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Published November 1, 2006 | Published
Journal Article Open

The Stellar Masses and Star Formation Histories of Galaxies at z ≈ 6: Constraints from Spitzer Observations in the Great Observatories Origins Deep Survey


Using the deep Spitzer Infrared Array Camera (IRAC) observations of the Great Observatories Origins Deep Survey (GOODS), we study the stellar masses and star formation histories of galaxies at z approx 6 based on the i_(775)-band dropout sample selected from the GOODS fields. In total, we derive stellar masses for 53 i_(775)-band dropouts that have robust IRAC detections. These galaxies have typical stellar masses of ~10^(10) M_⊙ and typical ages of a couple of hundred million years, consistent with earlier results based on a smaller sample of z ≈ 6 galaxies. The existence of such massive galaxies at z ≈ 6 can be explained by at least one set of N-body simulations of the hierarchical paradigm. We also study 79 i_(775)-band dropouts that are invisible in the IRAC data and find that they are typically less massive by a factor of 10. These galaxies are much bluer than those detected by the IRAC, indicating that their luminosities are dominated by stellar populations with ages ≾ 40 Myr. Based on our mass estimates, we derive a lower limit to the global stellar mass density at z ≈ 6, which is 1.1-6.7 × 10^6 M_⊙ Mpc^(-3). The prospect of detecting the progenitors of the most massive galaxies at yet higher redshifts is explored. We also investigate the implication of our results for reionization and find that the progenitors of the galaxies comparable to those in our sample, even in the most optimized (probably unrealistic) scenario, cannot sustain the reionization for a period longer than ~2 Myr. Thus most of the photons required for reionization must have been provided by other sources, such as the progenitors of the dwarf galaxies that are far below our current detection capability.

Additional Information

© 2006 The American Astronomical Society. Received 2006 March 31; accepted 2006 April 27. We thank the other members of the GOODS team, who have contributed to the success of the observations and data analysis. We also thank our referee, Rodger Thompson, for his prompt and insightful referee report. We are grateful to Kentaro Nagamine for his up-to-date MF. We wish to thank Ivo Labbe for useful discussion. Support for this work, part of the Spitzer Space Telescope Legacy Science Program, was provided by NASA through contract 1224666, issued by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. H. Y. acknowledges the support from the NASA grant HSTGO-09780.03.

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