Extended X-ray emission in spiral galaxy disks

We have studied the extended X-ray emission observed in the disks of seven nearby face-on spiral galaxies using XMM-Newton archival observations. Using a novel technique to remove the bulk of the contamination from bright X-ray point sources, we have isolated a residual disk component in each galaxy, comprising diffuse gas plus the integrated population of faint point sources. We have found soft X-ray emission from this component to be strongly correlated with FUV emission, unambiguously establishing a close link with recent star formation. We have found the residual X-ray/star formation rate (SFR) ratio across our sample to range from 1-5x10^39 erg s^{-1} ( M \odot yr{-1})^{-1}, with the ratio highest in regions with the highest SFR. This is consistent with the models of Ranalli et al. (2003), and matches the predicted X-ray emission ≈ 10 Myr after an extended burst of star formation. Our spectral analysis of the residual disk components indicates that a two-temperature thermal plasma fits the data well, with derived temperatures of 0:2 keV and 0:65 keV. This is consistent with previously derived results for spiral and starburst galaxies (e.g. Fraternali et al. 2002). We have shown the emission to be well modelled by a clumpy thin-disk distribution, with bubbles of hot gas and collections of faint point sources tracing the galactic spiral arms. We have found the excised bright point source populations to be evenly divided between old and young sources with the exception of M33, where young sources dominate. We have derived the residual X-ray/mass ratio in the inner disk of this system to be be 4x10^{28} erg s^{-1} M\odot^{-1} This is 5-10 times higher than the equivalent ratio in low star formation rate systems (Revnivtsev et al. 2008), with the excess attributed to contributions from the young point source population and diffuse gas. Our results have demonstrated a quantifiable link between diffuse X-ray emission and recent star formation in spiral galaxies. Further study is necessary to better constrain this relationship, in particular its dependence on the local environment.