CLEAR: The Ionization and Chemical-enrichment Properties of Galaxies at 1.1 < z < 2.3

Casey Papovich, Raymond C. Simons, Vicente Estrada-Carpenter, Jasleen Matharu, Ivelina Momcheva, Jonathan R. Trump, Bren E. Backhaus, Gabriel Brammer, Nikko J. Cleri, Steven L. Finkelstein, Mauro Giavalisco, Zhiyuan Ji, Intae Jung, Lisa J. Kewley, David C. Nicholls, Norbert Pirzkal, Marc Rafelski, Benjamin Weiner

Research output: Contribution to journalArticlepeer-review

18 Scopus citations


We use deep spectroscopy from the Hubble Space Telescope Wide-Field-Camera 3 IR grisms combined with broadband photometry to study the stellar populations, gas ionization and chemical abundances in star-forming galaxies at z ∼ 1.1-2.3. The data stem from the CANDELS Lyα Emission At Reionization (CLEAR) survey. At these redshifts, the grism spectroscopy measure the [O II] λ λ3727, 3729, [O III]λ λ4959, 5008, and Hβ strong emission features, which constrain the ionization parameter and oxygen abundance of the nebular gas. We compare the line-flux measurements to predictions from updated photoionization models (MAPPINGS V; Kewley et al.), which include an updated treatment of nebular gas pressure, log P / k = n e T e . Compared to low-redshift samples (z ∼ 0.2) at fixed stellar mass, log M * / M ⊙ = 9.4-9.8, the CLEAR galaxies at z = 1.35 (1.90) have lower gas-phase metallicity, Δ ( log Z ) = 0.25 (0.35) dex, and higher ionization parameters, Δ ( log q ) = 0.25 (0.35) dex, where U ≡ q/c. We provide updated analytic calibrations between the [O III], [O II], and Hβ emission-line ratios, metallicity, and ionization parameter. The CLEAR galaxies show that at fixed stellar mass, the gas ionization parameter is correlated with the galaxy specific star formation rates, where Δ log q ≃ 0.4 × Δ ( log sSFR ) , derived from changes in the strength of galaxy Hβ equivalent width. We interpret this as a consequence of higher gas densities, lower gas covering fractions, combined with a higher escape fraction of H-ionizing photons. We discuss both tests to confirm these assertions and implications this has for future observations of galaxies at higher redshifts.

Original languageEnglish (US)
Article number22
JournalAstrophysical Journal
Issue number1
StatePublished - Sep 1 2022

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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