Star Formation in Self-gravitating Disks in Active Galactic Nuclei. III. Efficient Production of Iron and Infrared Spectral Energy Distributions

Abstract

Strong iron lines are a common feature of the optical spectra of active galactic nuclei (AGNs) and quasars from z ∼ 6−7 to the local universe, and [Fe/Mg] ratios do not show cosmic evolution. During active episodes, accretion disks surrounding supermassive black holes (SMBHs) inevitably form stars in the self-gravitating part, and these stars accrete with high accretion rates. In this paper, we investigate the population evolution of accretion-modified stars (AMSs) to produce iron and magnesium in AGNs. The AMSs, as a new type of star, are allowed to have any metallicity but without significant loss from stellar winds, since the winds are choked by the dense medium of the disks and return to the core stars. Mass functions of the AMS population show a pile-up or cutoff pile-up shape in top-heavy or top-dominant forms if the stellar winds are strong, consistent with the narrow range of supernovae (SNe) explosions driven by the known pair-instability. This provides an efficient way to produce metals. Meanwhile, SN explosions support an inflated disk as a dusty torus. Furthermore, the evolving top-heavy initial mass functions lead to bright luminosity in infrared bands in dusty regions. This contributes a new component in infrared bands, which is independent of the emissions from the central part of accretion disks, appearing as a long-term trending of the NIR continuum compared to optical variations. Moreover, the model can be further tested through reverberation mapping of emission lines, including LIGO/LISA detections of gravitational waves and signatures from spatially resolved observations of GRAVITY+/VLTI.

Stellar Black Holes Can “Stretch” Supermassive Black Hole Accretion Disks

“…Previous studies have shown that star formation in the AGN accretion disk can revise the AGN SEDs (e.g., Goodman & Tan 2004;Thompson et al 2005;Wang et al 2023b), and the same is expected for accreting sBHs. The top panel of Figure 2 shows the SEDs of a pure SSD and an SSD embedded with sBHs for M • = 10 8 M e with f sBHs = 0.1 (for a pure SSD, f sBHs ≡ 0).…”

“…Star formation in the outer regions of the accretion disks of active galactic nuclei (AGNs) is a widely studied topic (e.g., Shlosman & Begelman 1989;Collin & Zahn 1999;Goodman & Tan 2004;Levin 2007;Wang et al 2011Wang et al , 2023bCantiello et al 2021;Chen et al 2023). The outer regions of the accretion disk are far from the central supermassive black hole (SMBH), where self-gravity plays a dominant role, and the disk inevitably collapses to form stars (e.g., Kolykhalov & Syunyaev 1980;Shlosman & Begelman 1989).…”

Accretion-modified Stars in Accretion Disks of Active Galactic Nuclei: The Low-luminosity Cases and an Application to Sgr A*

“…This interesting idea can be traced back to the early paper of Cheng & Wang (1999), who suggest that compact objects formed in AGN disks undergo mergers generating γ-ray bursts and GWs. The terminology of accretion-modified stars (AMS) used in this series are referred to as ones with accretion from AGN disks (Wang et al 2021a), where the accreting objects could be mainsequence stars (Wang et al 2023), stellar-mass BHs (sMBHs; Wang et al 2021b), neutron stars (Zhu et al 2021b), or white dwarf stars (Zhu et al 2021a;Zhang et al 2023). AMS phenomena exhibit distinguished features in different environments as predicted by the above papers.…”

“…Consequently, it may be an efficient way of fueling gas to galactic centers, triggering the activity of SMBHs (Shlosman & Begelman 1989;Thompson et al 2005;Wang et al 2010). Spectral energy distributions (SEDs) of AGNs could be revised by star formation (Goodman 2003;Goodman & Tan 2004;Thompson et al 2005) as well as the origins of high metallicity (Collin & Zahn 1999, 2008Wang et al 2011Wang et al , 2012bWang et al , 2023Grishin et al 2021;Fan & Wu 2023) observed in AGN broad-line regions (Hamann & Ferland 1999;Warner et al 2003;Nagao et al 2006;Shin et al 2013;Du & Wang 2014). After supernovae explosions of stars formed in the AGN disks, there remain compact objects as satellites of the central SMBHs.…”

The Interaction between Stars and Past AGN Disk: Possible Explanation for the Kinematic Distributions of S-stars in the Galactic Center

“…It has been suggested that the outer regions of accretion disks are gravitationally unstable and could trigger fast star formation (e.g., Paczynski 1978;Kolykhalov & Syunyaev 1980;Chen et al 2023). The broad emission lines of AGNs indicate that the vicinities of SMBHs are metal-rich (e.g., several times the solar abundances) and seem to be independent of redshifts (Hamann & Ferland 1992, 1999Nagao et al 2006;Xu et al 2018;Wang et al 2022), which is widely explained by rapid and intense star formation and evolution in AGN disk (Artymowicz et al 1993;Wang et al 2010Wang et al , 2011Wang et al , 2012Fan & Wu 2023;Wang et al 2023). Paumard et al (2006) suggested that in situ star formation in the accretion disk is in good agreement with the disk structure and stellar features of the stars in the CWD (see also Levin & Beloborodov 2003;Nayakshin et al 2007;Bonnell & Rice 2008;Mapelli et al 2012).…”