Gravitational wave background from Population III binary black holes consistent with cosmic reionization

Abstract

The recent discovery of the gravitational wave source GW150914 has revealed a coalescing binary black hole (BBH) with masses of ∼ 30 M ⊙ . Previous proposals for the origin of such a massive binary include Population III (PopIII) stars. PopIII stars are efficient producers of BBHs and of a gravitational wave background (GWB) in the 10 − 100 Hz band, and also of ionizing radiation in the early Universe. We quantify the relation between the amplitude of the GWB (Ω gw ) and the electron scattering optical depth (τ e ), produced by PopIII stars, assuming that f esc ≈ 10% of their ionizing radiation escapes into the intergalactic medium. We find that PopIII stars would produce a GWB that is detectable by the future O5 LIGO/Virgo if τ e 0.07, consistent with the recent Planck measurement of τ e = 0.055 ± 0.09. Moreover, the spectral index of the background from PopIII BBHs becomes as small as d ln Ω gw /d ln f 0.3 at f 30 Hz, which is significantly flatter than the value ∼ 2/3 generically produced by lower-redshift and less-massive BBHs. A detection of the unique flattening at such low frequencies by the O5 LIGO/Virgo will indicate the existence of a high-chirp mass, high-redshift BBH population, which is consistent with the PopIII origin. A precise characterization of the spectral shape near 30 − 50 Hz by the Einstein Telescope could also constrain the PopIII initial mass function and star formation rate.

Rates of Compact Object Coalescences

“…This mass retention is further aided by the drastically reduced wind-driven mass loss at zero meatallicity. Finally, the lack of metals has been conjectured to supress the fragmentation of a giant molecular cloud during star formation, increasing the typical mass of population III stars and making these prime progenitor candidates for binary black hole coalescences (Belczynski et al, 2004;Kinugawa et al, 2014;Inayoshi et al, 2016Inayoshi et al, , 2017.…”

eLISA eccentricity measurements as tracers of binary black hole formation

“…In these scenarios there would never be a giant phase: both stars would stay within their Roche lobes and eventually form massive BHs, because the cores that collapse would be large. Yet another scenario invokes a Population III origin for massive BH binaries [54][55][56], but semi-analytical models suggest that the probability of GW150914 having formed in the early Universe is ∼ 1% [57].…”

Black Hole Mergers in Galactic Nuclei Induced by the Eccentric Kozai–Lidov Effect

“…These merging BH binaries may also be a result of isolated binary evolution in the galactic field due to special modes of stellar evolution and evolution in active galactic nuclei Belczynski et al 2016a;Marchant et al 2016). Some studies also suggested that these LIGO detections are sourced in the first stars (Kinugawa et al 2014(Kinugawa et al , 2016Hartwig et al 2016;Inayoshi et al 2016;Dvorkin et al 2016), cores of massive stars (Reisswig et al 2013;Loeb 2016;Woosley 2016), or dark matter halos comprised of primordial black holes (Bird et al 2016; 1 http://www.ligo.org/ 2 http://www.virgo-gw.eu/ 3 http://gwcenter.icrr.u-tokyo.ac.jp/en/ Clesse & García-Bellido 2016;Sasaki et al 2016).…”