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

Abstract

This paper describes the {\it Chandra} observation of the diffuse emission in the face-on spiral NGC 6946. Overlaid on optical and H$\alpha$ 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$\pm$0.03 keV. Additional degrees of freedom are required to obtain a good fit and any of several models satisfy that need; one model uses a second continuum component with a temperature of 0.70$\pm$0.10 keV. An abundance measure of 3${}_{-1.90}^{+1.95}$ for Si differs from the solar value at the 90% confidence level; the net diffuse spectrum shows the line lies above the instrumental Si feature. For Fe, the abundance measure of 0.67$\pm$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 {\it XMM-Newton} RGS spectrum of diffuse gas in M81. The ratio of the two lines is $<$0.6-0.7 and suggests the possibility of non-equilibrium ionization conditions exist in the ISM of NGC 6946. An extrapolation of the point source luminosity distribution shows the diffuse component is not the sum of unresolved point sources; their contribution is at most 25%.

arXiv commentaccepted for ApJ; 16 pages; 12 figs; to meet Archive size limits, most converted to jpeg

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• Gas Accretion via Condensation and Fountains
• The Long-Term Variability of the X-Ray Sources in NGC 6946 and NGC 4485/4490
• A Deep X-ray View of the Hot Halo in the Edge-on Spiral Galaxy NGC 891
• Chemical enrichment of the complex hot ISM of the Antennae Galaxies: II. Physical properties of the hot gas and supernova feedback
• Half-Megasecond Chandra Spectral Imaging of the Hot Circumgalactic Nebula around Quasar Mrk 231
• A Chandra Observation of the Nearby Sculptor Group Sd Galaxy NGC 7793
• XMM-Newton observation of the interacting galaxies NGC1512 and NGC1510
• Hot gas and magnetic arms of NGC 6946: indications for reconnection heating?
• An optical search for supernova remnants in the nearby spiral galaxy NGC 2903
• Suzaku Observation of the Metallicity in the Interstellar Medium of NGC 4258