I. Labbe, P. A. Oesch, R. J. Bouwens, G. D. Illingworth, D. Magee, V. Gonzalez, C. M. Carollo, M. Franx, M. Trenti, P. G. van Dokkum, M. Stiavelli
Using new ultradeep Spitzer/IRAC photometry from the IRAC Ultradeep Field program (IUDF), we investigate the stellar populations of a sample of 63 Y-dropout galaxy candidates at z~8, only 650Myr 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 exposure time in [3.6] and [4.5] to ~120h over the HUDF reaching depths of ~28 (AB, 1 sigma). The improved depth and inclusion of brighter candidates result in direct > 3 sigma IRAC detections of 20/63 sources (32%), of which 9 (14%) are detected at > 5 sigma. Stacking fainter galaxies to estimate average properties we find that the typical L*(z = 8) galaxy has H-[3.6] = 0.35 +- 0.15 and [3.6]-[4.5] = 0.30 +- 0.15. These colors are significantly different from those of z-dropout galaxies at z~7, observed only 130Myr later. The simplest explanation is that we witness strong rest-frame optical emission lines ([OIII]4959,5007 + Hbeta) moving through the IRAC bandpasses with redshift. The presence of emission lines is further supported by evidence that z~8 galaxies with bluer J-H (rest-frame UV) show redder [3.6]-[4.5] colors (rest-frame B-V). We can obtain a model independent estimate of the equivalent width by assuming that the average rest-frame spectrum is the same at both z~7 and z~8, finding W([OIII]4959,5007+Hbeta)=500(-164,+226)\AA, contributing ~0.45 mag to [4.5] at z~8. Correcting for this, the typical L*(z=8) galaxy has rest-frame color U-V~0.2 (AB), mass-to-light ratios M/L_UV~0.07, stellar mass M~1x10^9 Msun, and specific star formation rate sSFR ~ 5Gyr^-1. The integrated stellar mass density (SMD) to MUV,AB<-18 is rho(z=8)=1.0(-0.5,+0.7)x10^6 Msun Mpc-3.
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http://arxiv.org/abs/1209.3037
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