The Direct-method Oxygen Abundance of Typical Dwarf Galaxies at Cosmic High Noon∗

Abstract

We present a Keck/MOSFIRE rest-optical composite spectrum of 16 typical gravitationally lensed star-forming dwarf galaxies at 1.7 ≲ z ≲ 2.6 (z mean = 2.30), all chosen independent of emission-line strength. These galaxies have a median stellar mass of log ( M * / M ⊙ ) med = 8.29 − 0.43 + 0.51 and a median star formation rate of S F R H α m e d = 2.25 − 1.26 + 2.15 M ⊙ y r − 1 . We measure the faint electron-temperature-sensitive [O iii] λ4363 emission line at 2.5σ (4.1σ) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of 12 + log ( O / H ) direct = 7.88 − 0.22 + 0.25 ( 0.15 − 0.06 + 0.12 Z ⊙ ). We investigate the applicability at high z of locally calibrated oxygen-based strong-line metallicity relations, finding that the local reference calibrations of Bian et al. best reproduce (≲0.12 dex) our composite metallicity at fixed strong-line ratio. At fixed M *, our composite is well represented by the z ∼ 2.3 direct-method stellar mass—gas-phase metallicity relation (MZR) of Sanders et al. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, having recalculated our stellar masses with more realistic nonparametric star formation histories ( log ( M * / M ⊙ ) med = 8.92 − 0.22 + 0.31 ) , we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at fixed M * and SFR, of the locally defined fundamental metallicity relation. We measure the doublet ratio [O ii] λ3729/[O ii] λ3726 = 1.56 ± 0.32 (1.51 ± 0.12) and a corresponding electron density of n e = 1 − 0 + 215 cm − 3 ( n e = 1 − 0 + 74 cm − 3 ) when considering the bootstrapped (statistical-only) error spectrum. This result suggests that lower-mass galaxies have lower densities than higher-mass galaxies at z ∼ 2.

A JWST/NIRSpec Exploration of the Connection between Ionization Parameter, Electron Density, and Star-formation-rate Surface Density in z = 2.7–6.3 Galaxies

“…Along these lines, accounting for sample variance suggests a large scatter in R(S II) at a given M * . Indeed, Gburek et al (2023) find an average n e consistent with the low-density limit (〈n e 〉 ; 1 cm −3 ) for a sample of lensed star-forming galaxies at z ∼ 2.3 with a median stellar mass of…”

Direct T e-based Metallicities of z = 2–9 Galaxies with JWST/NIRSpec: Empirical Metallicity Calibrations Applicable from Reionization to Cosmic Noon

“…We supplement this sample with a [O III]λ4364-detected galaxy from Gburek et al (2019) and two galaxies with detected [O II] auroral emission lines presented in Sanders et al (2023a). We do not include composite spectra (e.g., Steidel et al 2016;Gburek et al 2023), but instead limit to individual sources. The groundbased auroral-line sample thus includes 21 galaxies with a redshift distribution shown by the blue histogram in Figure 1, spanning z = 1.42 − 3.63.…”

“…The most robust resolution to this problem is to construct high-redshift metallicity calibrations using direct metallicity and strong-line measurements of high-redshift galaxies themselves. Based on deep ground-based spectroscopy of bright high-redshift line emitters, a sample of ∼20 star-forming galaxies at z = 1.4−3.7 with auroral-line detections and directmethod metallicities has been assembled, representing many nights of 8−10 m telescope time (Villar-Martín et al 2004;Christensen et al 2012aChristensen et al , 2012bBrammer et al 2012;Stark et al 2013;Bayliss et al 2014;James et al 2014;Stark et al 2014;Sanders et al 2016b;Kojima et al 2017;Berg et al 2018;Patrício et al 2018;Gburek et al 2019;Sanders et al 2020;Gburek et al 2023;Sanders et al 2023a). Sanders et al (2020) showed that, on average, this sample deviates from calibrations based on normal z ∼ 0 sources, but it is wellmatched by the local analog calibrations of Bian et al (2018).…”

The MUSE Ultra Deep Field (MUDF). V. Characterizing the Mass–Metallicity Relation for Low-mass Galaxies at z ∼ 1–2

“…At redshifts of z ∼ 1-3, the MZR has been characterized down to stellar masses of log(M å /M e ) ≈ 9 (Maiolino et al 2008;Henry et al 2013;Kacprzak et al 2016;Ly et al 2016;Onodera et al 2016;Suzuki et al 2017;Wang et al 2020;Sanders et al 2021;Gburek et al 2023). Studies beyond the local Universe were previously limited to higher-mass galaxies due to the difficulty of observing optical emission lines at z > 1 from the ground that are required for metallicities (Clarke et al 2023), combined with the intrinsic faintness of low-mass galaxies.…”