JWST & the Waz Arc I: Spatially Resolving the Physical Conditions within a Post-Starburst Galaxy at Redshift 5 with NIRSpec IFS
Taylor A. Hutchison, Gourav Khullar, Jane R. Rigby, Brian Welch, Michael K. Florian, Keren Sharon, Isaac Sierra, Julissa Sarmiento, Guillaume Mahler, Nikko J. Cleri, Rachel Bezanson, Michael D. Gladders, Matthew B. Bayliss, Juliana S. M. Karp, Dylan Berry, Alex Ross, T. Emil Rivera-Thorsen, Suhyeon C. Choe, Håkon Dahle, John Chisholm, Erini L. Lambrides, Rebecca L. Larson, Grace M. Olivier, Riley Owens, Erik Solhaug
TL;DR
We present JWST/NIRSpec IFS observations of the gravitationally lensed, rest-frame UV-bright galaxy COOL J1241+2219 (the Waz Arc) at $z\approx5.04$, revealing a post-starburst spectrum with aged stellar absorption features and weak nebular emission. Spatially resolved maps of rest-optical emission lines show heterogeneous nebular metallicities, low-ionization star-forming clumps, and evidence for possible fresh, metal-poor gas inflow, along with the first identification of diffuse ionized gas (DIG) at $z>5$ in this system. The study demonstrates the power of lensing to achieve ~10–100 pc scales, enabling dissection of gas-phase conditions, dust attenuation, and the relationship between gas and stellar populations in an early massive galaxy, shedding light on quenching processes in the early universe.
Abstract
We present NIRSpec/IFS observations of a rest-frame UV-bright, massive ($M_* \sim 10^{10}$ M$_\odot$, $z_{AB}=20.5$) galaxy highly magnified by gravitational-lensing observed just after the end of the epoch of reionization ($z=5.04$, $\barμ\sim90$). With JWST accessing the restframe UV and optical spectrum of this galaxy with high fidelity, we classify this UV-bright galaxy as post-starburst in nature -- due to weak/absent emission lines and strong absorption features -- making this an example of a new class of UV-bright but significantly quenched galaxies being discovered in this epoch. With a median $E(B-V)=0.44\pm0.14$, we identify the presence of stellar absorption across the arc both in Balmer lines and the MgII doublet, indicative of older stellar populations dominated by A stars (and potentially B stars). Using spatially-resolved maps of rest-optical strong emission lines, we find a heterogeneous distribution of nebular metallicities across the arc, potentially hinting at different enrichment processes. With a low median lensing-corrected H$α$ star formation rate of SFR$_{Hα} = 0.024 \pm 0.001$ M$_\odot$ yr$^{-1}$, we find in the most "star-forming" clumps indications of lower ionization (log$_{10}$U $\sim -3.2$), lower nebular metallicities (12+log$_{10}$O/H $\lesssim$ 8.3), and hints of higher densities that suggest a possible recent infall of more pristine (low metallicity) gas onto the galaxy. Investigating the regions with no detectable H$β$ emission, we find (for the first time at $z>5$) signatures of diffuse ionized gas (DIG). Separating DIG from HII regions within a galaxy has predominantly been demonstrated at lower redshifts, where such spatial resolution allows clear separation of such regions -- highlighting the immense power of gravitational lensing to enable studies at the smallest spatial scales at cosmic dawn.