The Nature of Extremely Red H - [4.5] > 4 Galaxies Revealed with SEDS and CANDELS

We have analyzed a sample of 25 extremely red H - [4.5] > 4 galaxies, selected using 4.5 μm data from the Spitzer SEDS survey and deep H-band data from the Hubble Space Telescope CANDELS survey, over ~180 arcmin2 of the UKIDSS Ultra-Deep Survey 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 colors. Using up to 17-band photometry (U through 8.0 μm), we have studied in detail their spectral energy distributions, including possible degeneracies in the photometric redshift/internal extinction (z phot-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 <z phot <5. All of the 24 μm detected sources in our sample are in this category. Two of these have S(24 \, \mu m)>300 \, \mu Jy, which at 3 <z phot <5 suggests that they probably host a dust-obscured active galactic nucleus. Our sample also contains four highly obscured (AV > 5) sources at z phot <1. Finally, we analyze in detail two z phot ~ 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.