Direct detection of galaxy stellar halos : NGC 3957 as a test case
  We present a direct detection of the stellar halo of the edge-on S0 galaxy
NGC 3957, using ultra-deep VLT/VIMOS V and R images. This is achieved with a
sky subtraction strategy based on infrared techniques. These observations allow
us to reach unprecedented high signal-to-noise ratios up to 15 kpc away from
the galaxy center, rendering photon-noise negligible. The 1 sigma detection
limits are R = 30.6 mag/arcsec^2 and V = 31.4 mag/arcsec^2. We conduct a
thorough analysis of the possible sources of systematic errors that could
affect the data: flat-fielding, differences in CCD responses, scaling of the
sky background, the extended halo itself, and PSF wings. We conclude that the
V-R colour of the NGC 3957 halo, calculated between 5 and 8 kpc above the disc
plane where the systematic errors are modest, is consistent with an old and
preferentially metal-poor normal stellar population, like that revealed in
nearby galaxy halos from studies of their resolved stellar content. We do not
find support for the extremely red colours found in earlier studies of diffuse
halo emission, which we suggest might have been due to residual systematic
errors.
