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

Abstract

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, σ _v , from O VIII (0.65, 0.77 keV), Ne X (1.02 keV), and Mg XII (1.47 keV). For the T ≈ 3 × 10⁶ K gas, O VIII yields a velocity dispersion of σv=1160‑90+100 km s^‑1, implying a wind speed that is significantly above the escape velocity (v _esc ≲ 450 km s^‑1). Ne X ( σv=550‑150+130 km s^‑1) and Mg XII (σ _v < 370 km s^‑1) show less velocity broadening than O VIII, 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.