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Today's Colloquium: Quantifying HI Morphology

Submitter: Colloquium
Description: Speaker: Benne Holwerda, ESA-ESTEC

Image: NGC6946 in optical (left) and HI emission (right). The optical image is a colour composite of the Digitized Sky Survey plates. The HI is a deep 192 hours WSRT observation. The images represent the same physical region.

The morphology of spiral galaxies hold much information on their history and environment. One important factor shaping morphology is recent gravitational interaction with other galaxies. To quantify the number of major interactions over cosmic times, several parameterizations of galaxy appearance have been developed. Typically, the images used are restframe ultraviolet, since these are available for high and low redshifts.

In the context of the upcoming grand radio surveys of 21cm line emission of the Local Universe, we are interested in quantifying the morphology of neutral hydrogen column density maps in a similar fashion. Radio observations of HI are spatially coarser than optical ones but the HI disk extends well beyond the optical one, and it is very sensitive to gravitational interaction as well as other processes.

We have applied the CAS and Gini-M20 image parametrizations to HI column density maps from several public HI surveys (WHISP, THINGS and VIVA), as well as those from simulated mergers. From these, we learn that there is a specific part of HI morphology space where mergers reside for a Gyr preceding the full merger. We estimate the local merger rate for spirals from the WHISP surveys. More accurate measures of the local merger rate will have to wait for the planned surveys with APERTIF and ASKAP.
Copyright: Rense Boomsma
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