Overview on spectral line source finding and visualisation
  Here I will outline successes and challenges for finding spectral line
sources in large data cubes that are dominated by noise. This is a 3D challenge
as the sources we wish to catalog are spread over several spatial pixels and
spectral channels. While 2D searches can be applied, e.g., channel by channel,
optimal searches take into account the 3-dimensional nature of the sources. In
this overview I will focus on HI 21-cm spectral line source detection in
extragalactic surveys, in particular HIPASS, the "HI Parkes All-Sky Survey" and
WALLABY, the "ASKAP HI All-Sky Survey". I use the original HIPASS data to
highlight the diversity of spectral signatures of galaxies and gaseous clouds,
both in emission and absorption. Among others, I report the discovery of a 680
km/s wide HI absorption trough in the megamaser galaxy NGC 5793. Issues such as
source confusion and baseline ripples, typically encountered in single-dish HI
surveys, are much reduced in interferometric HI surveys. Several large HI
emission and absorption surveys are planned for the Australian Square Kilometre
Array Pathfinder (ASKAP): here we focus on WALLABY, the 21-cm survey of the sky
(Dec < +30 degr; z < 0.26) which will take about one year of observing time
with ASKAP. Novel phased array feeds ("radio cameras") will provide 30 square
degrees instantaneous field-of-view. WALLABY is expected to detect more than
500 000 galaxies, unveil their large-scale structures and cosmological
parameters, detect their extended, low-surface brightness disks as well as gas
streams and filaments between galaxies. It is a precursor for future HI surveys
with SKA Phase I and II, exploring galaxy formation and evolution. The
compilation of highly reliable and complete source catalogs will require
sophisticated source-finding algorithms as well as accurate source
parametrisation.
