Quantified Hi morphology - IV. The merger fraction and rate in WHISP

Abstract

The morphology of the atomic hydrogen (Hi) disc 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 parametrization of the morphology of Hi column density maps of the Westerbork observations of neutral Hydrogen in Irregular and SPiral galaxies (WHISP) project 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 per cent 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 per cent of the WHISP galaxies are in an interaction (Concentration-M20) and only 7 per cent are 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 are identifiable for ∼1 Gyr by their disturbed Hi morphology. Expressed as volume merger rates, the two estimates agree very well: 7 and 6.8 × 10-3 mergers Gyr-1Mpc-3 for paired and morphologically disturbed Hi discs, respectively. The consistency of our merger fractions with 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 large Hi surveys. The relatively high value for the volume merger rate may be a bias in the selection or WHISP volume. The expected abundance in high-resolution Hi data by the planned South African Karoo Array Telescope (MeerKAT), Australian SKA Pathfinder (ASKAP) and Westerbork Synthesis Radio Telescope/APERture Tile In Focus instrument (WSRT/APERTIF) radio observatories will reveal the importance of mergers in the local Universe and, with the advent of the Square Kilometer Array (SKA), over cosmic times. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.