  Context. Massive stars are predicted to excite internal gravity waves (IGWs)
by turbulent core convection and from turbulent pressure fluctuations in their
near-surface layers. These IGWs are extremely efficient at transporting angular
momentum and chemical species within stellar interiors, but they remain largely
unconstrained observationally. Aims. We aim to characterise the photometric
detection of IGWs across a large number of O and early-B stars in the
Hertzsprung-Russell diagram, and explain the ubiquitous detection of stochastic
variability in the photospheres of massive stars. Methods. We combined
high-precision time-series photometry from the NASA Transiting Exoplanet Survey
Satellite with high-resolution ground-based spectroscopy of 70 stars with
spectral types O and B to probe the relationship between the photometric
signatures of IGWs and parameters such as spectroscopic mass, luminosity, and
macroturbulence. Results. A relationship is found between the location of a
star in the spectroscopic Hertzsprung-Russell diagram and the amplitudes and
frequencies of stochastic photometric variability in the light curves of
massive stars. Furthermore, the properties of the stochastic variability are
statistically correlated with macroturbulent velocity broadening in the
spectral lines of massive stars. Conclusions. The common ensemble morphology
for the stochastic low-frequency variability detected in space photometry and
its relationship to macroturbulence is strong evidence for IGWs in massive
stars, since these types of waves are unique in providing the dominant
tangential velocity field required to explain the observed spectroscopy.
