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 HI layer and the major features of the HI data cube can be successfully explained by a model dominated by a strong ( about 30') 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 HI 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 HI layer as the one presented here is needed in order to distinguish between these possibilities.