Spatial and kinematical lopsidedness of atomic hydrogen in the Ursa Major group of galaxies

We have carried out the harmonic analysis of the atomic hydrogen (H  i ) surface density maps and the velocity fields for 11 galaxies belonging to the Ursa Major group, over a radial range of 4–6 disc scalelengths in each galaxy. This analysis gives the radial variation of spatial lopsidedness, quantified by the Fourier amplitude A ₁ of the   m = 1  component normalized to the average value. The kinematical analysis gives a value for the elongation of the potential to be ～10 per cent. The mean amplitude of spatial lopsidedness is found to be ～0.14 in the inner disc, similar to the field galaxies, and is smaller by a factor of ～2 compared to the Eridanus group galaxies. It is also shown that the average value of A ₁ does not increase with the Hubble type, contrary to what is seen in field galaxies. We argue that the physical origin of lopsidedness in the Ursa Major group of galaxies is tidal interactions, albeit weaker and less frequent than in Eridanus. Thus systematic studies of lopsidedness in groups of galaxies can provide dynamical clues regarding the interactions and evolution of galaxies in a group environment.