We apply Principal Component Analysis (PCA)
to a sample of early-type galaxies from the Sloan Digital Sky Survey (SDSS)
in order to infer differences in their star formation histories
from their unresolved stellar populations.
We select a z < 0.1
volume-limited sample comprising ~7000 early-type galaxies
from SDSS/Data Release 4.
Out of the first few principal components (PC),
we study four which give information about stellar populations
and velocity dispersion.
We construct two parameters (η and ζ)
as linear combinations of PC1 and PC2.
The four components can be presented as “optimal filters”
to explore in detail the properties of the underlying stellar populations.
By comparing various photo-spectroscopic observables
— including NUV photometry from GALEX —
we find ζ to be most sensitive to recent episodes of star formation,
and η to be strongly dependent on
the average age of the stellar populations.
Both η and ζ also depend on metallicity.
We apply these optimal filters to composite spectra
assembled by Bernardi et al.
The distribution of the η component of the composites
appear to be indistinguishable between high and low density regions,
whereas the distribution of ζ parameters have a significant skew
towards lower values for galaxies in low density regions.
This result suggests that galaxies in lower density environments
are less likely to present weak episodes of recent star formation.
In contrast, a significant number of galaxies from our high density subsample
—
which includes clusters (both outer regions and centres) and groups
—
underwent small but detectable recent star formation at high metallicity,
in agreement with recent estimates targeting elliptical galaxies
in Hickson Compact Groups and in the field (Ferreras et al.).