Gravitational waves from supermassive black hole binaries in light of the NANOGrav 15-year data

The NANOGrav and other pulsar timing arrays (PTAs) have recently announced evidence for nHz gravitational waves (GWs) that may originate from supermassive black hole (SMBH) binaries. The spectral index of the GW signal differs from that predicted for binary evolution by GW emission alone, and we show that environmental effects such as dynamical friction with gas, stars, and dark matter improve the consistency of the SMBH binary model with the PTA data. We comment on the possible implications of environmental effects for PTA observations of fluctuations in the GW frequency spectrum and measurements of GWs at higher frequencies.