Evidence for a Fast Soft X-ray Wind in M82 from XMM-RGS

Starburst wind models predict that metals and energy are primarily carried out of the disk by hot gas (T > 10⁶ K), but the low energy resolution of X-ray CCD observations results in large uncertainties on the mass and energy loading. Here, we present evidence for a fast soft X-ray wind from the prototypical starburst galaxy M82 using deep archival observations from the Reflection Grating Spectrometer on XMM-Newton. After characterizing the complex line-spread function for the spatially extended outflow (≈ 4'), we perform emission-line fitting to measure the velocity dispersion, σᵥ, from OVIII (0.65, 0.77 keV), NeX (1.02 keV), and MgXII (1.47 keV). For the T ≈ 3 × 10⁶ K gas, OVIII yields a velocity dispersion of σᵥ = 1160⁺¹⁰⁰₋₉₀ km s⁻¹, implying a wind speed that is significantly above the escape velocity (vesc 450 km s⁻¹). NeX (σᵥ = 550⁺¹³⁰₋₁₅₀ km s⁻¹) and MgXII (σᵥ < 370 km s⁻¹) show less velocity broadening than OVIII, hinting at a lower wind speed or smaller opening angle on the more compact spatial scales traced by the T ≈ (0.6 - 1) × 10⁷ K gas. Alternatively, these higher energy emission lines may be dominated by shock-heated gas in the interstellar medium. Future synthesis of these measurements with Performance Verification observations of the $E = 2 - 12$ keV wind in M82 from the Resolve microcalorimeter on the X-ray Imaging and Spectroscopy Mission will inform the phase structure and energy budget of the hot starburst wind.