Early Results from GLASS-JWST. IV. Spatially Resolved Metallicity in a Low-mass z ∼ 3 Galaxy with NIRISS*

Abstract

We report the first gas-phase metallicity map of a distant galaxy measured with the James Webb Space Telescope (JWST). We use the NIRISS slitless spectroscopy acquired by the GLASS Early Release Science program to spatially resolve the rest-frame optical nebular emission lines in a gravitationally lensed galaxy at z = 3.06 behind the A2744 galaxy cluster. This galaxy (dubbed GLASS-Zgrad1) has stellar mass ∼10 ^8.6 M _⊙, instantaneous star formation rate ∼8.6 M _⊙ yr ⁻¹ (both corrected for lensing magnification), and global metallicity one-fourth solar. From its emission-line maps ([O iii], H β, H γ, [Ne iii], and [O ii]), we derive its spatial distribution of gas-phase metallicity using a well-established forward-modeling Bayesian inference method. The exquisite resolution and sensitivity of JWST/NIRISS, combined with lensing magnification, enable us to resolve this z ∼ 3 dwarf galaxy in ≳50 resolution elements with sufficient signal, an analysis hitherto not possible. We find that the radial metallicity gradient of GLASS-Zgrad1 is strongly inverted (i.e., positive): $\mathrm{\Delta }\log (\mathrm{O}/\mathrm{H})/\mathrm{\Delta }r$ = 0.165 ± 0.023 dex kpc ⁻¹. This measurement is robust at $\gtrsim 4-\sigma$ confidence level against known systematics. This positive gradient may be due to tidal torques induced by a massive nearby (∼15 kpc projected) galaxy, which can cause inflows of metal-poor gas into the central regions of GLASS-Zgrad1. These first results showcase the power of JWST wide-field slitless spectroscopic modes to resolve the mass assembly and chemical enrichment of low-mass galaxies in and beyond the peak epoch of cosmic star formation ( z ≳ 2). Reaching masses ≲ 10 ⁹ M _⊙ at these redshifts is especially valuable to constrain the effects of galactic feedback and environment and is possible only with JWST’s new capabilities.