Assisted Inspirals of Stellar Mass Black Holes Embedded in AGN Disks: Solving the "Final AU Problem" (1602.04226v2)

Abstract: We explore the evolution of stellar mass black hole binaries (BHBs) which are formed in the self-gravitating disks of active galactic nuclei (AGN). Hardening due to three-body scattering and gaseous drag are effective mechanisms that reduce the semi-major axis of a BHB to radii where gravitational waves take over, on timescales shorter than the typical lifetime of the AGN disk. Taking observationally-motivated assumptions for the rate of star formation in AGN disks, we find a rate of disk-induced BHB mergers ($\mathcal{R} \sim 3~{\rm yr}^{-1}~{\rm Gpc}^{-3}$, but with large uncertainties) that is comparable with existing estimates of the field rate of BHB mergers, and the approximate BHB merger rate implied by the recent Advanced LIGO detection of GW150914. BHBs formed thorough this channel will frequently be associated with luminous AGN, which are relatively rare within the sky error regions of future gravitational wave detector arrays. This channel could also possess a (potentially transient) electromagnetic counterpart due to super-Eddington accretion onto the stellar mass black hole following the merger.

• The paper demonstrates that AGN disk dynamics, combining gas torques and three-body interactions, effectively reduce binary separations to trigger gravitational wave emission.
• The study leverages established AGN disk models to estimate star formation rates and disk properties, aligning theoretical predictions with observed SMBH growth.
• The research estimates merger rates on par with gravitational wave detections and suggests potential electromagnetic counterparts to aid host galaxy identification.

Overview of "Assisted Inspirals of Stellar Mass Black Holes Embedded in AGN Disks: Solving the 'Final AU Problem'"

In this paper, Stone, Metzger, and Haiman propose a novel mechanism for the inspiral and merger of stellar mass black hole binaries (BHBs) within the accretion disks of active galactic nuclei (AGN). The paper addresses the challenge associated with the "final AU problem," wherein binaries must shrink to separations small enough for gravitational wave (GW) emission to drive their eventual coalescence. By situating these BHBs within star-forming AGN disks, the authors theorize that a combination of three-body interactions and gaseous torques can effectively assist the hardening process, reducing BHB separations to the necessary scales on timescales shorter than the AGN disk's typical lifespan.

The authors estimate that the rate of BHB mergers induced by AGN disks could potentially match or even exceed the field rate observed from GW detections such as GW150914. They further speculate on the potential for an electromagnetic (EM) counterpart to these mergers due to super-Eddington accretion, offering a unique observational opportunity distinct from EM-dark mergers in other environments.

Key Contributions and Analysis

1. AGN Disk-Induced BHB Hardening:
   • The paper presents two mechanisms: gravitational interactions within the star-forming disk and gas-induced torques resulting from interaction with AGN disk material. These processes harden the BHBs by reducing their semi-major axis, making it easier for GW forces to dominate the final stages.
2. Theoretical Modeling:
   • The authors utilize the star-forming, self-gravitating AGN disk models of Thompson et al. to estimate star formation rates and disk properties. They validate this theoretical approach by aligning it with SMBH growth observed in the local universe.
3. Rate Estimations:

-The research estimates a volumetric rate for disk-induced BHB mergers of about $\mathcal{R} \sim 3~{\rm Gpc}^{-3}~{\rm yr}^{-1}$, with significant uncertainties. Notably, the consideration of a top-heavy initial mass function (IMF) relevant to AGN disks has the potential to inflate these estimates further.

1. Observational Implications:
   • This BHB formation channel offers a potentially unique haLLMark: a transient EM counterpart in the presence of an AGN disk. Such association reduces the candidate host galaxies within the LIGO error volume, enhancing the prospects of EM follow-up and host galaxy identification.
2. Speculative and Future Directions:
   • The work highlights the need for more comprehensive simulations and enhanced astrophysical modelling to better understand the coupling between BHB dynamics and the disk environment. Future studies might refine these rate calculations and EM predictions, helping to interpret forthcoming observational data.

Conclusion

The paper underscores the feasibility of a star formation-driven BHB merger channel in AGN disks and its potential to contribute significantly to the rates of observable GW events. It challenges existing paradigms by suggesting that such embedded binaries not only have competitive merger rates but also offer a practical pathway for EM counterpart detection, an area typically lacking in other formation channels. While uncertainties remain, particularly regarding feedback processes and the exact nature of disk-binary interactions, this framework opens promising avenues for combined GW and multi-messenger astronomy.