Fig. 3: Scatter plots and spectra of [CII] and CO 1 → 0 emission.

a,b, The pixel-by-pixel correlations in the W75N (a) and HV (b) velocity ranges. The 3σ noise levels for [CII] emission (1.8 K km s⁻¹) and CO emission (3.6 K km s⁻¹) are indicated by red and blue dashed lines, respectively. The upper left area with red pixels indicates the intensity space that is below the CO noise level but bright in [CII], with an average value of 4.6 K km s⁻¹ (standard deviation 1.5 K km s⁻¹) and 5.1 K km s⁻¹ (standard deviation 2.2 K km s⁻¹) for the W75N and HV velocity ranges, respectively. c,d, The average spectra over all pixels in the map identified as CO-dark and [CII]-bright in the scatter plot in the W75N (c) and HV (d) velocity ranges, indicated by grey vertical lines. The r.m.s. noise of the spectra for both velocity ranges is 0.075 K for [CII] and 0.15 K for CO, respectively. The [CII] and CO line integrated emissions are 4.7 and 2.2 K km s⁻¹ for the W75N range and 5.4 and 1.3 K km s⁻¹ for the HV range, respectively.