The stellar mass density at z ~6 from Spitzer imaging of i'-drop galaxies

Creators: Eyles, Laurence P.; Bunker, Andrew J.; Ellis, Richard S.; Lacy, Mark; Stanway, Elizabeth R.; Stark, Daniel P.; Chiu, Kuenley

Abstract

We measure the ages, stellar masses, and star formation histories of z ~6 galaxies, observed within 1 Gyr of the big bang. We use imaging from the Hubble Space Telescope (HST) and the Spitzer Space Telescope from the public 'Great Observatories Origins Deep Survey' (GOODS), coupled with ground-based near-infrared imaging, to measure their spectral energy distributions (SEDs) from 0.8–5 μm, spanning the rest-frame ultraviolet (UV) and optical. From our sample of ≈50 'i'-drop' Lyman-break star-forming galaxies in GOODS-South with z'AB < 27, we focus on ≈30 with reliable photometric or spectroscopic redshifts. Half of these are confused with foreground sources at Spitzer resolution, but from the 16 with clean photometry we find that a surprisingly large fraction (40 per cent) have evidence for substantial Balmer/4000-Å spectral breaks. This indicates the presence of old underlying stellar populations that dominate the stellar masses. For these objects, we find ages of ~200–700 Myr, implying formation redshifts of 7 ≤ zf ≤ 18, and large stellar masses in the range ~1–3 × 10^10 M⊙. Analysis of seven i'-drops that are undetected at 3.6 μm indicates that these are younger, considerably less massive systems. We calculate that emission line contamination should not severely affect our photometry or derived results. Using SED fits out to 8 μm, we find little evidence for substantial intrinsic dust reddening in our sources. We use our individual galaxy results to obtain an estimate of the global stellar mass density at z ~ 6 . Correcting for incompleteness in our sample, we find the z ~ 6 comoving stellar mass density to be 2.5 × 10^6 M⊙ Mpc^−3. This is a lower limit, as post-starburst and dust-obscured objects, and also galaxies below our selection thresholds, are not accounted for. From our results, we are able to explore the star formation histories of our selected galaxies, and we suggest that the past global star formation rate may have been much higher than that observed at the z ~ 6 epoch. The associated UV flux we infer at z > 7 could have played a major role in reionizing the Universe.

Additional Information

© 2007 The Authors. Journal compilation © 2007 RAS. Accepted 2006 October 13. Received 2006 October 12; in original form 2006 July 24. Published Online: 6 December 2006. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Observations have been carried out using the Very Large Telescope at the ESO Paranal Observatory under Program ID: LP168. A-0485. This paper is based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with proposals #9425 and 9583 (the GOODS public imaging survey). We are grateful to the GOODS team for making their reduced images public. LPE acknowledges a Particle Physics and Astronomy Research Council (PPARC) studentship supporting this study. AJB is grateful for financial support from a Leverhulme Prize. ERS gratefully acknowledges support from NSF grant AST 02–39425. The compilation of T-dwarf spectra comes from Sandy Leggett. We acknowledge useful discussions with Richard McMahon, Karl Glazebrook and Michelle Doherty, and thank Carlton Baugh for providing electronic tables of the GALFORM stellar mass function. LPE thanks Rob King for useful discussions about low-mass T-dwarf objects. We are grateful to Andrea Grazian and Adriano Fontana for providing us with the GOODS-MUSIC sample: a multicolour catalogue of near-IR selected galaxies in the GOODS-South field. We thank the referee for useful comments on this manuscript.

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