Revisiting Stephan's Quintet with deep optical images
  Stephan's Quintet, a compact group of galaxies, is often used as a laboratory
to study a number of phenomena, including physical processes in the
interstellar medium, star formation, galaxy evolution, and the formation of
fossil groups. As such, it has been subject to intensive multi-wavelength
observation campaigns. Yet, models lack constrains to pin down the role of each
galaxy in the assembly of the group. We revisit here this system with
multi-band deep optical images obtained with MegaCam on the
Canada-France-Hawaii Telescope (CFHT), focusing on the detection of low surface
brightness (LSB) structures. They reveal a number of extended LSB features,
some new, and some already visible in published images but not discussed
before. An extended diffuse, reddish, lopsided, halo is detected towards the
early-type galaxy NGC 7317, the role of which had so far been ignored in
models. The presence of this halo made of old stars may indicate that the group
formed earlier than previously thought. Finally, a number of additional diffuse
filaments are visible, some close to the foreground galaxy NGC 7331 located in
the same field. Their structure and association with mid--IR emission suggest
contamination by emission from Galactic cirrus.
