H¹²C¹⁵N (HC¹⁵ N) J = 1 → 0 channel maps. For the units of velocity, ordinate, and wedge as well as the position of the crosses, see Fig. A.1. We note that the signal seen is very close to the noise level. This or a weak transition of SO or CH₃ OH may explain the emission at V ≈ 350 km s⁻¹. Near the systemic velocity, where the more abundant HCN isotopologues exhibit weak absorption, no signal is seen, which is likely also a consequence of S/Ns that are too low. The value 0.002 Jy corresponds to ≈ 85 mK. The beam size is shown in the lower right corner of the figure.