Line flux and ratio correlations. Left: I_{[C I](1−0)} versus I_{CO(2 − 1)} in the four galaxies we studied: NGC 3627, NGC 4321, NGC 1808 and NGC 7469. The I_{[C I](1−0)} and I_{CO(2 − 1)} integrated fluxes are moment-0 intensities extracted from their beam and pixel size matched data cubes. Only data points with propagated moment-0 S/N ≥ 5 are shown. The blue (with crossed hatch), green (with horizontal line hatch), gray (with dotted hatch) and light gray shadings indicate the 5σ thresholds for the four galaxies, respectively. Typical line widths of 10 km s⁻¹ (NGC 3627 and NGC 4321) and 30 km s⁻¹ (NGC 1808 and NGC 7469) in sigma are used to calculate the thresholds together with the channel rms in Table 1. For NGC 3627 and NGC 4321, their (uv-matched) CO(2−1) 5σ thresholds are out of the plotting range. Top axis shows the simply-converted H₂ surface density using a constant α_CO = 4.3 and CO excitation (i.e., CO(2−1) to CO(1−0) line ratio) R₂₁ = 0.65. Right: same data but shown as the line ratio R_C I/CO = I_{[C I](1−0)}/I_CO(2−1) versus I_{CO(2 − 1)}. The blue horizontal line indicates a star-forming (SF) disk-like R_C I/CO ∼ 0.1, i.e., the spiral arms of NGC 3627 and NGC 4321. The gray horizontal line indicates a starburst (SB) disk-like R_C I/CO ∼ 0.2, i.e., the ∼1 kpc SB disk/ring in NGC 1808 and NGC 7469. The red line with a slope of 0.8 in logarithm (0.035 (I_{CO(2 − 1)}/[100 K km s⁻¹])^0.8) visually guides the trend of increasing R_C I/CO in the central regions of these galaxies (see boxes in Figs. 1–3, sizes ∼300 − 500 pc).