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Astron. Astrophys. 320, 378-394 (1997)

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6. Conclusions

We have provided a detailed spectral investigation of the extended soft X-ray emission associated with the starburst galaxy M 82. Point sources have been located and removed from the diffuse wind emission, and the effects of contamination by the nuclear source allowed for. The diffuse emission was divided into a set of distinct regions as a function of distance from the galactic plane, allowing temperature, emission measure and absorbing column to be derived as a function of distance along the minor axis. The metallicity was found to be apparently low (0.00- [FORMULA]) throughout the wind, in agreement with results from ASCA. This work has shown the following:

  • The observed soft X-ray morphology differs significantly between north and south, and is not well described as a conical outflow. The emission extends out to [FORMULA] from the plane of the galaxy. There is no evidence for any limb brightening, as would be expected if the soft emission came from the shock heated halo surrounding the hot wind.
  • The emission from the wind is thermal. The temperature drops slowly along the wind, from [FORMULA] near the nucleus, to [FORMULA] in the outer wind. Numerical models of galactic winds predict the majority of the emission in the ROSAT band to be shocked halo, with effective temperatures in the range [FORMULA].
  • Since the entropy of the gas is not constant, at least in the south, the observed emission cannot originate from a free wind itself. Our baseline analysis is based on an assumed cylindrical geometry, but a more divergent geometry only makes the southern wind less isentropic.
  • The emission cannot come from a static halo of gas around M 82, given that the mass required to bind a hydrostatic halo is an order of magnitude higher than that inferred from the HI rotational velocity in the outer disk.
  • For reasonable mass and energy input from the starburst, a simple model for emission from shock heated clouds using the observed density of the X-ray emitting gas to predict post-shock temperatures is consistent with the observed temperatures.
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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998