When Are LIGO/Virgo’s Big Black Hole Mergers?
Maya Fishbach, Zoheyr Doctor, Thomas Callister, Bruce Edelman, Jiani Ye, Reed Essick, Will M. Farr, Ben Farr, Daniel E. Holz
Abstract
Abstract 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 ∼ 1, with component masses in the range ∼5–80 M ⊙ . In this catalog, the biggest BBHs, with m 1 ≳ 45 M ⊙ , are only found at the highest redshifts, z ≳ 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 at z < 0.4 to at z > 0.4. Alternatively, if the primary-mass spectrum has a break in the power law ( Broken Power Law ) at , 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 ∼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.