2012ApJ...750L..20C

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-A_V) 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(24µ) > 300µJy, which at 3 < z_phot< 5 suggests that they probably host a dust-obscured active galactic nucleus. Our sample also contains four highly obscured (A_V> 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.