Discovery of a giant HI tail in the galaxy group HCG 44

We report the discovery of a giant HI tail in the intragroup medium of HCG 44 as part of the ATLAS(3D) survey. The tail is similar to 300 kpc long in projection and contains similar to 5 x 10(8) M-circle dot of HI. We detect no diffuse stellar light at the location of the tail down to similar to 28.5 mag arcsec(-2) in g band. We speculate that the tail might have formed as gas was stripped from the outer regions of NGC 3187 (a member of HCG 44) by the group tidal field. In this case, a simple model indicates that about 1/3 of the galaxy's HI was stripped during a time interval of <1 Gyr. Alternatively, the tail may be the remnant of an interaction between HCG 44 and NGC 3162, a spiral galaxy now similar to 650 kpc away from the group. Regardless of the precise formation mechanism, the detected HI tail shows for the first time direct evidence of gas stripping in HCG 44. It also highlights that deep HI observations over a large field are needed to gather a complete census of this kind of events in the local Universe.