Quantified HI Morphology II : Lopsidedness and Interaction in WHISP
  Column Density Maps
  Lopsidedness of the gaseous disk of spiral galaxies is a common phenomenon in
disk morphology, profile and kinematics. Simultaneously, the asymmetry of a
galaxy's stellar disk, in combination with other morphological parameters, has
seen extensive use as an indication of recent merger or interaction in galaxy
samples. Quantified morphology of stellar spiral disks is one avenue to
determine the merger rate over much of the age of the Universe. In this paper,
we measure the quantitative morphology parameters for the HI column density
maps from the Westerbork observations of neutral Hydrogen in Irregular and
SPiral galaxies (WHISP). These are Concentration, Asymmetry, Smoothness, Gini,
M20, and one addition of our own, the Gini parameter of the second order moment
(GM). Our aim is to determine if lopsided or interacting disks can be
identified with these parameters. Our sample of 141 HI maps have all previous
classifications on their lopsidedness and interaction. We find that the
Asymmetry, M20, and our new GM parameter correlate only weakly with the
previous morphological lopsidedness quantification. These three parameters may
be used to compute a probability that an HI disk is morphologically lopsided
but not unequivocally to determine it. However, we do find that that the
question whether or not an HI disk is interacting can be settled well using
morphological parameters. Parameter cuts from the literature do not translate
from ultraviolet to HI directly but new selection criteria using combinations
of Asymmetry and M20 or Concentration and M20, work very well. We suggest that
future all-sky HI surveys may use these parameters of the column density maps
to determine the merger fraction and hence rate in the local Universe with a
high degree of accuracy.
