Big Galaxies and Big Black Holes: The Massive Ends of the Local Stellar and Black Hole Mass Functions and the Implications for Nanohertz Gravitational Waves

We construct the z = 0 galaxy stellar mass function (GSMF) by combining the GSMF at stellar masses M _* ≲ 10 ^11.3 M _⊙ from the census study of Leja et al. and the GSMF of massive galaxies at M _* ≳ 10 ^11.5 M _⊙ from the volume-limited MASSIVE galaxy survey. To obtain a robust estimate of M _* for local massive galaxies, we use MASSIVE galaxies with M _* measured from detailed dynamical modeling or stellar population synthesis modeling (incorporating a bottom-heavy initial mass function) with high-quality spatially resolved spectroscopy. These two independent sets of M _* agree to within ∼7%. Our new z = 0 GSMF has a higher amplitude at M _* ≳ 10 ^11.5 M _⊙ than previous studies, alleviating prior concerns of a lack of mass growth in massive galaxies between z ∼ 1 and 0. We derive a local black hole mass function (BHMF) from this GSMF and the scaling relation of supermassive black holes (SMBHs) and galaxy masses. The inferred abundance of local SMBHs above ∼10 ^10 M _⊙ is consistent with the number of currently known systems. The predicted amplitude of the nanohertz stochastic gravitational-wave background is also consistent with the levels reported by Pulsar Timing Array teams. Our z = 0 GSMF therefore leads to concordant results in the high-mass regime of the local galaxy and SMBH populations and the gravitational-wave amplitude from merging SMBHs. An exception is that our BHMF yields a z = 0 SMBH mass density that is notably higher than the value estimated from quasars at higher redshifts.