Four IRAC Sources with an Extremely Red H-[3.6] Color: Passive or Dusty Galaxies at z>4.5?

We report detection of four IRAC sources in the GOODS-South field with an extremely red color of H$-$[3.6]$>$4.5. The four sources are not detected in the deep HST WFC3 H-band image with H$_{limit}$=28.3 mag. We find that only 3 types of SED templates can produce such a red H$-$[3.6] color: a very dusty SED with the Calzetti extinction of A$_V$=16 mag at z=0.8; a very dusty SED with the SMC extinction of A$_V$=8 mag at z=2.0$\sim$2.2; and an 1Gyr SSP with A$_V \sim$0.8 at z=5.7. We argue that these sources are unlikely dusty galaxies at z$\leq$2.2 based on absent strong MIPS 24$μ$m emission. The old stellar population model at z$>$4.5 remains a possible solution for the 4 sources. At z$>$4.5, these sources have stellar masses of Log(M$_*$/M$_{\odot}$)=10.6$\sim$11.2. One source, ERS-1, is also a type-II X-ray QSO with L$_{2-8keV}$=1.6$\times 10^{44}$ 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.

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