BLACK HOLE GROWTH AND ACTIVE GALACTIC NUCLEI OBSCURATION BY INSTABILITY-DRIVEN INFLOWS IN HIGH-REDSHIFT DISK GALAXIES FED BY COLD STREAMS

Abstract

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