Deep Spitzer/IRAC Data for z similar to 10 Galaxies Reveal Blue Balmer Break Colors: Young Stellar Populations at similar to 500 Myr of Cosmic Time

We present the deepest constraints yet on the median rest-UV+optical spectral energy distribution (SED) of z similar to 10 galaxies prior to James Webb Space Telescope science operations. We constructed stacks based on four robust J (125) dropouts, previously identified across the GOODS fields. We used archival Hubble Space Telescope/Wide Field Camera 3 data and the full-depth Spitzer/IRAC mosaics from the GREATS program, the deepest coverage at similar to 3-5 mu m to date. The most remarkable feature of the SED is a blue IRAC [3.6]-[4.5] = -0.18 +/- 0.25 mag color. We also find a nearly flat H (160) - [3.6] = 0.07 +/- 0.22 mag color, corresponding to a UV slope beta = -1.92 +/- 0.25. This is consistent with previous studies and indicative of minimal dust absorption. The observed blue IRAC color and SED fitting suggest that z similar to 10 galaxies have very young (few x 10 Myr) stellar populations, with 80% of stars being formed in the last less than or similar to 160 Myr (2 sigma). While an exciting result, the uncertainties on the SED are too large to allow us to place strong constraints on the presence of a nebular continuum in z similar to 10 galaxies (as might be suggested by the blue [3.6]-[4.5] < 0 mag color). The resulting sSFR is consistent with the specific accretion rate of dark matter halos, indicative of a star formation efficiency showing quite limited evolution at such early epochs.