  The NANOGrav Collaboration has recently announced evidence for nHz
gravitational waves (GWs), in the form of a Hellings-Downs angular correlation
in the common-spectrum process that had been observed previously by them and
other Pulsar Timing Arrays (PTAs). We analyze the possibility that these GWs
originate from binary supermassive black holes (SMBHs) with total masses
$\gtrsim 10^9\, M_{\odot}$. The spectral index of the GW signal differs at 95 %
CL from that predicted for binary evolution by GW emission alone, and we find
$> 3 \sigma$ evidence that environmental effects such as dynamical friction
with gas, stars, and dark matter may be affecting the binary evolution. We
estimate the required magnitude and spectrum of such environmental effects and
comment on their possible implications for measurements of GWs at higher
frequencies.
