The Carnegie-Irvine Galaxy Survey. III. The Three-Component Structure of
  Nearby Elliptical Galaxies
  Motivated by recent developments in our understanding of the formation and
evolution of massive galaxies, we explore the detailed photometric structure of
a representative sample of 94 bright, nearby elliptical galaxies, using
high-quality optical images from the Carnegie-Irvine Galaxy Survey. The sample
spans a range of environments and stellar masses, from M* = 10^{10.2} to
10^{12.0} solar mass. We exploit the unique capabilities of two-dimensional
image decomposition to explore the possibility that local elliptical galaxies
may contain photometrically distinct substructure that can shed light on their
evolutionary history. Compared with the traditional one-dimensional approach,
these two-dimensional models are capable of consistently recovering the surface
brightness distribution and the systematic radial variation of geometric
information at the same time. Contrary to conventional perception, we find that
the global light distribution of the majority (>75%) of elliptical galaxies is
not well described by a single Sersic function. Instead, we propose that local
elliptical galaxies generically contain three subcomponents: a compact (R_e < 1
kpc) inner component with luminosity fraction f ~ 0.1-0.15; an
intermediate-scale (R_e ~ 2.5 kpc) middle component with f ~ 0.2-0.25; and a
dominant (f = 0.6), extended (R_e ~ 10 kpc) outer envelope. All subcomponents
have average Sersic indices n ~ 1-2, significantly lower than the values
typically obtained from single-component fits. The individual subcomponents
follow well-defined photometric scaling relations and the stellar mass-size
relation. We discuss the physical nature of the substructures and their
implications for the formation of massive elliptical galaxies.
