A mid-thirties crisis: dissecting the properties of gravitational wave sources near the 35 solar mass peak

Abstract One striking feature of binary black hole (BBH) mergers observed in the first decade of gravitational-wave astronomy is an excess of events with component masses around <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>35</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . Multiple formation channels have been proposed to explain this excess. To distinguish among these channels, it is essential to examine their predicted population-level distributions across additional parameters. In this work, we focus on BBH mergers near the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>35</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> peak and infer the population distributions of primary mass ( m 1 ), mass ratio ( q ), effective spin ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mstyle scriptlevel="0"/> <mml:msub> <mml:mi>χ</mml:mi> <mml:mrow> <mml:mrow> <mml:mi>eff</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ), and redshift ( z ). We observe a gradual increase in the merger rate with m 1 , rising by a factor of 3 from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>20</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> to a peak around <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>34</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , followed by a sharp, order-of-magnitude decline by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>50</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . This population also shows a weak preference for equal-mass mergers and has a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mstyle scriptlevel="0"/> <mml:msub> <mml:mi>χ</mml:mi> <mml:mrow> <mml:mrow> <mml:mi>eff</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> distribution skewed toward positive values, with a median of zero excluded at approximately 90% confidence. We find no significant <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>q</mml:mi> <mml:mo>−</mml:mo> <mml:mstyle scriptlevel="0"/> <mml:msub> <mml:mi>χ</mml:mi> <mml:mrow> <mml:mrow> <mml:mi>eff</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> correlation in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>35</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> peak population, suggesting that lower-mass systems ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>m</mml:mi> <mml:mn>1</mml:mn> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mn>20</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo>