X-ray emission from the extended discs of spiral galaxies

We present a study of the X-ray properties of a sample of six nearby late-type spiral galaxies based on XMM–Newton observations. Since our primary focus is on the linkage between X-ray emission and star formation in extended, extranuclear Galactic discs, we have selected galaxies with near face-on aspect and sufficient angular extent so as to be readily amenable to investigation with the moderate spatial resolution afforded by XMM–Newton. After excluding regions in each galaxy dominated by bright point sources, we study both the morphology and spectral properties of the residual X-ray emission, comprising both diffuse emission and the integrated signal of the fainter discrete source populations. The soft X-ray morphology generally traces the inner spiral arms and shows a strong correlation with the distribution of UV light, indicative of a close connection between the X-ray emission and recent star formation. The soft (0.3–2 keV) X-ray luminosity to star formation rate (SFR) ratio varies from 1–5 × 1039 erg s−1 (M⊙ yr−1)−1, with an indication that the lower range of this ratio relates to regions of lower SFR density. The X-ray spectra are well matched by a two-temperature thermal model with derived temperatures of typically ∼0.2 and ∼0.65 keV, in line with published results for other normal and star-forming galaxies. The hot component contributes a higher fraction of the soft luminosity in the galaxies with highest X-ray/SFR ratio, suggesting a link between plasma temperature and X-ray production efficiency. The physical properties of the gas present in the galactic discs are consistent with a clumpy thin-disc distribution, presumably composed of diffuse structures such as superbubbles together with the integrated emission of unresolved discrete sources including young supernova remnants.