Age and metallicity gradients in early-type galaxies: A dwarf to giant
  sequence
  We studied the stellar populations of 40 early-type galaxies using medium
resolution long-slit spectroscopy along their major axes (and along the minor
axis for two of them), from 10^7 Msol to 10^12 Msol (-9.2 > M_B > -22.4 mag).
All the studied galaxies lie on the mass-metallicity and age-mass relations.
The transition type dwarfs deviate from the latter relation having younger mean
age, and the low-mass dwarf spheroidals have older ages, marking a
discontinuity in the relation, possibly due to selection effects.
  In all mass regimes, the mean metallicity gradients are approximately -0.2
and the mean age gradients +0.1 dex per decade of radius. The individual
gradients are widely spread: $ -0.1 < \nabla_{\rm Age} < 0.4 $ and $-0.54 <
\nabla_{[{\rm Fe/H}]} < +0.2 $. We do not find evidence for a correlation
between the metallicity gradient and luminosity, velocity dispersion, central
age or age gradient. Likewise, we do not find a correlation between the age
gradient and any other parameter in bright early-type galaxies. In faint
early-types with $M_B \gtrsim -17$ mag, on the other hand, we find a
correlation between the age gradient and luminosity: the age gradient becomes
more positive for fainter galaxies.
  We conclude that various physical mechanisms can lead to similar gradients
and that these gradients are robust against the environmental effects. In
particular, the gradients observed in dwarfs galaxies certainly survived the
transformation of the progenitors through tidal harassment or/and ram-pressure
stripping. The diversity of metallicity gradients amongst dwarf elliptical
galaxies may reflect a plurality of progenitors' morphologies. The dwarfs with
steep metallicity gradients could have originated from blue compact dwarfs and
those with flat profiles from dwarf irregulars and late type spirals.
(Abridged)
