Radio continuum contours from the narrow-band 21-cm of NGC 3998 overlaid on an r-band optical image from the Sloan Digital Sky Survey (Alam et al. 2015). The contours from the radio continuum were produced from the 21-cm narrow-band data using uniform weighting – which corresponds to a robustness parameter in MIRIAD of –2. The beam-size is indicated in the lower-left corner of the image. The contours are given by S_c = S₀×2ⁿ mJy beam^-1, for n = 0,1,2,...,10, where S₀ = 0.150 mJy beam^-1 is the 3σ sensitivity as indicated in Table 1.