Absorption by associated ionised gas in the SNR Kes 73. Panel a: Spitzer MIPSGAL 24 μm image for Kes 73 (shown with a linear colour scale in MJy sr⁻¹) overlaid with black contours tracing 1.4 GHz radio continuum emission at levels 2.5, 4.8, 7.6, and 11 mJy beam⁻¹ from the MAGPIS survey. The morphology of the IR emission strongly mimics the radio emission. Panel b: ¹³CO (1–0) data from GRS integrated in the v_LSR = 95–105 km s⁻¹ range. The linear colour scale is in K km s⁻¹. Cyan and yellow contours represent the mid-IR (levels: 70 and 84 MJy sr⁻¹) and 74 MHzlow-radio frequency intensities (levels: 0.8, 1.3, 1.8, and 2.2 Jy beam⁻¹), respectively.The newly-identified CO cloud is detected within the boundaries of the Kes 73 shell. Panel c: optical depth towards Kes 73 at 74 MHz as a function of position. Cyan contours superimposed delineate the mid-IR emission as in panel b.