Gravitational Waves from Direct Collapse Black Holes Formation

TL;DR

This paper estimates the gravitational wave signals from the formation of direct collapse black holes in the early universe, showing they could be detectable by future observatories despite being buried in noise.

Contribution

It provides the first detailed estimate of gravitational wave signals from high-redshift DCBH formation using modern waveforms and formation rates.

Findings

Signal exceeds Ultimate-DECIGO sensitivity in the 0.8-300 mHz range.

Peak amplitude of gravitational wave background is 1.1 x 10^{-54}.

Proposes a method to detect these signals amidst noise.

Abstract

The possible formation of Direct Collapse Black Holes (DCBHs) in the first metal-free atomic cooling halos at high redshifts ($z > 10$) is nowadays object of intense study and several methods to prove their existence are currently under development. The abrupt collapse of a massive ($\sim 10^4 - 10^5 \, \mathrm{M_{\odot}}$) and rotating object is a powerful source of gravitational waves emission. In this work, we employ modern waveforms and the improved knowledge on the DCBHs formation rate to estimate the gravitational signal emitted by these sources at cosmological distances. Their formation rate is very high ($\sim 10^4 \, \mathrm{yr^{-1}}$ up to $z\sim20$), but due to a short duration of the collapse event ($\sim 2-30\, \mathrm{s}$, depending on the DCBH mass) the integrated signal from these sources is characterized by a very low duty-cycle (${\cal D}\sim 10^{-3}$), i.e. a shot-noise signal. Our results show that the estimated signal lies above the foreseen sensitivity of the Ultimate-DECIGO observatory in the frequency range $(0.8-300) \, \mathrm{mHz}$, with a peak amplitude $\Omega_{gw} = 1.1 \times 10^{-54}$ at $\nu_{max} = 0.9 \, \mathrm{mHz}$ and a peak Signal-to-Noise Ratio $\mathrm{SNR}\sim 22$ at $\nu = 20 \, \mathrm{mHz}$. This amplitude is lower than the Galactic confusion noise, generated by binary systems of compact objects in the same frequency band. For this reason, advanced techniques will be required to separate this signal from background and foreground noise components. As a proof-of-concept, we conclude by proposing a simple method, based on the auto-correlation function, to recognize the presence of a ${\cal D} \ll 1$ signal buried into the continuous noise. The aim of this work is to test the existence of a large population of high-z DCBHs, by observing the gravitational waves emitted during their infancy.