X-ray Spectroscopic Evidence of Charge Exchange Emission in the Disk of
  M51
  In the disks of spiral galaxies, diffuse soft X-ray emission is known to be
strongly correlated with star-forming regions. However, this emission is not
simply from a thermal-equilibrium plasma and its origin remains greatly
unclear. In this work, we present an X-ray spectroscopic analysis of the
emission from a star-forming enhanced region off the nucleus of the spiral
galaxy M51, named as the northern hot spot (NHS) hereafter. We analyze the high
spectral resolution data from XMM-Newton/RGS observations and unambiguously
detect an abnormally high G-ratio ($3.2^{+6.9}_{-1.5}$) of the O VII K$\alpha$
triplet emission from the NHS, which is spatially confirmed by oxygen emission
line maps from the RGS data. A physical model consisting of a thermal plasma
and its charge exchange (CX) with cold gas gives an excellent description of
the high G-ratio and the entire RGS spectra. Complementary to the RGS data, the
Chandra/ACIS-S data are also analyzed because its high spatial resolution is
beneficial for decomposing the diffuse and discrete source contributions. Its
imaging spectra of the diffuse emission either from the NHS or other parts of
the M51 disk can be well interpreted by the same physical model. Though the
surface brightness of the diffuse emission from the NHS is about twice that
from other parts of the disk, the hot plasma has the similar temperature of
~0.34 keV and approximately solar metallicity. For both parts of the disk, the
CX contributes ~50% to the diffuse soft X-ray emission in the 7-30 angstrom
band, and has an interface area of about five times the geometric area of the
M51 disk. Our results suggest that the hot plasma can interact with the cold
interstellar medium efficiently, and have strong implications for our
understanding of soft X-ray emission as a tracer of the stellar feedback in
active star-forming galaxies.
