Using new ultradeep Spitzer/InfraRed Array Camera (IRAC) photometry from the IRAC Ultra Deep Field program, we investigate the stellar populations of a sample of 63 Y-dropout galaxy candidates at z... Show moreUsing new ultradeep Spitzer/InfraRed Array Camera (IRAC) photometry from the IRAC Ultra Deep Field program, we investigate the stellar populations of a sample of 63 Y-dropout galaxy candidates at z \~{} 8, only 650 Myr after the big bang. The sources are selected from HST/ACS+WFC3/IR data over the Hubble Ultra Deep Field (HUDF), two HUDF parallel fields, and wide area data over the CANDELS/GOODS-South. The new Spitzer/IRAC data increase the coverage in [3.6] and [4.5] to \~{}120h over the HUDF reaching depths of \~{}28 (AB,1{$σ$}). The improved depth and inclusion of brighter candidates result in direct {\gt}=3{$σ$} InfraRed Array Camera (IRAC) detections of 20/63 sources, of which 11/63 are detected at {\gt}=5{$σ$}. The average [3.6]-[4.5] colors of IRAC detected galaxies at z \~{} 8 are markedly redder than those at z \~{} 7, observed only 130 Myr later. The simplest explanation is that we witness strong rest-frame optical emission lines (in particular [O III] {$λ$}{$λ$}4959, 5007 + H{$β$}) moving through the IRAC bandpasses with redshift. Assuming that the average rest-frame spectrum is the same at both z \~{} 7 and z \~{} 8 we estimate a rest-frame equivalent width of $\{$W$\}$\_$\{$[O$\backslash$,$\backslash$scriptsize$\{$III$\}$]$\backslash$ $\backslash$lambda $\backslash$lambda 4959,5007+H$\backslash$beta $\}$=670\^{}$\{$+260$\}$\_$\{$-170$\}$ Å contributing 0.56\^{}$\{$+0.16$\}$\_$\{$-0.11$\}$ mag to the [4.5] filter at z \~{} 8. The corresponding $\{$W$\}$\_$\{$H$\backslash$alpha $\}$=430\^{}$\{$+160$\}$\_$\{$-110$\}$ Å implies an average specific star formation rate of sSFR=11\_$\{$-5$\}$\^{}$\{$+11$\}$ Gyr$^{–1}$ and a stellar population age of 100\_$\{$-50$\}$\^{}$\{$+100$\}$ Myr. Correcting the spectral energy distribution for the contribution of emission lines lowers the average best-fit stellar masses and mass-to-light ratios by \~{}3 {\times}, decreasing the integrated stellar mass density to $\backslash$rho \^{}*(z=8,M\_$\{$$\backslash$rm$\{$UV$\}$$\}${\lt}-18)=0.6\^{}$\{$+0.4$\}$\_$\{$-0.3$\}$$\backslash$times 10\^{}6 $\backslash$,M\_$\backslash$odot Mpc$^{–3}$. Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs \#11563, 9797. Based on observations with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work was provided by NASA through contract 125790 issued by JPL/Caltech. Based on service mode observations collected at the European Southern Observatory, Paranal, Chile (ESO Program 073.A-0764A). Based on data gathered with the 6.5{\nbsp}m Magellan Telescopes located at Las Campanas Observatory, Chile. Show less
