Multiscale mass transport in z~6 galactic discs: fueling black holes

TL;DR

This study uses high-resolution cosmological simulations to analyze mass transport mechanisms fueling supermassive black holes in galactic centers at high redshift, revealing dominant processes and accretion rates consistent with early quasar formation.

Contribution

It provides a detailed multiscale analysis of mass transport processes and their role in SMBH growth at high redshift, incorporating both Reynolds and gravitational stresses.

Findings

Reynolds stress dominates the alpha parameter with values much greater than 1.

Black holes grow at the Eddington limit during certain periods.

Average accretion rates are a few solar masses per year, supporting early quasar formation models.

Abstract

By using AMR cosmological hydrodynamic N-body zoom-in simulations, with the RAMSES code, we studied the mass transport processes onto galactic nuclei from high redshift up to $z\sim6$. Due to the large dynamical range of the simulations we were able to study the mass accretion process on scales from $\sim50[kpc]$ to $\sim$ few $1[pc]$. We studied the BH growth on to the galactic center in relation with the mass transport processes associated to both the Reynolds stress and the gravitational stress on the disc. Such methodology allowed us to identify the main mass transport process as a function of the scales of the problem. We found that in simulations that include radiative cooling and SNe feedback, the SMBH grows at the Eddington limit for some periods of time presenting $\langle f_{EDD}\rangle\approx 0.5$ throughout its evolution. The $\alpha$ parameter is dominated by the Reynolds term, $\alpha_R$, with $\alpha_R\gg 1$. The gravitational part of the $\alpha$ parameter, $\alpha_G$, has an increasing trend toward the galactic center at higher redshifts, with values $\alpha_G\sim 1$ at radii <$\sim$ few $ 10^1[pc]$ contributing to the BH fueling. In terms of torques, we also found that gravity has an increasing contribution toward the galactic center at earlier epochs with a mixed contribution above $\sim 100 [pc]$. This complementary work between pressure gradients and gravitational potential gradients allows an efficient mass transport on the disc with average mass accretion rates of the order $\sim$ few $1 [M_{\odot}/yr]$. These level of SMBH accretion rates found in our cosmological simulations are needed in all models of SMBH growth that attempt to explain the formation of redshift $6-7$ quasars.