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Summer student project: Detecting the IGM-ISM interface with the WSRT

Submitter: Andreas Johansson
Description: At the region of the interface of the interstellar and intergalactic matter, the density of the HI drops to a critical level and the gas gets ionized by the UV background, since the optical depth decreases and the gas seizes to be self-shielding. We know that the HI disk in most galaxies stretches far outside the visible disk, but the characteristics and extent of the HI disk outside this cut-off region still needs attention.

This summer-student project aimed to develop and test a method to extrapolate the kinematics of a galaxy to extend far beyond the visible disk in order to make a guess of where this faint, outer gas should be in the data sets. Doing this for a large number of galaxies, and averaging the results, could lead to a detection of the faint outer gas. This was performed on 56 galaxies from the WHISP survey, those that are big enough and not to edge-on.

The conclusion is that this method can be used to reach a sensitivity lower than the 10^19 atoms cm^-1 (0.08 Msol/pc^2), and some previously unknown features are visible at very large radius. When data is coming in from the APERTIF HI survey, this method can be applied to many thousands of galaxies and will provide a powerful tool to study the outskirts of the HI disks.

I would acknowledge my gratitude to my supervisors Tom Oosterloo and Gyula Jozsa for the help given and patience shown. I would also like to thank ASTRON for hosting me during these eleven weeks.

Left image: Azimuthal integration of column density
Right image: Intensity plot green line: 1 Msol/pc^2 (1.25x10^20 atoms cm^-1)
Final image: Logarithm of azimuthal integration of column density
Copyright: A Johansson / Astron
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