Aims. We present the analysis of photometric, spectroscopic, and morphological properties for differently selected samples of passive galaxies up to z = 1 extracted from the zCOSMOS-20k spectroscopic survey. This analysis intends toexplore the dependence of galaxy properties on the selection criterion adopted, study the degree of contamination due to star-forming outliers, and provide a comparison between different commonly used selection criteria. This work is a first step to fully investigating the selection effects of passive galaxies for future massive surveys such as Euclid. Methods. We extracted from the zCOSMOS-20k catalog six different samples of passive galaxies, based on morphology (3336 "morphological" early-type galaxies), optical colors (4889 "red-sequence" galaxies and 4882 "red UVJ" galaxies), specific star-formation rate (2937 "quiescent" galaxies), a best fit to the observed spectral energy distribution (2603 "red SED" galaxies), and a criterion that combines morphological, spectroscopic, and photometric information (1530 "red & passive early-type galaxies"). For all the samples, we studied optical and infrared colors, morphological properties, specific star-formation rates (SFRs), and the equivalent widths of the residual emission lines; this analysis was performed as a function of redshift and stellar mass to inspect further possible dependencies. Results. We find that each passive galaxy sample displays a certain level of contamination due to blue/star-forming/nonpassive outliers. The morphological sample is the one that presents the higher percentage of contamination, with ∼12-65% (depending on the mass range) of galaxies not located in the red sequence, ∼25-80% of galaxies with a specific SFR up to ∼25 times higher than the adopted definition of passive, and significant emission lines found in the median stacked spectra, at least for log (M/M Š™) < 10.25. The red & passive ETGs sample is the purest, with a percentage of contamination in color <10% for stellar masses log (M/MŠ™) > 10.25, very limited tails in sSFR, a median value ∼20% higher than the chosen passive cut, and equivalent widths of emission lines mostly compatible with no star-formation activity. However, it is also the less economic criterion in terms of information used. Among the other criteria, we found that the best performing are the red SED and the quiescent ones, providing a percentage of contamination only slightly higher than the red & passive ETGs criterion (on average of a factor of ∼2) but with absolute values of the properties of contaminants still compatible with a red, passively evolving population. We also find a strong dependence of the contamination on the stellar mass and conclude that, almost irrespective of the adopted selection criteria, a cut at log (M/MŠ™) > 10.75 provides a significantly purer sample in terms of star-forming contaminants. By studying the restframe color-mass and color-color diagrams, we provided two revised definitions of passive galaxies based on these criteria that better reproduce the observed bimodality in the properties of zCOSMOS-20k galaxies. The analysis of the number densities of the various samples shows evidences of mass-assembly "downsizing", with galaxies at 10.25 < log (M/MŠ™) < 10.75 increasing their number by a factor ∼2-4 from z = 0.6 to z = 0.2, by a factor ∼2-3 from z = 1 to z = 0.2 at 10.75 < log (M/MŠ™) < 11, and by only ∼10-50% from z = 1 to z = 0.2 at 11 < log (M/MŠ™) < 11.5.
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