Origin of Disk Lopsidedness in Spiral Galaxies

In our work we have used the atomic hydrogen [H I] gas distribution in the H I 21-cm line emission to study the dark matter halo perturbations. For this analysis, the 2-D H I surface density and velocity maps (archival) of the galaxies in the Eridanus group (obtained using the GMRT) and in the Ursa Major group (obtained from WSRT) were used. In addition a few Hickson Compact Groups of galaxies were also studied using the GMRT. The H I maps of these galaxies were Fourier analysed to estimate the asymmetry in the distribution and motion of gas. The average asymmetry parameter in the 1.5 to 2.5 K'-band scale lengths was found to be ˜ 0.27 for the Eridanus group of galaxies while it was ˜ 0.14 for the Ursa Major group of galaxies. The asymmetries in the distribution of H I as a function of Hubble type of galaxies were also studied and was found to be directly correlated with the compactness of the groups. In addition, the trend in the asymmetry as a function of the Hubble type of galaxies was opposite to that seen in the field galaxies, i.e., in the group galaxies, the early type galaxies showed more asymmetry than late type. These two aspects indicated that tidal interactions between the galaxies in a group environment to be the major cause of asymmetries. The observed asymmetry parameters were consistent with recent numerical simulations of asymmetries of gas disk caused by fly-by interactions. We have also estimated the perturbation of dark matter halo using the asymmetry parameter obtained from the Fourier series analysis of the surface density maps