Hiding Out at the Low End: No Gap and a Peak in the Black-Hole Mass Spectrum

In recent years, the existence of a gap in the mass spectrum of compact objects formed from stellar collapse, between the heaviest neutron stars and the lightest black holes, has been a matter of significant debate. The presence or absence of a mass gap has implications for the supernova mechanism, as well as being a fundamental property of the compact object mass function. In X-ray binaries containing black holes a gap is observed, but it is not known whether this is representative of a true gap in the mass function or due to selection effects or systematic biases in mass estimation. Binary black hole mergers detected from gravitational waves in the GWTC-3 transient catalog furnish a large sample of several tens of low-mass black holes with a well-understood selection function. Here we analyze the \nevts{} GWTC-3 merger events (along with GW230529\_181500) with at least one black hole ($3 \, M_\odot < m_1$) and chirp masses below those of a $20\,M_\odot$--$20\,M_\odot$ merger ($\mathcal{M} < 17.41 M_{\odot}$) to uncover the structure of the low-mass black hole mass function. Using flexible parameterized models for the mass function, we find, similar to existing studies, a sharp peak in the mass function at $m \simeq (8-10 M_{\odot})$. We observe a steady decline in the merger rate to lower masses, but by less than an order of magnitude in total, and find that the first percentile of black hole masses in our most flexible model is $m_{1\%} =3.13^{+0.18}_{-0.04}$. In other words, this sample of low-mass black holes is not consistent with the existence of a mass gap.