Intermediate data products in the subtraction of thermal emission in M 51. Left panel: Initial continuum-subtracted Hα map obtained with the Kitt Peak National Observatory 2.1-m telescope using the narrow-band Hα-filter KP1563 (Kennicutt et al. 2003). The angular resolution is $1_{\cdot }^{\mathrm{\prime }\mathrm{\prime }}35\times 1_{\cdot }^{\mathrm{\prime }\mathrm{\prime }}35$ and the rms noise is ≈50 μJy beam⁻¹. The map was downloaded from the ancillary data at the SINGS webpage.⁶ Right panel: Extinction corrected prediction of the free–free emission at 1370 MHz in M51. This emission was scaled to the appropriate frequency with the spectral index index α = 0.1. The map shown here was subtracted from the radio maps to get the non-thermal emission. The beam size is ${17}_{\cdot }^{\mathrm{\prime }\mathrm{\prime }}5\times {15}_{\cdot }^{\mathrm{\prime }\mathrm{\prime }}0$.