X-ray emission from the extended disks 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 disks, 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, comprised of
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 x 10^39 erg/s(/Msun/yr), 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 keV 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 disks are consistent
with a clumpy thin-disk distribution, presumably composed of diffuse structures
such as superbubbles together with the integrated emission of unresolved
discrete sources including young supernova remnants.
