Star/Galaxy separation

In an image of the sky, what makes a single star look different from a galaxy, a system of millions (sometimes trillions) of stars? To the human eye, and to many telescopes, they both look like single points of light in the night sky.

This seemingly very simple question hides the much more complicated issue of allocating a size and a scale to objects observed in the sky, which has concerned observers and theorists throughout the twentieth century. Perhaps the most dramatic illustration of this long-standing issue is Heber Curtis and Harlow Shapley so called great debate in the 1920s, which solved the question of the size of our Galaxy in relation to cosmic scales; whereas Shapley was arguing in favor of the milky way embracing the entirety of the universe and spiral nebulae being part of it, Curtis saw our galaxy as one object among many other island universes.

One common denominator of the wide variety of observational probes constraining Dark Energy is the necessity to select pure samples of galaxies. More specifically, all the surveys must differentiate galaxies at cosmological distances from local objects, to obtain pure, or at least well-understood, samples. In the area of ''precision cosmology'', any source of systematic error is likely to play a decisive role and needs to be taken into account in order to refine the standard inflationary Big Bang picture.

During my PhD, I have been leading the star/galaxy separation task force in the Dark Energy Survey. In particular, I have designed a new tool for star galaxy separation, proved its performance on simulations (Soumagnac et. al 2014) and tested it on the DES data (publication in preparation).