H I, HISA, and ¹³CO spectra. The black curve shows an example spectrum of H I emission (T_on) from −80 to 160 km s⁻¹ averaged over an area of 180′′ × 180′′ centered in ℓ = 19.9°, b = −0.5°. The dashed red curve is a second-order polynomial fit (T_off) to the absorption-free channels of the H I spectrum at 33.5–43.0 and 56.0–65.5 km s⁻¹ (see Sect. 2.2). We estimated the HISA spectrum by then subtracting the H I spectra from the fitted background emission. The GRS ¹³CO spectrum (Jackson et al. 2006) covering velocities from −5 to 135 km s⁻¹ is shown in blue and has been multiplied by a factor of ten for better visibility.