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

Abstract

We use deep spectroscopy from the Hubble Space Telescope Wide-Field-Camera 3 IR grisms combined with broad-band 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 Lyman-α Emission At Reionization (CLEAR) survey. At these redshifts the grism spectroscopy measure the [O II] λλ3727, 3729, [O III] λλ4959, 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. 2019a), 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, ∆(log Z) = 0.25 (0.35) dex, and higher ionization parameters, ∆(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 (sSFRs), where ∆ log q 0.4 × ∆(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 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.

Bridging Optical and Far-infrared Emission-line Diagrams of Galaxies from Local to the Epoch of Reionization: Characteristic High [O iii] 88 μm/SFR at z > 6

“…We used the same lower limit as Equation (3) because the increase in the lower limit makes it difficult to model the z ∼ 2 galaxies exhibiting relatively low [O III]/Hβ ratios. These modifications are consistent with a recent observational study of the U−Z gas relation at z ∼ 1−2 (Papovich et al 2022). For the FIR sample, the z > 6 galaxies were assumed to have subsolar metallicities because highredshift galaxies likely have low metallicities according to the redshift evolution of the mass-metallicity relation (e.g., Zahid et al 2013).…”

The Physical Conditions of Emission-Line Galaxies at Cosmic Dawn from JWST/NIRSpec Spectroscopy in the SMACS 0723 Early Release Observations

“…We compare our z > 5 mass-metallicity properties to several lower-redshift comparison samples of "direct" metallicity estimates from [O III] auroral lines: the Sanders [O III]λ1663 (gray diamonds) galaxies at z ∼ 2.5, stacked HST/WFC3 grism measurements of z ∼ 2 galaxies (blue squares) from Henry et al (2021), and the best-fit line describing z ∼ 0 galaxies (dotted line) from Berg et al (2012). The Figure also includes the parametric mass-metallicity relations at z ∼ 0.2, z ∼ 1.3, z ∼ 1.9 determined by Papovich et al (2022), although these are only constrained over 9 < log(M /M ) < 11 (e.g. at higher stellar masses than most of the z > 5 galaxies.…”

“…Galaxies at 1 < z < 3.5 have lower metallicity than z ∼ 0 galaxies of the same stellar mass (Henry et al 2013;Steidel et al 2014;Maiolino & Mannucci 2019;Sanders et al 2021), as expected from enrichment by star formation. But beyond the metallicity evolution they also have higher ionization (Liu et al 2008;Kewley et al 2015;Shapley et al 2015;Strom et al 2018;Backhaus et al 2022;Papovich et al 2022). Compared to the current epoch, galaxies at 1 < z < 2 have higher AGN content (Trump et al 2011;Juneau et al 2014;Coil et al 2015), higher-density H II regions (Brinchmann et al 2008;Liu et al 2008;Davies et al 2021), and more α-enrichment from Wolf-Rayet stars and/or massive binaries (Masters et al 2014;Strom et al 2017;Sanders et al 2020).…”

CLEAR: Boosted Lyα Transmission of the Intergalactic Medium in UV-bright Galaxies

“…Particularly, they find a higher Lyα detection rate at z ; 7 from massive galaxies with strong [O III]+Hβ emission, which reflects enhanced ionizing photoproduction rates (e.g., Roberts-Borsani et al 2016;Tang et al 2019Tang et al , 2021aEndsley et al 2021b), arguing for higher Lyα equivalent widths (EWs) from the strong [O III]+Hβ emitting population as similar as shown at lower redshifts of z ; 2-3 (Tang et al 2021b). This may be related to the galaxies' specific star formation rates (sSFRs) and ionization (Backhaus et al 2022;Papovich et al 2022), although this remains tenuous as there is not yet any conclusive evidence for a significant enhancement of Lyman continuum escape fraction found for galaxies with higher [O III] +Hβ galaxies at z ∼ 3 (Saxena et al 2022). Therefore, it is prudent to look for indications that evolution of Lyα emission in galaxies depends on UV luminosity in the epoch of reionization.…”