The Spectral Energy Distributions Of z~8 Galaxies From The IRAC Ultra
  Deep Fields: Emission Lines, Stellar Masses, And Specific Star Formation
  Rates At 650 Myr
  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 coverage at 3.6 micron and 4.5 micron 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, of which 11/63 are detected at > 5 sigma. The average
[3.6]-[4.5] colors of IRAC detected galaxies at z~8 are markedly redder than
those at z~7, observed only 130Myr later. The simplest explanation is that we
witness strong rest-frame optical emission lines (in particular
[OIII]4959,5007+Hbeta) 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([OIII]4959,5007+Hbeta) = 670
(+260,-170) Angstrom contributing 0.56 (+0.16,-0.11) mag to the 4.5 micron
filter at z~8. The corresponding W(Halpha) = 430 (+160,-110) Angstrom implies
an average specific star formation rate of sSFR = 11 (+11,-5) Gyr^-1 and a
stellar population age of 100 (+100,-50) 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 x3, decreasing the integrated
stellar mass density to rho*(z=8,MUV<-18)=0.6 (+0.4,-0.3) x 10^6 Msun Mpc^-3.
