We present the Zurich Extragalactic Bayesian Redshift Analyzer (zebra). The current version of zebra combines and extends several of the classical approaches to produce accurate photometric redshifts down to faint magnitudes. In particular, zebra uses the template‐fitting approach to produce Maximum Likelihood and Bayesian redshift estimates based on the following points. An automatic iterative technique to correct the original set of galaxy templates to best represent the Spectral Energy Distributions (SEDs) of real galaxies at different redshifts. A training set of spectroscopic redshifts for a small fraction of the photometric sample to improve the robustness of the photometric redshift estimates. An iterative technique for Bayesian redshift estimates, which extracts the full two‐dimensional redshift and template probability function for each galaxy. We demonstrate the performance of zebra by applying it to a sample of 866 IAB≤ 22.5 COSMOS galaxies with available u*, B, V, g′, r′, i′, z′ and Ks photometry and zCOSMOS spectroscopic redshifts in the range 0 < z < 1.3. Adopting a 5σ clipping that excludes ≤10 galaxies, both the Maximum Likelihood and Bayesian zebra estimates for this sample have an accuracy σΔz/(1+z) smaller than 0.03. Similar accuracies are recovered using mock galaxies. zebra is made available at http://www.exp-astro.phys.ethz.ch/ZEBRA.
Motivated by the desire to reliably and automatically classify structure of thousands of COSMOS galaxies, we present ZEST, the Zurich Estimator of Structural Types. To classify galaxy structure, ZEST uses (1) five nonparametric diagnostics: asymmetry, concentration, Gini coefficient, second-order moment of the brightest 20% of galaxy pixels, and ellipticity; and (2) the exponent n of single-Sérsic fits to the two-dimensional surface brightness distributions. To fully exploit the wealth of information while reducing the redundancy present in these diagnostics, ZEST performs a principal component (PC) analysis. We use a sample of $56,000 I AB 24 COSMOS galaxies to show that the first three PCs fully describe the key aspects of the galaxy structure, i.e., to calibrate a three-dimensional classification grid of axes PC 1 , PC 2 , and PC 3 . We demonstrate the robustness of the ZEST grid on the z ¼ 0 sample of Frei et al. The ZEST classification breaks most of the degeneracy between different galaxy populations that affects morphological classifications based on only some of the diagnostics included in ZEST. As a first application, we present the evolution since z $ 1 of the luminosity functions (LFs) of COSMOS galaxies of early, disk, and irregular galaxies and, for disk galaxies, of different bulge-to-disk ratios. Overall, we find that the LF up to a redshift z ¼ 1 is consistent with a pure luminosity evolution (of about 0.95 mag at z $ 0:7). We highlight, however, two trends that are in general agreement with a downsizing scenario for galaxy formation, i.e., (1) a deficit of a factor of about 2 at z $ 0:7 of M B > À20:5 structurally classified earlytype galaxies and (2) an excess of a factor of about 3, at a similar redshift, of irregular galaxies.
We study the evolution since z ∼ 1 of the rest-frame B luminosity function of the early-type galaxies (ETGs) in ∼ 0.7 degrees 2 in the COSMOS field. In order ⋆ Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA Inc, under NASA contract NAS 5-26555.-2to identify all plausible progenitors of local ETGs we construct the sample of high−z galaxies using two complementary criteria: (i) A morphological selection based on the Zurich Estimator of Structural Types, and (ii) A photometric selection based on the galaxy properties in the rest-frame (U − V )-M V colormagnitude diagram. We furthermore constrain both samples so as to ensure that the selected high−z progenitors of ETGs are compatible with evolving into systems which obey a fundamental z = 0 scaling relation for early-type galaxies, i.e., the µ B − r hl Kormendy relation. Assuming the luminosity evolution derived from studies of the fundamental plane for high−z ETGs, our analysis shows no evidence for a decrease in the number density of the most massive ETGs out to z ∼ 0.7: Both the morphologically-and the photometrically-selected sub-samples show no evolution in the number density of bright (≈ L > 2.5L * ) ETGs. Allowing for different star formation histories, and cosmic variance, we estimate a maximum decrease in the number density of massive galaxies at that redshift of ∼ 30%. We observe, however, both in the photometrical and in the morphological samples, a deficit of up to ∼ 2 − 3 of fainter early-type galaxies over the same cosmic period. Our results argue against a significant contribution of recent dissipationless "dry" mergers to the formation of the most massive early-type galaxies. We suggest that the mass growth in low luminosity ETGs can be explained with a conversion from z ∼ 0.7 to z = 0 of blue, irregular and disk galaxies into low-and intermediate-mass "red", early-type galaxies, possibly also through gas rich mergers. This interpretation is consistent with the observed increase of a factor of the order of ∼ 2 − 3, from z = 0 to z = 0.7, of the rest-frame B-band luminosity function of blue irregular galaxies.
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