Abstract

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 similar to 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 similar to 120h over the HUDF reaching depths of similar to 28 (AB, 1 sigma). The improved depth and inclusion of brighter candidates result in direct >= 3 sigma InfraRed Array Camera (IRAC) detections of 20/63 sources, of which 11/63 are detected at >= 5 sigma. The average [3.6]-[4.5] colors of IRAC detected galaxies at z similar to 8 are markedly redder than those at z similar to 7, observed only 130 Myr later. The simplest explanation is that we witness strong rest-frame optical emission lines (in particular [O III] lambda lambda 4959, 5007 + H beta) moving through the IRAC bandpasses with redshift. Assuming that the average rest-frame spectrum is the same at both z similar to 7 and z similar to 8 we estimate a rest-frame equivalent width of W-[OIII] lambda lambda 4959,W-5007+H beta = 670(-170)(+260) angstrom contributing 0.56(-0.11)(+0.16) mag to the [4.5] filter at z similar to 8. The corresponding W-H alpha = 430(-110)(+160) angstrom 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 similar to 3x, decreasing the integrated stellar mass density rho*(z = 8, M-UV -18) = 0.6(-0.3)(+0.4) x 10(6) M(circle dot)Mpc(-3).