[Paper Review] Gravitational wave source populations: Disentangling an AGN component

V. Gayathri, D. M. Wysocki+5 more|arXiv (Cornell University)|Jan 10, 2023

Pulsars and Gravitational Waves Research4 citations

TL;DR

This paper introduces a hierarchical population analysis framework to disentangle the contribution of active galactic nucleus (AGN) disks to gravitational wave source populations, comparing AGN-based formation models with phenomenological models. It finds that high-mass and high-mass-ratio binary black hole mergers are more likely to originate in AGN disks, with a significant AGN component inferred at ~O(5 Gpc⁻³ yr⁻¹), while lower-mass events are better explained by other channels.

ABSTRACT

The astrophysical origin of the over 90 compact binary mergers discovered by the LIGO and Virgo gravitational wave observatories is an open question. While the unusual mass and spin of some of the discovered objects constrain progenitor scenarios, the observed mergers are consistent with multiple interpretations. A promising approach to solve this question is to consider the observed distributions of binary properties and compare them to expectations from different origin scenarios. Here we describe a new hierarchical population analysis framework to assess the relative contribution of different formation channels simultaneously. For this study we considered binary formation in AGN disks along with phenomenological models, but the same framework can be extended to other models. We find that high-mass and high-mass-ratio binaries appear more likely to have an AGN origin compared to the same origin as lower-mass events. Future observations of high-mass black hole mergers could further disentangle the AGN component from other channels.

Motivation & Objective

• To determine the relative contribution of AGN disk formation to the observed gravitational wave binary black hole population.
• To address the ambiguity in astrophysical origins of LIGO-Virgo detected mergers, particularly for high-mass and high-mass-ratio events.
• To develop a flexible, hierarchical inference framework that simultaneously models multiple formation channels, including AGN and phenomenological models.
• To assess how the presence of AGN formation affects the inferred parameters of other formation channels, such as power-law distributions.
• To evaluate whether current gravitational wave data can distinguish AGN-originated binaries from other formation scenarios.

Proposed method

• Proposes a hierarchical Bayesian population inference framework to model multiple formation channels simultaneously.
• Uses a one-parameter AGN disk model parameterized by the maximum natal black hole mass ($m_{\rm max}$), with Salpeter-like mass function for seed black holes.
• Incorporates phenomenological models (e.g., power-law and Gaussian distributions) for non-AGN formation channels.
• Employs fiducial AGN disk parameters: $\alpha = 0.1$, $\epsilon = 0.1$, $M_\bullet = 10^6 \, M_\odot$, and $\dot{M} = 0.1 \, \dot{M}_{\rm Edd}$.
• Performs statistical inference on observed merger rate distributions in mass ratio and total mass space, comparing model predictions to LIGO-Virgo-KAGRA GWTC-3 data.
• Uses Markov Chain Monte Carlo (MCMC) sampling to estimate posterior distributions of model parameters, including power-law slope $\alpha$ and minimum mass $m_{\rm min}$.

Experimental results

Research questions

• RQ1 What is the relative contribution of AGN disk formation to the observed gravitational wave binary black hole population?
• RQ2 How do the inferred properties of AGN-formed binaries (e.g., mass ratio, spin) compare to those of other formation channels?
• RQ3 To what extent does the inclusion of an AGN component affect the inferred parameters of phenomenological models like power-law and Gaussian distributions?
• RQ4 Are high-mass and high-mass-ratio binaries more likely to originate in AGN disks than in other environments?
• RQ5 Can current gravitational wave data distinguish AGN-originated binaries from those formed via isolated stellar evolution or other channels?

Key findings

• The AGN component contributes approximately $O(5 \, \text{Gpc}^{-3} \text{yr}^{-1})$ to the total merger rate, with a significant contribution to high-mass and high-mass-ratio binaries.
• High-mass binaries ($m_{\rm max} \gtrsim 50 \, M_\odot$) and high-mass-ratio mergers ($q < 1/10$) are more likely to originate in AGN disks than lower-mass events.
• The inferred power-law slope $\alpha$ varies widely depending on model composition: $1.6^{+0.2}_{-0.2}$ for PL-only, but increases to $8.4^{+2.3}_{-3.4}$ when AGN is included with $m_{\rm min} = 50 \, M_\odot$.
• The minimum mass $m_{\rm min}$ of the power-law component shifts from $2.5^{+0.3}_{-0.3} \, M_\odot$ in PL-only to $8.5^{+0.2}_{-0.4} \, M_\odot$ when AGN is included, indicating strong degeneracy between models.
• The power-law model remains stable in overall merger rate but its parameters are highly sensitive to confounding contributions from AGN and Gaussian components.
• Future observations of high-mass black hole mergers are expected to further disentangle the AGN component from other formation channels.