On the diffuse soft X-ray emission from the nuclear region of M51
  We present an analysis of the diffuse soft X-ray emission from the nuclear
region of M51 combining both XMM-Newton RGS and Chandra data. Most of the RGS
spectrum of M51 can be fitted with a thermal model with a temperature of
$\sim0.5$ keV except for the OVII triplet, which is forbidden-line dominated.
The Fe L-shell lines peak around the southern cloud, where the OVIII and NVII
Lya lines also peak. In contrast, the peak of the OVII forbidden line is about
10$"$ offset from that of the other lines, indicating that it is from a
spatially distinct component. The spatial distribution of the OVII triplet
mapped by the Chandra data shows that most of the OVII triplet flux is located
at faint regions near edges, instead of the southern cloud where other lines
peak. This distribution of the OVII triplet is inconsistent with the
photoionization model. Other mechanisms that could produce the anomalous OVII
triplet, including a recombining plasma and charge exchange X-ray emission, are
discussed.
