Black Hole growth and AGN obscuration by instability-driven inflows in high-redshift disk galaxies fed by cold streams

TL;DR

High-redshift disk galaxies experience instability-driven inflows from cold streams that fuel black hole growth and cause AGN obscuration, with implications for galaxy evolution and detection in surveys.

Contribution

This paper provides analytic estimates and simulations showing how cold stream-fed inflows drive black hole growth and obscuration in high-redshift galaxies, a novel connection between inflow dynamics and AGN properties.

Findings

Inflow rate ~10 Msun/yr for z~2 galaxies of 10^11 Msun.

Black hole mass ~10^8 Msun with moderate accretion rates.

High gas densities can obscure AGN in high-redshift disks.

Abstract

Disk galaxies at high redshift have been predicted to maintain high gas surface densities due to continuous feeding by intense cold streams leading to violent gravitational instability, transient features and giant clumps. Gravitational torques between the perturbations drive angular momentum out and mass in, and the inflow provides the energy for keeping strong turbulence. We use analytic estimates of the inflow for a self-regulated unstable disk at a Toomre stability parameter Q~1, and isolated galaxy simulations capable of resolving the nuclear inflow down to the central parsec. We predict an average inflow rate ~10 Msun/yr through the disk of a 10^11 Msun galaxy, with conditions representative of z~2 stream-fed disks. The inflow rate scales with disk mass and (1+z)^{3/2}. It includes clump migration and inflow of the smoother component, valid even if clumps disrupt. This inflow grows the bulge, while only a fraction ~ 10^-3 of it needs to accrete onto a central black hole (BH), in order to obey the observed BH-bulge relation. A galaxy of 10^11 Msun at z~2 is expected to host a BH of ~10^8 Msun, accreting on average with moderate sub-Eddington luminosity L_X ~ 10^42-43 erg/s, accompanied by brighter episodes when dense clumps coalesce. We note that in rare massive galaxies at z~6, the same process may feed 10^9 Msun BH at the Eddington rate. High central gas column densities can severely obscure AGN in high-redshift disks, possibly hindering their detection in deep X-ray surveys.