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