Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published December 1, 2005 | Accepted Version + Published
Journal Article Open

Spitzer imaging of i′-drop galaxies: old stars at z ≈ 6

Abstract

We present new evidence for mature stellar populations with ages >100 Myr in massive galaxies (M_(stellar) > 10¹⁰ M_⊙) seen at a time when the Universe was less than 1 Gyr old. We analyse the prominent detections of two z ≈ 6 star-forming galaxies (SBM03#1 and #3) made at wavelengths corresponding to the rest-frame optical using the Infrared Array Camera camera onboard the Spitzer Space Telescope. We had previously identified these galaxies in Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) / Great Observatories Origins Deep Survey (GOODS) images of Chandra Deep Field South through the 'i′-drop' Lyman-break technique, and subsequently confirmed the identification spectroscopically with the Keck telescope. The new Spitzer photometry reveals significant Balmer/4000-Å discontinuities, indicative of dominant stellar populations with ages >100 Myr. Fitting a range of population synthesis models (for normal initial mass functions) to the HST/Spitzer photometry yields ages of 250–650 Myr and implied formation redshifts z_f≈ 7.5–13.5 in presently-accepted world models. Remarkably, our sources have best-fitting stellar masses of 1.3–3.8 × 10¹⁰ M_⊙ (95 per cent confidence) assuming a Salpeter IMF. This indicates that at least some galaxies with stellar masses >20 per cent of those of a present-day L* galaxy had already assembled within the first Gyr after the Big Bang. We also deduce that the past average star formation rate must be comparable to the current observed rate (SFRᵤᵥ ~ 5–30 M_⊙ yr−¹), suggesting that there may have been more vigorous episodes of star formation in such systems at higher redshifts. Although a small sample, limited primarily by Spitzer's detection efficiency, our result lends support to the hypothesis advocated in our earlier analyses of the Ultra Deep Field and GOODS HST/ACS data. The presence of established systems at z ≈ 6 suggests that long-lived sources at earlier epochs (z > 7) played a key role in reionizing the Universe.

Additional Information

© 2005 RAS. Received: 13 July 2005. Accepted: 14 July 2005. Published: 01 December 2005. We thank Karl Glazebrook, Rychard Bouwens, Richard McMahon, Rodger Thompson and the anonymous referee for very useful comments. This work is based [in part] 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 & 9583 (the GOODS public imaging survey). We are grateful to the GOODS team for making their reduced images public — a very useful resource. LPE acknowledges a Particle Physics and Astronomy Research Council (PPARC) studentship supporting this study.

Attached Files

Published - 364-2-443.pdf

Accepted Version - 0502385.pdf

Files

0502385.pdf
Files (1.3 MB)
Name Size Download all
md5:9157e6e941dca8bbc29d5af05d8ddd2b
868.5 kB Preview Download
md5:4eb70d147f0f17c3c0ca752fa6eae43d
412.0 kB Preview Download

Additional details

Created:
August 19, 2023
Modified:
October 20, 2023