Origin of disc lopsidedness in the Eridanus group of galaxies

The H-I surface density maps for a sample of 18 galaxies in the Eridanus group are Fourier analysed. This analysis gives the radial variation of the lopsidedness in the H-I spatial distribution. The lopsidedness is quantified by the Fourier amplitude A(1) of the m = 1 component normalized to the average value. It is also shown that in the radial region where the stellar disc and H-I overlap, their A(1) coefficients are comparable. All the galaxies studied show significant lopsidedness in H-I. The mean value of A(1) in the inner regions of the galaxies (1.5-2.5 scalelengths) is >= 0.2. This value of A(1) is twice the average value seen in the field galaxies. Also, the lopsidedness is found to be smaller for late-type galaxies; this is opposite to the trend seen in the field galaxies. These two results indicate a different physical origin for disc lopsidedness in galaxies in a group environment compared to the field galaxies. Further, a large fraction (similar to 30 per cent) shows a higher degree of lopsidedness (A(1) >= 0.3). It is also seen that the disc lopsidedness increases with the radius as demonstrated in earlier studies, but over a radial range that is two times larger than done in the previous studies. The average lopsidedness of the halo potential is estimated to be similar to 10 per cent, assuming that the lopsidedness in H-I disc is due to its response to the halo asymmetry.