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 ($T > 10^{6}$ 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 ($\approx 4'$), we perform emission-line fitting to measure the velocity dispersion, $\sigma_{\text{v}}$, from OVIII (0.65, 0.77 keV), NeX (1.02 keV), and MgXII (1.47 keV). For the $T \approx 3 \times 10^{6}$ K gas, OVIII yields a velocity dispersion of $\sigma_{\text{v}} = 1160^{+100}_{-90}$ km s$^{-1}$, implying a wind speed that is significantly above the escape velocity ($v_{\text{esc}} \lesssim 450$ km s$^{-1}$). NeX ($\sigma_{\text{v}} = 550^{+130}_{-150}$ km s$^{-1}$) and MgXII ($\sigma_{\text{v}} < 370$ 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 $T \approx (0.6 - 1) \times 10^{7}$ 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.