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The distribution of stars and gas in many galaxies is asymmetric. Thisso-called lopsidedness is expected to significantly affect the dynamics andevolution of the disc, including the star formation activity. Here, we measurethe degree of lopsidedness for the gas distribution in a selected sample of 70galaxies from the Westerbork HI Survey of Spiral and Irregular Galaxies. Thiscomplements our earlier work (Paper I) where the kinematic lopsidedness wasderived for the same galaxies. The morphological lopsidedness is measured byperforming a harmonic decomposition of the surface density maps. The amplitudeof lopsidedness A_1, the fractional value of the first Fourier component, istypically quite high (about 0.1) within the optical disc and has a constantphase. Thus, lopsidedness is a common feature in galaxies and indicates aglobal mode. We measure A_1 out to typically one to four optical radii,sometimes even further. This is, on average, four times larger than thedistance to which lopsidedness was measured in the past using near-IR as atracer for the old stellar component, and will therefore provide a new, morestringent constraint on the mechanism for the origin of lopsidedness.Interestingly, the value of A_1 saturates beyond the optical radius.Furthermore, the plot of A_1 vs. radius shows fluctuations which we argue aredue to local spiral features. We also try to explain the physical origin ofthis observed disc lopsidedness. No clear trend is found when the degree oflopsidedness is compared to a measure of the isolation or interactionprobability of the sample galaxies. However, this does not rule out a tidalorigin if the lopsidedness is long-lived. Additionally, we find that theearly-type galaxies tend to be more morphologically lopsided than late-typegalaxies. Both results together indicate a tidal origin for the lopsidedness.

Lopsidedness in WHISP galaxies: II. Morphological lopsidedness