A Chandra Observation of the Diffuse Emission in the Face-on Spiral NGC 6946

This paper describes the Chandra observation of the diffuse emission in the face-on spiral NGC 6946. Overlaid on optical and Hα images, the diffuse emission follows the spiral structure of the galaxy. An overlay on a 6 cm polarized radio intensity map confirms the phase offset of the polarized emission. We then extract and fit the spectrum of the unresolved emission with several spectral models. All model fits show a consistent continuum thermal temperature with a mean value of 0.25 ± 0.03 keV. Additional degrees of freedom are required to obtain a good fit, and any of several models satisfy that need; one model uses a second continuum component with a temperature of 0.70 ± 0.10 keV. An abundance measure of 3 for Si differs from the solar value at the 90% confidence level; the net diffuse spectrum shows that the line lies above the instrumental Si feature. For Fe the abundance measure of 0.67 ± 0.13 is significant at 99%. Multiple Gaussians also provide a good fit. Two of the fitted Gaussians capture the O VII and O VIII emission; the fitted emission is consistent with an XMM-Newton RGS spectrum of diffuse gas in M81. The ratio of the two lines is less than 0.6-0.7 and suggests the possibility of nonequilibrium ionization conditions existing in the interstellar medium of NGC 6946. An extrapolation of the point-source luminosity distribution shows that the diffuse component is not the sum of unresolved point sources; their contribution is at most 25%.