Science cases for the velocity-resolved mid-J CO and 13CO and [NII] emissions

When studying PDRs, the contribution of shocks to the emission of mid-J CO lines is often debated. Plane-parallel PDR models fail to reproduce strong mid-J CO emissions, frequently leading to the conclusion of a shock contribution, while clumpy PDR models predict a flatter CO ladder in agreement with observations. One way to assign the origin of the mid-J CO lines is to investigate their velocity profile. Observations of mid-J 13CO emission profiles provide additional information as these lines are more likely optically thin. With AtLAST located on Cerro Chajnantor, a better atmospheric transparency compared to the ALMA plateau would enable efficient observations of the CO(6-5) to (8-7) lines. Under best weather conditions, it would be possible to observe up to CO(13-12) and the [NII]205 micron emission, which is essential to distinguish the ionized gas contribution to the [CII] emission observed from SOFIA. High angular resolution mapping in nearby galaxies with a large single dish telescope at high frequencies has a good synergy with interferometric observations at lower frequencies. Note that being at the high site does not compromise the zero-spacing-filling at low frequencies.