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The XMM-RGS View of M82: First Detection of Velocity Broadening in a Soft X-ray Starburst Wind

Presentation #104.15 in the session ISM/Galaxies - Poster Session.

Published onMay 03, 2024
The XMM-RGS View of M82: First Detection of Velocity Broadening in a Soft X-ray Starburst Wind

We present the first measurement of velocity broadening in the soft X-ray wind from the prototypical starburst galaxy M82. Starburst winds are believed to shape the stellar mass - halo mass relation below L*, and multiphase wind models suggest that the hot phase (T > 106 K) dominates the energy loading. However, the volume filling factor, mass, and total energy of the hot gas are highly uncertain at the low energy resolution of X-ray CCD observations. The velocity distribution provides an important diagnostic of these quantities and can distinguish between soft X-rays originating from interfaces with the cooler gas and from a volume-filling phase. Using archival observations from the Reflection Grating Spectrometer (RGS) on XMM-Newton, we measure the velocity dispersion of the starburst wind in M82 using several soft X-ray lines, including a correction for the instrumental broadening due to the extended and structured wind. We find a velocity dispersion of σ = 850 ± 250 km/s from OVIII (0.65 keV) and σ < 600 km/s from NeX (1.02 keV) and MgXII (1.47 keV). The OVIII measurement significantly exceeds the velocity dispersion in the cool gas (T ~ 104 K; σ < 200 km/s), suggesting a volume-filling phase with a fast outflow speed (v > 1000 km/s) or a large opening angle (θ > 30°). We present the inferred energy and multi-phase structure of the wind and discuss implications for interpreting soft X-ray emission from starburst galaxies, including a performance verification observation of M82 with the X-Ray Imaging and Spectroscopy Mission.

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