科研之友
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摘要
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 similar to 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 (eta and zeta) 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 photospectroscopic observables - including near-ultraviolet (NUV) photometry from Galaxy Evolution Explorer (GALEX) - we find zeta to be most sensitive to recent episodes of star formation, and eta to be strongly dependent on the average age of the stellar populations. Both eta and zeta also depend on metallicity. We apply these optimal filters to composite spectra assembled by Bernardi et al. The distribution of the eta component of the composites appear to be indistinguishable between high- and low-density regions, whereas the distribution of zeta 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.
- 出版日期2007-12-1
