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Published May 1, 2016 | Published + Submitted
Journal Article Open

ISM excitation and metallicity of star-forming galaxies at z ~ 3.3 from near-IR spectroscopy


We study the relationship between stellar mass, star formation rate (SFR), ionization state, and gas-phase metallicity for a sample of 41 normal star-forming galaxies at 3 ≾ z ≾ 3.7. The gas-phase oxygen abundance, ionization parameter, and electron density of ionized gas are derived from rest-frame optical strong emission lines measured on near-infrared spectra obtained with Keck/Multi-Object Spectrograph for Infra-Red Exploration. We remove the effect of these strong emission lines in the broadband fluxes to compute stellar masses via spectral energy distribution fitting, while the SFR is derived from the dust-corrected ultraviolet luminosity. The ionization parameter is weakly correlated with the specific SFR, but otherwise the ionization parameter and electron density do not correlate with other global galaxy properties such as stellar mass, SFR, and metallicity. The mass–metallicity relation (MZR) at z ≃ 3.3 shows lower metallicity by ≃ 0.7 dex than that at z = 0 at the same stellar mass. Our sample shows an offset by ≃ 0.3 dex from the locally defined mass–metallicity–SFR relation, indicating that simply extrapolating such a relation to higher redshift may predict an incorrect evolution of MZR. Furthermore, within the uncertainties we find no SFR–metallicity correlation, suggesting a less important role of SFR in controlling the metallicity at high redshift. We finally investigate the redshift evolution of the MZR by using the model by Lilly et al., finding that the observed evolution from z = 0 to z ≃ 3.3 can be accounted for by the model assuming a weak redshift evolution of the star formation efficiency.

Additional Information

© 2016 The American Astronomical Society. Received 2015 December 18; accepted 2016 February 5; published 2016 May 3. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. 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. The observations were partly carried out within the framework of the Subaru–Keck time exchange program. This research made use of Astropy, 16 a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013), APLpy, 17 an open-source plotting package for Python, and matplotlib, 18 a Python 2D plotting library (Hunter 2007). We thank Scott Dahm, Marc Kassis, Jim Lyke, Greg Wirth, and the rest of the staff at the Keck Observatory for supporting the observations; Andreas Faisst, Nicholas Konidaris, Luca Rizzi, and Benny Trakhtenbrot for the assistance on the MOSFIRE data reduction; and Roberto Maiolino, Claudia Scarlata, and Maryam Shirazi for insightful discussions. We thank the anonymous referee for providing constructive comments. A.R. is grateful to the Institute for Astronomy at ETH Zurich for its kind hospitality while working on this project. Facility: Keck I (MOSFIRE).

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Published - apj_822_1_42.pdf

Submitted - 1602.02779v1.pdf


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