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We search for passive galaxies at $z>3$ in the GOODS-South field, usingdifferent techniques based on photometric data, and paying attention to developmethods that are sensitive to objects that have become passive shortly beforethe epoch of observation. We use CANDELS HST catalogues, ultra-deep $Ks$ dataand new IRAC photometry, performing spectral energy distribution fitting usingmodels with abruptly quenched star formation histories. We then single outgalaxies which are best fitted by a passively evolving model, and having onlylow probability $( < 5 \%)$ star-forming solutions. We verify the effects ofincluding nebular lines emission, and we consider possible solutions atdifferent redshifts. The number of selected sources dramatically depends on themodels used in the SED fitting. Without including emission lines and withphotometric redshifts fixed at the CANDELS estimate, we single out 30candidates; the inclusion of nebular lines emission reduces the sample to 10objects; allowing for solutions at different redshifts, only 2 galaxies surviveas robust candidates. Most of the candidates are not far-infrared emitters,corroborating their association with passive galaxies. Our results translateinto an upper limit in the number density of $\sim 0.173$ arcmin$^2$ above thedetection limit. However, we conclude that the selection of passive galaxies at$z>3$ is still subject to significant uncertainties, being sensitive toassumptions in the SED modeling adopted and to the relatively low S/N of theobjects. By means of dedicated simulations, we show that JWST will greatlyenhance the accuracy, allowing for a much more robust classification.

Chasing passive galaxies in the early Universe: a critical analysis in CANDELS GOODS-South