Correlation in velocity between DIB 18621, ¹²CO and H I. Upper and middle panels: correlation between the DIB λ8621 velocity (V_DIB) and the in tensity-weighted velocity of the nearest¹² CO (Dame et al. 2001) and H I (HI4PI Collaboration 2016) component. The color scale indicates the number density of the data points estimated by the Gaussian KDE. The error of V_DIB is simply estimated by the uncertainty of λ₈₆₂₁. The red line is the linear fit to the LSR velocities. The fit slope (α) and intercept (t), as well as the number of the points, are indicated. The dashed green line traces the one-to-one correspondence. Lower panel: example of the peak-finding method (see Sect. 5.1). The red and blue lines are the spectra of ¹²CO and H I, respectively, toward the same sightline. The red star indicates the LSR velocity of the DIB signal detected on this sightline. The Galactic coordinate (ℓ,b) and the Gaia source ID of the background star are marked.