DeepDive: Tracing the early quenching pathways of massive quiescent galaxies at $z>3$ from their star-formation histories and chemical abundances

We investigate the chemical abundances and star-formation histories (SFH) of ten massive ($\mathrm{log}{10} (M{\star}/\mathrm{M}{\odot})>10.5$) quiescent galaxies at $3<z<4$ using deep, medium-resolution spectroscopic data obtained as part of the \textit{JWST DeepDive} Cycle 2 GO program. Our \textit{DeepDive} sample demonstrates early formation and quenching times inferred from spectro-photometric fitting, with most galaxies having formed 50% of their stellar mass by $z \sim 5$, and quenching by $z \sim 4$, showing good agreement across the various SFH parameterizations explored in this work. Though they differ slightly between SFH parameterizations, the inferred formation timescales for the {\it DeepDive} sample span both rapid ($\lesssim$ 100 Myr) and more extended ($\gtrsim$ 200 Myr) episodes, corresponding to star formation occurring over a few to several dynamical times given their compact sizes and high densities at $z\sim3-4$. On average, massive quiescent galaxies at $3<z<4$ are $α$-enhanced ($\langle [α/\mathrm{Fe}]\rangle$= $0.22^{+0.22}{-0.17}$), although there is strong diversity ($\sim0.3$ dex in scatter) among individual [$α$/Fe] values. Our results for $α$-enhancement are consistent with lower-redshift studies, implying weak evolution in [$α$/Fe] from $z \sim 4$ to $z\sim 1$. The SFH timescales associated with the low [$α$/Fe] measurements suggest longer formation timescales, potentially pointing to earlier enrichment by Type Ia supernovae, or metals preferentially being removed via outflows driven either by powerful early active galactic nuclei or supernovae. Overall, this work represents the first, statistically representative combined study of the star-formation histories and chemical abundances of massive quiescent galaxies at $z>3$.

💡 Research Summary

This paper presents the first statistically robust combined analysis of star‑formation histories (SFHs) and chemical abundances for massive quiescent galaxies (MQGs) at redshifts greater than three, using deep, medium‑resolution JWST spectroscopy from the Cycle 2 GO program “DeepDive”. Ten galaxies with stellar masses log M★/M⊙ > 10.5 and spectroscopic redshifts 3 < z < 4 were observed with NIRCam imaging (four filters) and NIRSpec G235M (F170LP) spectroscopy, achieving 1–3 hours of integration per target. The authors performed simultaneous spectro‑photometric fitting with the Bagpipes code, employing BC03 stellar population models (solar abundance ratios, Kroupa IMF) and a flexible, piecewise continuous SFH parameterization. In parallel, they applied the alfα framework, which incorporates α‑enhanced simple stellar population (SSP) models (Knowles 2023) to directly measure