Stellar content of extremely red quiescent galaxies at z>2
  CONTEXT. A set of 20 extremely red galaxies at 2.5<z<3.8 with photometric
features of old passive-evolving galaxies without dust, with stellar masses of
~10^{11} M_sun, have colors that could be related to passive-evolving galaxies
with mean ages larger than 1 Gyr. This suggests they have been formed, on
average, when the Universe was very young (<1 Gyr).
  AIMS. We provide new estimates for the stellar content of these 20 galaxies,
with a deeper analysis for two of them that includes spectroscopy.
  METHODS. We obtained, with the GRANTECAN-10.4 m, ultraviolet rest-frame
spectra of two galaxies and analyzed them together with photometric data. The
remaining 18 galaxies are analyzed only with photometry. We fit the data with
models of a single-burst stellar population (SSP), combinations of two SSPs, as
well as with extended star formation.
  RESULTS. Fits based on one SSP do not provide consistent results for the blue
and red wavelengths. Moreover, the absence in the spectra of a break at ~2,000
angstroms indicates that a rather young component is necessary. Using two SSPs
we can match the photometric and spectroscopic data, with the bulk of the
stellar population being very old (several Gyr) and the remaining contribution
(<5% of stellar mass fraction) from a young, likely residual star formation
component with age <~0.1 Gyr. Exponentially decaying extended star formation
models improve slightly the fits with respect to the single burst model, but
they are considerably worse than the two SSP based fits, further supporting the
residual star formation scenario.
  CONCLUSIONS. The fact that one SSP cannot match these early-type galaxies
highlights the limitations for the use of age estimators based on single lines
or breaks, such as the Balmer break used in cosmic chronometers, thus
questioning this approach for cosmological purposes.
