An XMM - Newton view of M 101 - III . Diffuse X - ray emission

We present a study of the X-ray properties of the nearby face-on Scd spiral galaxy M101 based on recent XMM-Newton observations. In this third and final paper in the present series, we focus on the spatial and spectral properties of the residual emission, after excluding bright X-ray sources with L X > 10 37 erg s −1. Within a central region of radius 10 ′ (21 kpc), the X-ray emission broadly traces the pattern of the spiral arms, establishing a strong link with recent star formation, but it also exhibits a radial scale length of ≈ 2.6 ′ (5.4 kpc) consistent with optical data. We estimate the soft X-ray luminosity within the central 5 ′ (10.5 kpc) region to be L X ≈ 2.1 × 10 39 erg s −1 (0.5–2 keV), the bulk of which appears to originate as diffuse emission. We find a two-temperature thermal model best fits the spectral data with derived temperatures of 0.20 −0.04 keV which are very typical of the diffuse components seen in other normal and starburst galaxies. More detailed investigation of the X-ray morphology reveals a strong correlation with images recorded in the far-UV through to V band, with the best match being with the U band. We interpret these results in terms of a clumpy thin-disk component which traces the spiral arms of M101 plus an extended lower-halo component with large filling factor.