Spectroscopic bulge-disc decomposition: a new method to study the
  evolution of lenticular galaxies
  A new method for spectroscopic bulge-disc decomposition is presented, in
which the spatial light profile in a two-dimensional spectrum is decomposed
wavelength-by-wavelength into bulge and disc components, allowing separate
one-dimensional spectra for each component to be constructed. This method has
been applied to observations of a sample of nine S0s in the Fornax Cluster in
order to obtain clean high-quality spectra of their individual bulge and disc
components. So far this decomposition has only been fully successful when
applied to galaxies with clean light profiles, consequently limiting the number
of galaxies that could be separated into bulge and disc components. Lick index
stellar population analysis of the component spectra reveals that in those
galaxies where the bulge and disc could be distinguished, the bulges have
systematically higher metallicities and younger stellar populations than the
discs. This correlation is consistent with a picture in which S0 formation
comprises the shutting down of star formation in the disc accompanied by a
final burst of star formation in the bulge. The variation in spatial-fit
parameters with wavelength also allows us to measure approximate colour
gradients in the individual components. Such gradients were detected separately
in both bulges and discs, in the sense that redder light is systematically more
centrally concentrated in all components. However, a search for radial
variations in the absorption line strengths determined for the individual
components revealed that they are absent from the vast majority of S0 discs and
bulges. The absence of gradients in line indices for most galaxies implies that
the colour gradient cannot be attributed to age or metallicity variations, and
is therefore most likely associated with varying degrees of obscuration by
dust.
