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Abstract: Post-starburst E+A or k+a spectra, characterized by their exceptionallystrong Balmer lines in absorption and the lack of emission lines, belong togalaxies in which the star formation activity ended abruptly sometime duringthe past Gyr. We perform a spectral analysis of galaxies in clusters, groups,poor groups and the field at z=0.4-0.8 based on the ESO Distant Cluster Survey.The incidence of k+a-s at these redshifts depends strongly on environment.K+a-s reside preferentially in clusters and, unexpectedly, in a subset of thesigma = 200-400 km-s groups, those that have a low fraction of OII emitters.In these environments, 20-30% of the recently star-forming galaxies have hadtheir star formation activity recently truncated. In contrast, there areproportionally fewer k+a-s in the field, the poor groups and groups with a highOII fraction. The incidence of k+a galaxies correlates with the clustervelocity dispersion: more massive clusters have higher proportions of k+a-s.Spectra of dusty starburst candidates, with strong Balmer absorption andemission lines, present a very different environmental dependence from k+a-s.They are numerous in all environments at z=0.4-0.8, but they are especiallynumerous in all types of groups, favoring the hypothesis of triggering by amerger. Our observations are consistent with previous suggestions that clusterk+a galaxies are observed in a transition phase as massive S0 and Sa galaxies,evolving from star-forming later types to passively evolving early-typegalaxies. The correlation between k+a fraction and cluster sigma supports thehypothesis that k+a galaxies in clusters originate from processes related tothe intracluster medium, while several possibilities are discussed for theorigin of the k+a frequency in low-OII groups.abr.

Autor: Bianca M. Poggianti, Alfonso Aragon-Salamanca, Dennis Zaritsky, Gabriella De Lucia, Bo Milvang-Jensen, Vandana Desai, Pascale Jab


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