The line-of-sight warp of the spiral galaxy ESO 123-G23

We present 3-D modelling of the distribution and kinematics of the neutral hydrogen in the spiral galaxy ESO 123-G23. The optical appearance of this galaxy is an almost perfectly edge-on disk, while the neutral hydrogen is found to extend vertically out to about 15 kpc on either side of the galactic plane. The H I layer and the major features of the H I data cube can be successfully explained by a model dominated by a strong (about 30(degree)) line-of-sight warp. Other models were tried, including a flare model and a two-component model, but they clearly do not reproduce the data. This is the first unambiguous detection of a galactic warp that has the maximum deviation from the central plane almost along the line-of-sight. No evidence for the presence of any companion galaxy is found in the H I data cube. Line-of-sight warps in edge-on galaxies are probably frequent, but escape detection as they are too weak. Moreover they may easily be mistaken as flares or "thick disks". A 3-D modelling of the H I layer as the one presented here is needed in order to distinguish between these possibilities.