2009ApJ...690..319W

GOODS 850-5 is a hyperluminous radio-faint submillimeter source in the Great Observatories Origins Deep Survey-North. Although it is generally agreed that GOODS 850-5 is at a high redshift, z ≳ 4, its exact redshift is unknown. While its stellar spectral energy distribution (SED) suggests z ∼ 6, its radio/far-IR (FIR) SED suggests a lower redshift of z ∼ 4. To better constrain its stellar SED and redshift, we carried out nano-Jansky sensitivity ultradeep near-IR (NIR) observations between 1.2 and 2.1 µm with the Hubble Space Telescope and the 8 m Subaru Telescope. Even with such great depths we did not detect GOODS 850-5, and the results show that it is an extremely curious source. Between the K_s and 3.6 µm bands, its spectral slope is greater than 3 times that of an extremely red object, and the flux ratio between the two bands is greater than 8 times that of Lyman breaks. It is quite challenging to explain this unusually red color without a Lyman break (which would imply z > 17). It requires a large amount (M_{sstarf}∼ 10^11.5 M_☉) of reddened old stars at z ∼ 6, coexisting with an even more extinguished violent ∼2400-4400 M_☉/yr starburst, which does not have any associated detectable rest-frame UV radiation. We discuss the discrepancy between the NIR and radio/FIR photometric redshifts. We conclude that GOODS 850-5 is at least at z > 4 and is more likely at z ≳ 6. We describe the unusual properties of GOODS 850-5, including its SED and formation history, and we discuss the implications of such massive z > 6 galaxies.