas inflows and outflows, SMBHs do not grow to more than $10^{10} M_{\odot}$ by $z=0$, because of gas depletion from galaxy centers driven by AGN feedback. In terms of SMBH - host galaxy coevolution along the $M_{\rm BH} - M_{\star}$ relation, we find that they initially lie below and thereby move upward toward the relation. We make some physical implications of the growth of high-$z$ Little Red Dots recently observed by JWST: the normal-mass SMBHs had predominantly undergone BH merger driven evolution, whereas the overmassive BHs underwent periods of Eddington-limited or super-Eddington bursts of gas accretion.

Figures (8)

• Figure 1: Cosmic build-up of the BH mass versus stellar mass of our simulated galaxies in run BHs5e0.1v5, each panel indicating one redshift $z = 5, 3, 2, 1, 0.5, 0$. The blue circles are galaxies from our simulation. The black lines indicate the observed BH mass versus galaxy stellar mass relation for: local galaxies showing the correlation with the bulge mass as the solid Marconi03, dashed McConnell13, and dotted Reines15 lines; as well as $z \sim 6$ quasars Wang10 as the dash-dotted line.
• Figure 2: Growth with redshift of BH mass (top row) and Eddington accretion ratio (bottom row) of some of the massive BHs in our standard simulation BHs5e0.1v5. The left column indicate six most-massive BHs with final mass $M_{\rm BH (z=0)} > 10^{9} M_{\odot}$, and the right column shows six intermediate-mass BHs of $M_{\rm BH (z=0)} = (10^{7} - 10^{8}) M_{\odot}$.
• Figure 3: Growth of BH mass due to Mergers or Accretion of the same BHs as plotted in Fig. \ref{['fig-BH-Mass-EddRatio-vs-z']}: six most-massive BHs (top row) and six intermediate-mass BHs (bottom row) from our standard simulation BHs5e0.1v5. The left column shows the Merger-Dominated mass growth regime and the right column presents the Accretion-Dominated mass growth regime; the two regimes distinguished with a limiting Eddington accretion ratio $= 0.1$ as described in §\ref{['sec-res-MD-vs -AD']}.
• Figure 4: Left column: Build up with redshift of the stellar mass of the host galaxy of the six most-massive BHs (top row) and six intermediate-mass BHs (bottom row); same BHs which were plotted in Fig. \ref{['fig-BH-Mass-EddRatio-vs-z']}, distinguished by the plotting colours. Right column: Redshift track or the evolution with cosmic time of the BH mass versus host galaxy stellar mass of the black holes. The black lines indicate the observed BH mass versus galaxy stellar mass relation of: local galaxies Marconi03 as the dashed line, and $z=6$ quasars Wang10 as the solid line.
• Figure 5: Gas overdensity in our standard simulation BHs5e0.1v5 at six epochs $z = 3, 2, 1, 0.5, 0.2, 0$. The BH which would become most-massive at $z=0$ is back tracked, and each panel shows a projected $(2000 h^{-1}$ kpc$)^3$ comoving volume around the location of this BH. The red cross symbols indicate the positions of all the BHs inside the plotted region, with the symbol size proportional to BH mass. The black circle is the virial radius $R_{\rm 200}$ of the host galaxy of the back-tracked BH, the red circles depict the $R_{\rm 200}$ of the galaxies with the mass range $M_{\rm halo} > 10^{12} M_{\odot}$, while the blue circles show the $R_{\rm 200}$ of $(10^{11} < M_{\rm halo} < 10^{12}) M_{\odot}$ galaxies.