A Coherent Study of Emission Lines from Broadband Photometry: Specific Star Formation Rates and [O III]/Hβ Ratio at 3 < z < 6

Creators: Faisst, A. L.; Capak, P.; Hsieh, B. C.; Laigle, C.; Salvato, M.; Tasca, L.; Cassata, P.; Davidzon, I.; Ilbert, O.; Le Fèvre, O.; Masters, D.; McCracken, H. J.; Steinhardt, C.; Silverman, J. D.; de Barros, S.; Hasinger, G.; Scoville, N. Z.

Abstract

We measure the Hα and [O III] emission line properties as well as specific star formation rates (sSFRs) of spectroscopically confirmed 3 < z < 6 galaxies in COSMOS from their observed colors versus redshift evolution. Our model describes consistently the ensemble of galaxies including intrinsic properties (age, metallicity, star formation history), dust attenuation, and optical emission lines. We forward-model the measured Hα equivalent widths (EW) to obtain the sSFR out to z ~ 6 without stellar mass fitting. We find a strongly increasing rest-frame Hα EW that is flattening off above z ~ 2.5 with average EWs of 300–600 Å at z ~ 6. The sSFR is increasing proportionally to (1+z)^(2.4) at z < 2.2 and to (1+z)^(1.5) at higher redshifts, indicative of a fast build-up of mass in high-z galaxies within e-folding times of 100–200 Myr at z ~ 6. The redshift evolution at z > 3 cannot be fully explained in a picture of growth driven by cold accretion. We find a progressively increasing [O III]λ5007/Hβ ratio out to z ~ 6, consistent with the ratios in local galaxies selected by increasing Hα EW (i.e., sSFR). This demonstrates the potential of using "local high-z analogs" to investigate the spectroscopic properties and relations of galaxies in the re-ionization epoch.

Additional Information

© 2016 The American Astronomical Society. Received 2016 January 15; accepted 2016 March 7; published 2016 April 20. We would like to thank Josh Speagle, Micaela Bagley, and Bahram Mobasher for valuable discussions. We also would like to thank the referee for valuable comments that have improved this paper. A.F. acknowledges support from the Swiss National Science Foundation. C.L. is supported by the ILP LABEX (under reference ANR-10-LABX-63 and ANR-11-IDEX-0004- 02). J.D.S. is supported by JSPS KAKENHI Grant Number 26400221, the World Premier International Research Center Initiative (WPI), MEXT, Japan and by CREST, JST. Part of this work is based on data obtained with the European Southern Observatory Very Large Telescope, Paranal, Chile, under Large Programs 175.A-0839 and 185.A-0791. This work is supported by funding from the European Research Council Advanced Grant ERC-2010-AdG-268107-EARLY. Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under ESO programme ID 179.A-2005 and on data products produced by TERAPIX and the Cambridge Astronomy Survey Unit on behalf of the UltraVISTA consortium. Also, the authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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