Determining the true colour of the extended halos of early-type galaxies

Deep optical images provide unprecedented surface brightness level so that we can probe the stellar populations in the very outskirts of early-type galaxies, in an environment which is crucial to determine the history of these galaxies. The deep images change our outlook on the nearby early-type galaxies by revealing stellar shells, rings, streams, tails and extended halos.

At the very low surface brightness levels reached by deep optical images, a number of artefacts and ghosts show up. By using a new method, we correct for these artificial effects. This was done for a large sample of massive early-type galaxies observed with the MegaCam camera at the Canada - France - Hawaii Telescope under the MATLAS (Mass Assembly of early-Type GaLAxies with their fine Structures) programme. We calculate the colour profiles at large galactic radii, study the age and metallicity gradients, calibrated with the central ancillary data, and compare it with predictions from numerical simulations.