The Hydra I cluster core. I. Stellar populations in the cD galaxy NGC
  3311
  (Abridged for arXiv) The history of the mass assembly of brightest cluster
galaxies may be studied by the mapping the stellar populations at large radial
distances from the galaxy centre. We provide extended and robust measurements
of the stellar population parameters in NGC 3311, the cD galaxy at the centre
of the Hydra I cluster and out to three effective radii. Using seven
absorption-features defined in the Lick/IDS system and single stellar
populations models, we obtained luminosity-weighted ages, metallicities and
alpha element abundances. The trends in the Lick indices and the distribution
of the stellar population parameters indicate that the stars of NGC 3311 may be
divided into two radial regimes, one within and the another beyond one
effective radius, $R_e = 8.4$ kpc, similar to the distinction between inner
galaxy and external halo derived from the NGC 3311 velocity dispersion profile.
The inner galaxy ($R\leq R_e$) is old (age $\sim 14$ Gyr), have negative
metallicity gradients and positive alpha element gradients. The external halo
is also very old, but the metal and element abundances of the external halo
have both a large scatter, indicating that stars from a variety of satellites
with different masses have been accreted. The region in the extended halo
associated with the off-centred envelope at 0$^o$ < P.A.< 90$^o$ (Arnaboldi et
al. 2012) has higher metallicity with respect to the symmetric external halo.
The different stellar populations in the inner galaxy and extended halo reflect
the dominance of in situ stars in the former and the accreted origin for the
large majority of the stars in the latter. These results provide supporting
evidence to the recent theoretical models of formation of massive ellipticals
as a two-phase process.
