When, where, and how star formation happens in a galaxy pair at cosmic noon using CANUCS JWST/NIRISS grism spectroscopy

Abstract

Spatially resolved studies are key to understanding when, where, and how stars form within galaxies. Using slitless grism spectra and broad-band imaging from the CAnadian NIRISS Unbiased Cluster Survey (CANUCS), we study the spatially resolved properties of a strongly lensed (μ = 5.4±1.8) z = 0.8718 galaxy pair consisting of a blue face-on galaxy (10.2 ± 0.2 log(M/M ⊙ )) with multiple star-forming clumps and a dusty red edge-on galaxy (9.9 ± 0.3 log(M / M ⊙)). We produce accurate H α maps from JWST/NIRISS grism data using a new methodology that accurately models spatially varying continuum and emission line strengths. With spatially resolved indicators, we probe star formation on time-scales of ∼ 10 Myr (NIRISS H emission line maps) and ∼ 100 Myr (UV imaging and broad-band SED fits). Taking the ratio of the H α to UV flux (η), we measure spatially resolved star formation burstiness. We find that in the face-on galaxy both H α and broad-band star formation rates (SFRs) drop at large galactocentric radii by a factor of ∼ 4.7 and 3.8, respectively, while SFR over the last ∼ 100 Myrs has increased by a factor of 1.6. Additionally, of the 20 clumps identified in the galaxy pair we find that 7 are experiencing bursty star formation, while 10 clumps are quenching, and 3 are in equilibrium (either being in a state of steady star formation or post-burst). Our analysis reveals that the blue face-on galaxy disc is predominantly in a quenching or equilibrium phase. However, the most intense quenching within the galaxy is seen in the quenching clumps. This pilot study demonstrates what JWST/NIRISS data can reveal about spatially varying star formation in galaxies at Cosmic Noon.