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, Nor Pirzkal, Marc Rafelski, Benjamin J. Weiner
Abstract
Abstract 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 Ly α Emission At Reionization (CLEAR) survey. At these redshifts, the grism spectroscopy measure the [O II ] λ λ 3727, 3729, [O III ] λ λ 4959, 5008, and H β 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, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mi>P</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi>k</mml:mi> <mml:mo>=</mml:mo> <mml:msub> <mml:mrow> <mml:mi>n</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> </mml:math> . Compared to low-redshift samples ( z ∼ 0.2) at fixed stellar mass, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mspace width="0.25em"/> <mml:mo>=</mml:mo> </mml:math> 9.4–9.8, the CLEAR galaxies at z = 1.35 (1.90) have lower gas-phase metallicity, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>log</mml:mi> <mml:mi>Z</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> = 0.25 (0.35) dex, and higher ionization parameters, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>log</mml:mi> <mml:mi>q</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> = 0.25 (0.35) dex, where U ≡ q / c . We provide updated analytic calibrations between the [O III ], [O II ], and H β 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>log</mml:mi> <mml:mi>q</mml:mi> <mml:mo>≃</mml:mo> <mml:mn>0.4</mml:mn> <mml:mo>×</mml:mo> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>log</mml:mi> <mml:mspace width="0.25em"/> <mml:mi>sSFR</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> , derived from changes in the strength of galaxy H β 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.