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16-04-2010
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Finding galaxies

Submitter: Paolo Serra
Description: The coming years will see a revolution in radio astronomy. Among the many new radio telescopes under commissioning or construction, APERTIF and ASKAP will provide unprecedented survey capabilities for the study of neutral hydrogen gas (HI) in galaxies. For the first time, it will be possible to survey the entire sky at high resolution (a few tens of arcseconds) and with large instantaneous redshift coverage, obtaining the most detailed, statistical picture of the HI content of galaxies during the past few billion years.

HI surveys with APERTIF and ASKAP will detect tens of thousands of galaxies, making object-by-object analysis impossible. Even before being able to analyse the HI sources, a major challenge will be finding them! This particular problem is being investigated within the WALLABY team (WALLABY is the planned ASKAP all-sky HI survey). Source-finding software is being tested on simulated data-cubes, and here we show the result of a first test of the Duchamp source finder on a simulated WALLABY data-cube based on WSRT data. About 100 galaxies observed for the WHISP project with the WSRT were distributed over a 1-square-degree field and in the redshift range 0.02-0.04. WALLABY-like noise based on WSRT data was then added to the simulated cube before running Duchamp. The movie shows:
- grey scale: total HI image of the noise-free cube
- red contours: total HI image of the noise-free cube using only pixels where the flux density is larger than twice the r.m.s. noise expected for WALLABY
- yellow contours: total HI image reconstructed by Duchamp

The movie shows that Duchamp is able to recover most of the flux above 2 sigma. However, this depends on the kinematics of the HI. For example, an expected problem is that source finders may see faint, nearly edge-on discs as two separate sources, detecting only the brightest emission at the edge of the disc (see galaxy in the bottom left). It is clear that source finders will mostly be working very close to the noise level, and extensive testing of different software on different simulated cubes will be crucial for the success of future surveys.

Websites:
WALLABY: http://www.atnf.csiro.au/people/bkoribal/askap/
WHISP: http://www.astro.rug.nl/~whisp/
DUCHAMP: http://www.atnf.csiro.au/people/Matthew.Whiting/Duchamp/
Copyright: WALLABY team
 
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