A fast starburst wind consumes most of the energy from supernovae

Starburst galaxies often host multiphase, galaxy-scale winds thought to enrich the circumgalactic medium and limit further star formation by disrupting interstellar gas clouds^{1, 2─3}. These winds are primarily powered by supernovae^{4, 5─6}, but it remains unclear how supernova energy forms an organized flow. Here we use the Resolve spectrometer on the X-ray Imaging and Spectroscopy Mission to show that the hot (T = 2 × 10⁷ K) gas in the nucleus of the starburst galaxy M82 is moving quickly, with a line-of-sight velocity dispersion σ=595-128+464kms-1. This is consistent with a hot, nuclear wind generated by thermal pressure. We show that a free-wind model reproduces the measured temperature but underpredicts the velocity. The inferred mass and energy outflow rates from the nucleus, about 7 M_⊙ yr⁻¹ and 4 × 10⁴² erg s⁻¹, require that most supernova energy is thermalized. These outflow rates provide enough energy to power the ≳30 M_⊙ yr⁻¹ cool outflow and still transport up to 3 M_⊙ yr⁻¹ to the intergalactic medium, suggesting that thermal gas pressure is sufficient to power the multiphase wind without additional support from cosmic rays⁷. We also show that the nuclear gas is hotter and faster than the plasma seen on larger scales (kT=0.72-0.08+0.10keV, σ=175-73+86kms-1), suggesting a distinct origin for the latter.