AChandraObservation of the Diffuse Emission in the Face‐on Spiral NGC 6946

This paper describes the {\it Chandra} observation of the diffuse emission inthe face-on spiral NGC 6946. Overlaid on optical and H${\alpha}$ images, thediffuse emission follows the spiral structure of the galaxy. An overlay on a 6cm polarized radio intensity map confirms the phase offset of the polarizedemission. We then extract and fit the spectrum of the unresolved emission withseveral spectral models. All model fits show a consistent continuum thermaltemperature with a mean value of 0.25$\pm$0.03 keV. Additional degrees offreedom are required to obtain a good fit and any of several models satisfythat need; one model uses a second continuum component with a temperature of0.70$\pm$0.10 keV. An abundance measure of 3$^{+1.95}_{-1.90}$ for Si differsfrom the solar value at the 90% confidence level; the net diffuse spectrumshows the line lies above the instrumental Si feature. For Fe, the abundancemeasure of 0.67$\pm$0.13 is significant at 99%. Multiple gaussians also providea good fit. Two of the fitted gaussians capture the O VII and O VIII emission;the fitted emission is consistent with an {\it XMM-Newton} RGS spectrum ofdiffuse gas in M81. The ratio of the two lines is $<$0.6-0.7 and suggests thepossibility of non-equilibrium ionization conditions exist in the ISM of NGC6946. An extrapolation of the point source luminosity distribution shows thediffuse component is not the sum of unresolved point sources; theircontribution is at most 25%.Comment: accepted for ApJ; 16 pages; 12 figs; to meet Archive size limits, most converted to jpe