Origin of disc lopsidedness in the Eridanus group of galaxies

Abstract

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 A₁ 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 A₁ coefficients are comparable. All the galaxies studied show significant lopsidedness in H I. The mean value of A₁ in the inner regions of the galaxies (1.5–2.5 scale- lengths) is ≥ 0.2. This value of A₁ 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 (∼30 per cent) shows a higher degree of lopsidedness (A₁ ≥ 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 ∼10 per cent, assuming that the lopsidedness in H I disc is due to its response to the halo asymmetry.