A robust sample of galaxies at redshifts 6.0 < z < 8.7: stellar populations, star formation rates and stellar masses

Abstract

We present the results of a photometric redshift analysis designed to identify z≥ 6 galaxies from the near-infrared Hubble Space Telescope imaging in three deep fields [Hubble Ultra Deep Field (HUDF), HUDF09-2 and Early Release Science] covering a total area of 45 square arcmin. By adopting a rigorous set of criteria for rejecting low-redshift interlopers, and by employing a deconfusion technique to allow the available ultradeep IRAC imaging to be included in the candidate-selection process, we have derived a robust sample of 70 Lyman break galaxies (LBGs) spanning the redshift range 6.0 < z < 8.7. Based on our final sample, we investigate the distribution of ultraviolet (UV) spectral slopes (f_λ∝λ^β), finding a variance-weighted mean value of 〈β〉=−2.05 ± 0.09 which, contrary to some previous results, is not significantly bluer than displayed by lower redshift starburst galaxies. We confirm the correlation between UV luminosity and stellar mass reported elsewhere, but based on fitting galaxy templates featuring a range of star formation histories (SFHs), metallicities and reddening, we find that, at z≥ 6, the range in mass-to-light ratio (M_★/L_(UV)) at a given UV luminosity could span a factor of ≃50. Focusing on a subsample of 21 candidates with IRAC detections at 3.6µm, we find that L^★ LBGs at z≃ 6.5 have a median stellar mass of M_★= (2.1 ± 1.1) × 10^9 M_⊙ (Chabrier initial mass function) and a median specific star formation rate (sSFR) of 1.9 ± 0.8 Gyr^(−1). Using the same subsample, we have investigated the influence of nebular continuum and line emission, finding that for the majority of candidates (16 out of 21), the best-fitting stellar masses are reduced by less than a factor of 2.5. However, galaxy template fits exploring a plausible range of SFHs and metallicities provide no compelling evidence of a clear connection between SFR and stellar mass at these redshifts. Finally, a detailed comparison of our final sample with the results of previous studies suggests that, at faint magnitudes, several high-redshift galaxy samples in the literature are significantly contaminated by low-redshift interlopers.

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