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

TL;DR

This study identifies four extremely red IRAC sources in GOODS-South, exploring their nature as either passive or dusty galaxies at z>4.5, with some being active X-ray QSOs or HyperLIRGs, indicating early galaxy formation and merging.

Contribution

It presents the discovery and analysis of four high-redshift IRAC sources with extreme colors, proposing their possible passive or dusty nature, and linking some to active galactic nuclei and galaxy mergers.

Findings

Four IRAC sources with H-[3.6]>4.5 detected in GOODS-South.

Possible models include dusty galaxies at z≤2.2 or passive galaxies at z>4.5.

One source is an X-ray QSO; another is a HyperLIRG.

Abstract

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$2.2 based on absent strong MIPS 24$\mu$m emission. The old stellar population model at z$>$4.5 remains a possible solution for the 4 sources. At z$>$4.5, these sources have stellar masses of Log(M$_*$/M$_{\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.