Maps of blueshifted (left) and redshifted (right) C⁺ emission in HDF850.1, with the 0.98mm continuum subtracted. These two channels are each 470 km s^-1 wide, centered on −205 km s^-1 and +265 km s^-1, with integration ranges of −440 km s^-1 to + 30 km s^-1 (left) and + 30 km s^-1 to + 500 km s^-1 (right). These velocities are relative to 307.267GHz (C⁺158 μm at z = 5.1853). Contours are −3σ (dashed), + 3σ, and then go up in steps of 1σ = 0.20Jybeam^-1 km s^-1. The blueshifted, C⁺ north peak (left) is 1.6Jybeam^-1 km s^-1, and its spatially integrated flux is 6.9Jy km s^-1. The redshifted, C⁺ south peak (right) is 1.4Jybeam^-1 km s^-1, and its spatially integrated flux is 3.0Jy km s^-1. The beam (lower left, in each panel), is 0.38′′ × 0.29′′ at PA = 59° (FWHM), with KJy^-1. The cross marks the phase reference position, at 12:36:51.980, +62:12:25.70 (J2000). The C⁺-line contours are superimposed on a greyscale version (Downes et al. 1999) of the BVI image from the Hubble Deep Field.