October 15, 2012 Meeting

Speaker: Benne Holwerda, ESA

Title: Quantifying HI Morphology

Abstract: The classification of the morphology of galaxies at any wavelength has for a long time been the realm of the human eye. While powerful, this made the classification inherently subjective. One approach to quantify and objectify morphological classification is to use several scale-invariant parameters to span the space of galaxy types: concentration, asymmetry, smoothness, Gini distribution index, and the second order moment of brightest regions (M20).

These parameters have proven themselves useful in identifying ongoing major merger events in the near and distant Universe by quantifying the morphology of galaxies in star-formation dominated wavelengths (ultraviolet and blue optical). I applied these parameters for the first time to samples of atomic hydrogen (HI) column density maps, reasoning that before the triggered star-formation, the HI disks of late-type galaxies is severely affected by the merger or tidal event. The motivation was to find a more sensitive, reliable and possibly longer lasting morphological identification of merger events, initially in the nearby Universe but with SKA possibly applicable further afield.

I will present results from several HI surveys, identifying the fraction of mergers through their HI morphology, the visibility time scale from simulations and an first estimate of the volume merger rate based on the WHISP survey for the local Universe. In addition, I identified Extended UltraViolet Disks (XUV) through their ultra-violet and HI maps and discuss what can be said about their origin. Two surveys are poised to map the entire sky in HI at high resolution and sensitivity, the WNSHS survey with WSRT/APERTIF  in the Netherlands and the WALLABY survey with the ASKAP telescope in Australia. Quantified morphology of the thousands of observed galaxies will provide the statistics to identify ongoing mergers, unique systems, stripped HI disks etc.