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

Karina I. Caputi, James S. Dunlop, Ross J. McLure, Jiasheng Huang, Giovanni G. Fazio, Matthew L. N. Ashby, Marco Castellano, Adriano Fontana, Michele Cirasuolo, Omar Almaini, Eric F. Bell, Mark Dickinson, Jennifer L. Donley, Sandra M. Faber, Henry C. Ferguson, Mauro Giavalisco, Norman A. Grogin, Dale D. Kocevski, Anton M. Koekemoer, David C. Koo, Kamson Lai, Jeffrey A. Newman, Rachel S. Somerville

Published 2012-02-02, updated 2012-03-29Version 2

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.