Charge Exchange X-ray Emission of Nearby Star-forming Galaxies
  Properties of hot gas outflows from galaxies are generally measured from
associated X-ray line emission assuming that it represents atomic transitions
in thermally excited hot gas. X-ray line emission, however, can also arise from
the charge exchange between highly ionised ions and neutral species. The
K\alpha\ triplet of He-like ions can be used as a powerful diagnostic, because
the charge exchange X-ray emission (CXE) favours the inter-combination and
forbidden lines, while the thermal emission favours the resonance line. We
analyse the OVII triplet of a sample of nine nearby star-forming galaxies
observed by the XMM-Newton reflection grating spectrometers. For most galaxies,
the forbidden lines are comparable to or stronger than the resonance lines,
which is in contrast to the thermal prediction. For NGC 253, M51, M83, M61, NGC
4631, and the Antennae galaxy, the observed line ratios are consistent with the
ratio of the CXE; for M94 and NGC 2903, the observed ratios indicate multiple
origins; for M82, different regions show different line ratios, also indicating
multiple origins. We discuss other possible mechanisms that can produce a
relatively strong forbidden line, such as a collisional
non-equilibrium-ionization recombining/ionizing plasma, which are not favoured.
These results suggest that the CXE may be a common process and contribute a
significant fraction of the soft X-ray line emission for galaxies with massive
star formation.
