The Stellar Masses and Star Formation Histories of Galaxies atz≈ 6: Constraints fromSpitzerObservations in the Great Observatories Origins Deep Survey

Using the deep Spitzer IRAC observations of the Great Observatories OriginsDeep Survey (GOODS), we study the stellar masses and star formation historiesof galaxies at z ~ 6. Our study is based on the i-band dropout sample selectedfrom the GOODS southern and northern fields (~ 330 arcmin^2 in total), severalof which already have spectroscopic confirmations. In total, we derive stellarmasses for 53 i-band dropouts that have robust IRAC detections. These galaxieshave typical stellar masses of ~ 1e10 M_sun and typical ages of a couple ofhundred million years, consistent with earlier results based on a smallersample of z ~ 6 galaxies in the HUDF. We also study 79 i-band dropouts that areinvisible in the IRAC data and find that they are typically less massive by afactor of ten. Based on our mass estimates, we derive a lower limit to theglobal stellar mass density at z ~ 6. Considering the range of systematicuncertainties in the derived stellar masses, this lower limit is 1.1--6.7e6M_sun/Mpc^3 (co-moving), which is 0.2--1.1% of the present-day value. Theprospect of detecting the progenitors of the most massive galaxies at yethigher redshifts is explored: a deep, wide-field near-IR survey using ourcurrent technology could possibly result in positive detections at z > 7. Wealso investigate the implication of our results for reionzation, and find thatthe progenitors of the galaxies comparable to those in our sample, even in themost optimized (probably unrealistic) scenario, cannot sustain the reionizationfor a period longer than ~ 2 million years. Thus most of the photons requiredfor reionization must have been provided by other sources, such as theprogenitors of the dwarf galaxies that are far below our current detectioncapability. (Abridged)Comment: Accepted for publication in ApJ. A correction regarding comparions to LCDM models has been made; models now agree with observations better. All other conclusions unaffecte