Four IRAC sources with an extremely red H - [3.6] color: Passive or dusty galaxies at z > 4.5?

Abstract

We report the detection of four IRAC sources in the GOODS-South field with an extremely red color of H - [3.6] > 4.5. The four sources are not detected in the deep Hubble Space Telescope WFC3 H-band image with H limit = 28.3mag. We find that only three types of SED templates can produce such a red H - [3.6] color: a very dusty SED with the Calzetti extinction of AV = 16mag at z = 0.8; a very dusty SED with the SMC extinction of AV = 8mag at z = 2.0- 2.2; and an 1Gyr SSP with AV ∼0.8 at z = 5.7. We argue that these sources are unlikely dusty galaxies at z ≤ 2.2 based on absent strong MIPS 24 μm emission. The old stellar population model at z > 4.5 remains a possible solution for the 4 sources. At z > 4.5, these sources have stellar masses of log(M */M ⊙) = 10.6-11.2. One source, ERS-1, is also a type-II X-ray QSO with L 2 - 8 keV = 1.6 × 1044 erg s-1. One of the four sources is an X-ray QSO and another one is a HyperLIRG, suggesting a galaxy-merging scenario for the formation of these massive galaxies at high redshifts. © 2011. The American Astronomical Society. All rights reserved.