Astronomers! Do You Know Where Your Galaxies are?

Large-scale spectral-line radio surveys, particularly those using the H i transition, are an effective means of studying the populations of galaxies in the local Universe. Surveying on a large scale allows one to both probe to the fainter flux levels and remain sensitive to the rare but interesting objects at the brightest fluxes. Effective use of large-scale surveys is dictated by the efficiency of detection of the objects of interest. Finding the extreme bright objects is often the easy part – to find the more typical sources you need to push down to fainter fluxes close to the noise level. The complexity and volume of three-dimensional survey data also requires a large degree of automation and reliability to find the desired sources quickly and in a uniform way. We present the Duchamp source finder as a solution to these problems. It is a stand-along software package, designed to detect objects in threedimensional, specifically spectral-line, FITS data cubes, and produce source lists and graphical results showing the detected objects (see Fig. 1). It is optimised for the case of a large number of separated sources embedded in a cube dominated by noise: the typical situation expected for extra-galactic H i surveys.