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 the northern hot spot; a region with enhanced star formation off the nucleus of M51. Based on the high spectral resolution data from XMM-Newton/Reflection Grating Spectrometer (RGS) observations, we unambiguously detect a high G ratio (3.2( -1.5 )(+6.9)) of the O VII He alpha triplet. This high G ratio is also spatially confirmed by oxygen emission-line maps from the same data. A physical model consisting of a thermal plasma and its charge exchange (CX) with neutral cool gas gives a good explanation for the G ratio and the entire RGS spectra. This model also gives a satisfactory characterization of the complementary Chandra ACIS-S data, which enables a direct imaging of the diffuse emission, tracing the hot plasma across the galaxy. The hot plasma has a similar characteristic temperature of similar to 0.34 keV and an approximately solar metallicity. The CX contributes similar to 50% to the diffuse emission in the 0.4-1.8 keV band, suggesting an effective hot/cool gas interface area about five times the geometric area of the M51 disk. Therefore, the CX appears to play a major role in the soft X-ray production and may be used as a powerful tool to probe the interface astrophysics, important for studying galactic ecosystems.

Automated summary

In the disks of spiral galaxies, diffuse soft X-ray emission is strongly correlated with star-forming regions. However, the origin of this emission is unclear. This study analyzed X-ray emission from a region with enhanced star formation in the northern hot spot of M51 and detected a high G ratio of the O VII He alpha triplet. A physical model involving a thermal plasma and charge exchange with cool gas provides a good explanation for the emission. The study also suggests that charge exchange plays a major role in soft X-ray production.