Abstract
Abstract We focus on gravitational-wave events of binary black-hole mergers up to the third observing run with the minimum false alarm rate smaller than 10−5 yr−1. These events tell us that the mass ratio of two black holes follows m2/m1 = 0.723 with the chance probability of 0.00301% for the chirp mass Mchirp > 18 M⊙. We show that the relation of m2/m1 = 0.723 is consistent with the binaries originated from population III stars which are the first stars in the universe. On the other hand, it is found for Mchirp < 18M⊙ that the mass ratio follows m2/m1 = 0.601 with the chance probability of 0.117% if we ignore GW190412 with m2/m1 ∼ 0.32. This suggests a different origin from that for Mchirp > 18M⊙.
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