Quantified HI Morphology IV: The Merger Fraction and Rate in WHISP
  The morphology of the atomic hydrogen (HI) disk of a spiral galaxy is the
first component to be disturbed by a gravitational interaction such as a merger
between two galaxies. We use a simple parametrisation of the morphology of HI
column density maps of Westerbork HI Spiral Project (WHISP) to select those
galaxies that are likely undergoing a significant interaction. Merging galaxies
occupy a particular part of parameter space defined by Asymmetry (A), the
relative contribution of the 20% brightest pixels to the second order moment of
the column density map (M20) and the distribution of the second order moment
over all the pixels (GM). Based on their HI morphology, we find that 13% of the
WHISP galaxies are in an interaction (Concentration-M20) and only 7% based on
close companions in the data-cube. This apparent discrepancy can be attributed
to the difference in visibility time scales: mergers are identifiable as close
pairs for 0.5 Gyr but ~1 Gyr by their disturbed HI morphology. Expressed as
volume merger rates, the two estimates agree very well: 7 and 6.8 x 10^-3
mergers Gyr^-1 Mpc^-3 for paired and morphologically disturbed HI disks
respectively. The consistency of our merger fractions to those published for
bigger surveys such as the Sloan Digital Sky Survey, shows that HI morphology
can be a very viable way to identify mergers in a large HI survey. The
relatively high value for the volume merger rate may be a bias in the selection
or WHISP volume. The expected boon in high-resolution HI data by the planned
MeerKAT, ASKAP and WSRT/APERTIF radio observatories will reveal the importance
of mergers in the local Universe and, with the advent of SKA, over cosmic
times.
