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Studying The Star Formation Histories of Galaxies in Clusters from Composite Spectra

By Alan Dressler, Augustus Oemler, Bianca M. Poggianti, Ian Smail, Scott Trager, Stephen A. Shectman, Warrick J. Couch and Richard S. Ellis

Abstract

We have formed “composite spectra ” by combining the integrated–light spectra of individual galaxies in 8 intermediate–redshift and 12 low–redshift clusters of galaxies. Because these composite spectra have much higher signal–to–noise ratios than individual galaxy spectra, they are particularly useful in quantifying general trends in star formation for galaxy populations in distant clusters, z> 0.3. By measuring diagnostic features that represent stellar populations of very different ages, a grand-composite spectrum can reflect the fractions of those populations as accurately as if excellent spectral measurements were available for each galaxy. Such composite spectra can also be useful in the study of finer spectral signatures, for example, spectral indices that break the age–metallicity degeneracy, and the shape of the Hδ absorption line as an indicator of the age and duration of an epoch of starbursting galaxies in a cluster. Measuring the equivalent widths of spectral features in composite spectra is especially well–suited for comparing cosmic variance of star formation in cluster

Year: 2004
OAI identifier: oai:CiteSeerX.psu:10.1.1.317.475
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