SIMBAD references

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 ∼ 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,1σ). The improved depth and inclusion of brighter candidates result in direct ≥ 3σ InfraRed Array Camera (IRAC) detections of 20/63 sources, of which 11/63 are detected at ≥ 5σ. 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] λλ4959, 5007 + Hβ) 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]λ λ 4959,5007+H_β=670⁺²⁶⁰_–170 Å contributing 0.56^+0.16_–0.11mag to the [4.5] filter at z ∼ 8. The corresponding W_Hα = 430⁺¹⁶⁰_–110 Å implies an average specific star formation rate of s_SFR = 11⁺¹¹_–5 G/yr 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 ∼3 x, decreasing the integrated stellar mass density to ρ^*(z = 8, M_UV < -18 ) = 0.6 ^+0.4_–0.3 x 10⁶M_☉ M/pc3.