Lower-left panel: naturally-weighted VLA DnA-array 22 GHz radio continuum contour map of NGC 4151 produced by using the line-free channels in the maser data cube. Contours: –1, 1, 2, 3, 4, 5  ×  1.7 mJy (1σ rms = 0.6 mJy beam^-1). Upper panels: 22 GHz spectra from the IF1 (left) and IF2 (right) VLA DnA-array data cube. The two IFs sample the velocity ranges where narrow water maser lines (their velocities are marked by arrows) were detected with the GBT by Braatz et al. (2004). The systemic velocity of the galaxy is 995 km s^-1.