Star Formation in Self-Gravitating Disks in Active Galactic Nuclei. I. Metallicity Gradients in Broad-Line Regions

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 ∼ 1M ⊙ yr −1 . 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.

The evolution of chemical abundance in quasar broad line region

“…Besides the formation of the stars, whether the stars will eventually enrich the gas is another issue. A few theoretical studies show that the fragmentation of the unstable gaseous disk is able to give rise to the formation of protostars and consequently results in supernova explosion producing strong enriched outflows (Collin & Zahn 1999b, 2008Wang et al 2011b). Another possible scenario is that if the formed stars exceed a few hundred solar masses, the stars may disrupt themselves immediately upon reaching the zeroage main sequence due to the pulsational instabilities and then enrich CNO abundance of the surrounding diffuse gas by returning the mass after disruption (Jiang & Goodman 2011).…”

Abundance anomalies in tidal disruption events

“…A more general puzzle about abundances inferred from quasar BLRs is that they appear to require very high metallicities, Z > ∼ 5Z⊙ (e.g., Dietrich et al 2003, Nagao et al 2006. It is somewhat challenging to reach these metallicities with global star formation models (e.g., Hamann & Ferland 1993, Friaca & Terlevich 1998, Romano et al 2002, leading to an alternative picture of star formation associated with the outer, self-gravitating parts of the accretion flow (e.g., Collin & Zahn 1999, Wang et al 2011. BLR metallicity estimates use the steady increase of nitrogen relative to oxygen and carbon as the overall metallicity increases (Hamann & Ferland 1993).…”

Interpretation of Departure from the Broad-line Region Scaling in Active Galactic Nuclei

“…Some models specified the inner BLR radius on some physical grounds (e.g. disk local self-gravity, Wang et al 2011Wang et al , 2012; dust presence in the accretion disk atmosphere, Czerny et al 2015Czerny et al , 2017.…”