Longitude-velocity and Latitude-velocity plots Observing velocity profiles of the HI lines at different longitudes, we can derive a diagram in which the intensity distribution is mapped in the position-velocity space, namely in the l-vr plane. This is called a longitude-velocity diagram. The LV diagram is used for studying the rotation characteristics of the galactic disk as well as to derive the spiral structure.[ Y.Sofue, Galactic Radio Astronomy ]. The HI spectrums that I observed give us the information needed to construct the l-v diagrams. Data I'll write more about the "observation" part in a later post. But for now, just take it that we get the velocities and the corresponding latitudes from this data. Longitude-velocity plots l-v plot at latitude b=0 l-v plot using data at b=0 latitude In the longitude-velocity plot above you can see that the gas isn't spread everywhere i.e not randomly scattered across the velocity space but instead it forms these "str...