When Are LIGO/Virgo's Big Black Hole Mergers?

We study the evolution of the binary black hole (BBH) mass distribution across cosmic time. The second gravitational-wave transient catalog (GWTC-2) from LIGO/Virgo contains BBH events out to redshifts z similar to 1, with component masses in the range similar to 5-80 M-circle dot. In this catalog, the biggest BBHs, with m(1) greater than or similar to 45 M-circle dot, are only found at the highest redshifts, z greater than or similar to 0.4. We ask whether the absence of high-mass observations at low redshift indicates that the mass distribution evolves: the biggest BBHs only merge at high redshift, and cease merging at low redshift. Modeling the BBH primary-mass spectrum as a power law with a sharp maximum mass cutoff (TRUNCATED model), we find that the cutoff increases with redshift (> 99.9% credibility). An abrupt cutoff in the mass spectrum is expected from (pulsational) pair-instability supernova simulations; however, GWTC-2 is only consistent with a Truncated mass model if the location of the cutoff increases from 45(5)(+13) M-circle dot at z < 0.4 to 80(13)(+16) M-circle dot at z > 0.4. Alternatively, if the primary-mass spectrum has a break in the power law (BROKEN POWER LAW) at 38(-8)(+15) M-circle dot, rather than a sharp cutoff, the data are consistent with a nonevolving mass distribution. In this case, the overall rate of mergers, at all masses, increases with redshift. Future observations will distinguish between a sharp mass cutoff that evolves with redshift and a nonevolving mass distribution with a gradual taper, such as a BROKEN POWER LAW. After similar to 100 BBH merger observations, a continued absence of high-mass, low-redshift events would provide a clear signature that the mass distribution evolves with redshift.

Automated summary

The study indicates that the cutoff value of the binary black hole mass distribution increases with redshift, suggesting an evolving mass distribution. However, if the mass distribution exhibits a broken power law instead of a sharp cutoff, it may signify a non-evolving mass distribution, with the merger rate increasing with redshift. Future observations will be able to distinguish between these two scenarios based on the presence or absence of high-mass, low-redshift events.