Star formation in self-gravitating disks in active galactic nuclei. I. Metallicity gradients in broad line regions

TL;DR

This paper models how star formation in accretion disks around supermassive black holes leads to metallicity gradients and hot gas diffusion, explaining observed features in active galactic nuclei.

Contribution

It introduces a new equation describing metallicity evolution in star-forming disks and links hot gas diffusion to metallicity gradients in broad line regions.

Findings

Metallicity gradients are established and evolve in steady-state AGNs.

Hot gas escapes from the star-forming disk, contributing to broad line regions.

Metallicity can serve as a probe of the star-forming disk age.

Abstract

It has been suggested that the high metallicity generally observed in active galactic nuclei (AGNs) and quasars originates from ongoing star formation in the self-gravitating part of accretion disks around the supermassive black holes. We designate this region as the star forming (SF) disk, in which metals are produced from supernova explosions (SNexp) while at the same time inflows are driven by SNexp-excited turbulent viscosity to accrete onto the SMBHs. In this paper, an equation of metallicity governed by SNexp and radial advection is established to describe the metal distribution and evolution in the SF disk. We find that the metal abundance is enriched at different rates at different positions in the disk, and that a metallicity gradient is set up that evolves for steady-state AGNs. Metallicity as an integrated physical parameter can be used as a probe of the SF disk age during one episode of SMBH activity. In the SF disk, evaporation of molecular clouds heated by SNexp blast waves unavoidably forms hot gas. This heating is eventually balanced by the cooling of the hot gas, but we show that the hot gas will escape from the SF disk before being cooled, and diffuse into the BLRs forming with a typical rate of $\sim 1\sunmyr$. The diffusion of hot gas from a SF disk depends on ongoing star formation, leading to the metallicity gradients in BLR observed in AGNs. We discuss this and other observable consequences of this scenario.