  We have analysed a sample of 25 extremely red H-[4.5]>4 galaxies, selected
using 4.5 micron data from the Spitzer SEDS survey and deep H-band data from
the Hubble Space Telescope (HST) CANDELS survey, over ~180 square arcmin of the
UKIDSS Ultra Deep Survey (UDS) field. Our aim is to investigate the nature of
this rare population of mid-infrared (mid-IR) sources that display such extreme
near-to-mid-IR colours. Using up to 17-band photometry (U through 8.0 microns),
we have studied in detail their spectral energy distributions, including
possible degeneracies in the photometric redshift/internal extinction
(zphot-Av) plane. Our sample appears to include sources of very different
nature. Between 45% and 75% of them are dust-obscured, massive galaxies at
3<zphot<5. All of the 24 micron-detected sources in our sample are in this
category. Two of these have S(24 micron)>300 microJy, which at 3<zphot<5
suggests that they probably host a dust-obscured active galactic nucleus (AGN).
Our sample also contains four highly obscured (Av>5) sources at zphot<1.
Finally, we analyse in detail two zphot~6 galaxy candidates, and discuss their
plausibility and implications. Overall, our red galaxy sample contains the tip
of the iceberg of a larger population of z>3 galaxies to be discovered with the
future James Webb Space Telescope.
