Gravitational-Wave Signatures of Massive Black Hole Formation

TL;DR

The paper addresses the problem of predicting gravitational-wave signals from direct-collapse black hole (DCBH) formation, a key ingredient in the early growth of supermassive black holes and a potential LISA source. It introduces a three-stage, multi-code pipeline that spans cosmological simulations (RenSims), mesoscale protostellar evolution with MESA, and strong-gravity numerical relativity using the Einstein Toolkit to follow collapse and GW emission. The authors outline data-transfer methods between scales (RenSims to MESA to NR), demonstrate a proof-of-concept collapse using RNSID-based initial data, and report preliminary GW signals dominated by the $(l,m)=(2,0)$ mode with a modest final BH spin ($\chi \sim 0.2$) in uniform-rotation tests. This framework enables prediction and interpretation of LISA-detectable DCBH formation signals, informing population modeling and future GW data analyses, while highlighting key challenges in EOS consistency, initialization, and cross-code coupling.

Abstract

Direct-collapse black holes (DCBHs) are an important component of the massive black hole population of the early universe, and their formation and early mergers will be prominent in the data stream of the Laser Interferometer Space Antenna (LISA). However, the population and binary properties of these early black holes are poorly understood, with masses, mass ratios, spins, and orbital eccentricities strongly dependent on the details of their formation, and the properties of the remaining exterior material (baryonic and non-baryonic), which may be substantial to the point of merger. We report on initial work to simulate the formation, collapse, and/or merger of such DCBH regions in order to extract the resulting gravitational-wave signals.

Figures (6)

• Figure 1: Top: density and temperature profiles extracted from the LWH clump of WiseEtAl_2019. Bottom: angular velocity specific angular momentum profiles for the same clump demonstrating a strong differential rotation law.
• Figure 2: Top: Lapse function $\alpha$ evolution along $x$-axis from the same simulation. Bottom: density evolution along $x$-axis from Saijo uniform rotation collapse simulation.
• Figure 3: Top: irreducible and total horizon masses of the DCBH after formation. Bottom: dimensionless spin $\chi \equiv J/M_{\rm tot}^2$ after formation.
• Figure 4: Dominant $(l = 2, m = 0)$ mode components of the outgoing Weyl scalar $\psi_4$, extracted at coordinate radii $40 M$, $60 M$, $80 M$, and $100 M$, and translated in time and scaled in amplitude to correct for the different extraction radii.
• Figure 5: Minimum of the lapse function $\alpha$ as a function of evolution time $t$, for three different size perturbations of polytropic $K$.