The Fornax Deep Survey with VST. I. The extended and diffuse stellar
  halo of NGC~1399 out to 192 kpc
  [Abrigded] We have started a new deep, multi-imaging survey of the Fornax
cluster, dubbed Fornax Deep Survey (FDS), at the VLT Survey Telescope. In this
paper we present the deep photometry inside two square degrees around the
bright galaxy NGC1399 in the core of the cluster. We found a very extended and
diffuse envelope surrounding the luminous galaxy NGC1399: we map the surface
brightness out to 33 arcmin (~ 192 kpc) from the galaxy center and down to
about 31 mag/arcsec^2 in the g band. The deep photometry allows us to detect a
faint stellar bridge in the intracluster region between NGC1399 and NGC1387. By
analyzing the integrated colors of this feature, we argue that it could be due
to the ongoing interaction between the two galaxies, where the outer envelope
of NGC1387 on its east side is stripped away. By fitting the light profile, we
found that it exists a physical break radius in the total light distribution at
R=10 arcmin (~58 kpc) that sets the transition region between the bright
central galaxy and the outer exponential stellar halo. We discuss the main
implications of this work on the build-up of the stellar halo at the center of
the Fornax cluster. By comparing with the numerical simulations of the stellar
halo formation for the most massive BCGs, we find that the observed stellar
halo mass fraction is consistent with a halo formed through the multiple
accretion of progenitors with a stellar mass in the range 10^8 - 10^11 M_sun.
This might suggest that the halo of NGC1399 has also gone through a major
merging event. The absence of a significant number of luminous stellar streams
and tidal tails out to 192 kpc suggests that the epoch of this strong
interaction goes back to an early formation epoch. Therefore, differently from
the Virgo cluster, the extended stellar halo around NGC1399 is characterised by
a more diffuse and well-mixed component, including the ICL.
