Evolution of galaxy spectral properties up to redshift one: searching for AGN-feedback signature.

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Scientific paper

Among the possible explanations for the galaxy color bimodality, we explore the hypothesis of AGN feedback. For this purpose, we have utilized the zCOSMOS 20k catalogue to create samples of galaxies without broad-line AGNs and investigate the evolution of host galaxy spectral properties up to redshift 1. These samples have been divided in 24 mass-redshift bins (about 200 galaxies each) to make average spectra with high S/N ratio, allowing accurate flux measurements. Then, we have applied a continuum subtraction procedure to the average composites by using templates of stellar population synthesis. This provides new spectra that are free from absorption contaminations by stars, plus information on age of the stellar populations and composition (metallicity). At the end, we have exploited four different types of BPT diagnostic diagrams, which are used to display which is the main ionizing mechanism originating emission lines: stars or central active nuclei. Looking at these diagrams, we have found a clear role for the galaxy total stellar mass in leading galaxy evolution. The composite galaxies show a progressive shift towards the AGNs region with increasing stellar mass, except for the sample in which there is a lower cut in EW (no passive galaxy) for detected emission lines. This difference between the diagnostic diagrams might be due to the lack of low ionized sources like LINERs, which are maybe AGNs but present such weak emission lines that they could have been excluded by the latter sample. If we consider a hierarchical scenario in which young low mass galaxies are undergoing star formation and high mass galaxies are the product of a later mass assembly, the latter are red passively-evolving galaxies. In this way, AGNs might have just suppressed the star formation with their feedback, by means of transforming blue galaxies into red ones that continue to accrete mass.

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