Longitude-velocity diagram for DIB λ8621 and ¹²CO. Upper panel: variation of the radial velocity of DIB λ8621 carrier (V_DIB) along with the Galactic longitude for 3592 selected DIB measurements. The points are colored by their number density estimated by the Gaussian KDE. The red dots with error bars are the median V_DIB calculated in each longitude bin with a step of 10°. The colored lines are theoretical rotation curves calculated with the rotation model in Reid et al. (2019) with different distances from the Sun. Lower panel: median V_DIB superimposed on the longitude-velocity map of ¹²CO J = (1−0) emission from Dame et al. (2001). The color scale displays the ¹²CO latitude-integrated intensity on a logarithmic scale.