On stellar-mass black hole mergers in AGN disks detectable with LIGO (1702.07818v2)

Abstract: Black hole mergers detectable with LIGO can occur in active galactic nucleus (AGN) disks. Here we parameterize the merger rates, the mass spectrum and the spin spectrum of black holes (BH) in AGN disks. The predicted merger rate spans $\sim 10^{-4}-10^{4} \rm{Gpc}^{-1} \rm{yr}^{-1}$, so upper limits from LIGO ($<212\rm{Gpc}^{-1}\rm{yr}^{-1}$) already constrain it. The predicted mass spectrum has the form of a broken power-law consisting of a pre-existing BH powerlaw mass spectrum and a harder powerlaw mass spectrum resulting from mergers. The predicted spin spectrum is multi-peaked with the evolution of retrograde spin BH in the gas disk playing a key role. We outline the large uncertainties in each of these LIGO observables for this channel and we discuss ways in which they can be constrained in the future.

• The paper demonstrates that LIGO data constrains AGN disk merger rates to below 212 Gpc⁻³ yr⁻¹.
• It employs theoretical models to predict a dual power-law mass spectrum and complex spin distributions shaped by accretion and mergers.
• The study highlights how observed mass and spin features could validate AGN disk evolutionary pathways in gravitational wave observations.

Constraining Stellar Mass Black Hole Mergers in AGN Disks Detectable with LIGO

The paper explores the occurrence of stellar mass black hole (BH) mergers within active galactic nucleus (AGN) disks, which are detectable by the Laser Interferometer Gravitational-Wave Observatory (LIGO). This investigation is encapsulated in a comprehensive analysis of the merger rates, alongside the mass and spin spectrum of BHs, aiming to interpret these occurrences within the context of current and future LIGO observations.

Merger Rates in AGN Disks

Black hole mergers in AGN disks display a theoretical rate spanning several orders of magnitude, from $10^{-4}$ to $10^4$ Gpc$^{-3}$ yr$^{-1}$. This range sharply contradicts the upper limit discerned from LIGO, which is $<212$ Gpc$^{-3}$ yr$^{-1}$. This incongruity provides a natural constraint on the merger rates, indicating the necessity to narrow down the contributing parameters. The parameters influencing these rates include the density of galactic nuclei, the proportion of such nuclei hosting active AGNs, and the fraction of BHs embedded in disk structures.

Mass and Spin Spectrum

The mass spectrum predicted for BH mergers features a dual power-law distribution, originating from both pre-existing and post-merger BHs. BH mass increase due to accretion and merger activity within the disk suggests a transformation to this broken power-law structure with $\gamma_2 < \gamma_1$. Notably, the investigation adopts a toy model that simulates how BHs uniformly distributed across an AGN disk mass spectrum may evolve through the successive stages.

Similarly, the spin spectrum is remarkably complex, as orbital dynamics in AGN disks foster multi-peaked spin distributions. The comparative rates of prograde and retrograde mergers within these frameworks highlight how differential accretion and mergers transform initial uniform spin distributions into systems dominated by extreme spin values.

Observational and Theoretical Implications

From an observational perspective, these findings present a pivotal opportunity for employing LIGO data to constrain AGN disk models. In particular, the identification of spin alignments and mass distributions within detected LIGO events could validate or refute certain AGN evolutionary pathways.

Furthermore, the emergence of "overweight" BHs merging into intermediate-mass BHs (IMBH) introduces prospects for LISA (Laser Interferometer Space Antenna) to detect such mergers around supermassive black holes, which provides an off-line complement to LIGO's high-frequency domain.

Future Directions

As LIGO and other GW observatories refine their results, they may provide key insights into the dynamics within AGN disks. There is a potential for studies and simulations that address uncertainties in the initial conditions of BHs in galactic centers, nuances in their mass accretion rates, and the dynamical importance of retrograde vs. prograde migrations.

Additionally, evolving observation strategies for electromagnetic signatures linked to BH mergers could yield complementary restrictions in constraining the phenomenology of AGNs. Likewise, growing databases of stellar and BH dynamics in galactic centers would offer empirical data crucial for refining the proposed models and enhancing our understanding of complex gravitational interactions in these violet large-scale structures.

In conclusion, this paper underscores the intricate dance of black holes within AGN disks and elucidates how such phenomena may be constrained using gravitational wave data. As research advances, constraints on BH characteristics in AGN environments will undoubtedly enhance our grasp of galactic evolution and the universe's dynamic gravitation blueprint.