Origin of disc lopsidedness in the Eridanus group of galaxies

Abstract

The Hi 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 Hi 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 Hi. The mean value of A 1 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 Hi disc is due to its response to the halo asymmetry. © 2006 RAS.