X-ray spectral study of the extended emission, 'the Cap' located 11.6 kpc above the disk of M 82

摘要

The extended X-ray emission from 'the Cap' region, located 11' (11.6 kpc) above the disk of the starburst galaxy M 82, has been observed with Suzaku and XMM-Newton. Owing to the good energy resolution and the large collecting area of the XIS on Suzaku, combined with similar properties of the EPIC instrument on XMM-Newton, we have clearly detected K-shell emission lines from O VII, O VIII, Ne X, Mg XI, Mg XII, Sr XIII, and the Fe L complex. Two optically thin thermal plasma components are required to fit the observed X-ray spectra. We have determined the metal abundances of O, Ne, Mg, Si, and Fe in this region for the first time. Their metal abundance ratios agree well with those of metal-poor stars and the model prediction of metals synthesized by type-II supernovae, but they are not consistent with the metallicities of type-la supernovae. This result is support for the idea that the origin of the metals in the Cap is type-II supernova explosions occurring in the starburst regions in the M 82 galaxy. We discuss the possible contribution from sputtered dust grains to the metals in the Cap. An emission line consistent with the C VI transition of n = 4 to 1 at 0.459 keV has been marginally detected, although it is not statistically significant at the 99% confidence level; the presence of this line would suggest charge-exchange processes in the Cap.