Galaxy Stellar Mass Functions from z 10 to z 6 using the Deepest Spitzer/Infrared Array Camera Data: No Significant Evolution in the Stellar-to-halo Mass Ratio of Galaxies in the First Gigayear of Cosmic Time

We present new stellar mass functions at z ~ 6, z ~ 7, z ~ 8, z ~ 9 and, for the first time, z ~ 10, constructed from ~800 Lyman-break galaxies previously identified over the eXtreme Deep Field and Hubble Ultra-Deep Field parallel fields and the five Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey fields. Our study is distinctive due to (1) the much deeper (~200 hr) wide-area Spitzer/Infrared Array Camera (IRAC) imaging at 3.6 μm and 4.5 μm from the Great Observatories Origins Deep Survey Re-ionization Era Wide-area Treasury from Spitzer program (GREATS) and (2) consideration of z ~ 6-10 sources over a 3× larger area than those of previous Hubble Space Telescope+Spitzer studies. The Spitzer/IRAC data enable ≥2σ rest-frame optical detections for an unprecedented 50% of galaxies down to a stellar mass limit of $\sim {10}^{8}{{ \mathcal M }}_{\odot }$ across all redshifts. Schechter fits to our volume densities suggest a combined evolution in the characteristic mass ${{ \mathcal M }}^{* }$ and normalization factor φ^* between z ~ 6 and z ~ 8. The stellar mass density (SMD) increases by ~1000× in the ~500 Myr between z ~ 10 and z ~ 6, with indications of a steeper evolution between z ~ 10 and z ~ 8, similar to the previously reported trend of the star formation rate density. Strikingly, abundance matching to the Bolshoi-Planck simulation indicates halo mass densities evolving at approximately the same rate as the SMD between z ~ 10 and z ~ 4. Our results show that the stellar-to-halo mass ratios, a proxy for the star formation efficiency, do not change significantly over the huge stellar mass buildup occurred from z ~ 10 to z ~ 6, indicating that the assembly of stellar mass closely mirrors the buildup in halo mass in the first ~1 Gyr of cosmic history. The James Webb Space Telescope is poised to extend these results into the "first galaxy" epoch at z ≳ 10.