arXiv subjects

Rion Oh

Publications and source records attributed to Rion Oh.

4 recordsLinked to original sources

arXiv2026

JWST Spectra Conclusively Show an Excess of Neutral Gas Outflows in Quiescent Galaxies at z=2-5

Galaxies exhibit a broad range of star formation activity, from actively star-forming to quiescent systems. Yet, the mechanisms responsible for the rapid shutdown and continued suppression of star formation remain poorly understood, particularly for the quiescent galaxies recently uncovered by JWST at $z>3$. Neutral gas outflows provide a direct tracer of gas removal and regulation in quiescent galaxies. We therefore present a NaID $\lambda\lambda5891,5897$ absorption--line stacking analysis of 274 galaxies at $z=2$--5 drawn from the DAWN JWST Archive (DJA) and the EMBER JWST program, using JWST/NIRSpec medium-resolution grating spectroscopy and NIRCam photometry. NaID absorption is detected in all quiescent galaxy stacks spanning $z=2$--5, providing the first statistical evidence that neutral gas outflows are a ubiquitous feature of quiescent galaxies in the early Universe. Compared to the non-quiescent galaxy stacks, the quiescent stacks show mass loading factors higher by 2--4 dex, with mass outflow rates $\dot{M}_{\rm out} \sim 9$--$30\,M_\odot\,{\rm yr}^{-1}$ elevated by $\sim0.5$--$1$ dex and outflow velocities $v_{\rm out} \sim 310$--$570\,{\rm km\,s}^{-1}$ higher by a factor of $\sim2$. The extreme mass loading factor values and declining star formation histories strongly indicate that these outflows cannot be driven by current star formation alone. In the statistically reliable quiescent redshift bins at $2 \leq z < 3$ and $3 \leq z < 4$, the elevated $[\mathrm{N\,II}]/\mathrm{H}\alpha$ and $[\mathrm{O\,III}]/\mathrm{H}\beta$ ratios are consistent with a possible AGN contribution, suggesting that non-stellar feedback may play a role in sustaining quiescence in galaxies at these epochs.

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arXiv2026

LEGGOS: A Shocking Lack of Evidence for Shocks at sub-kiloparsec Scales at 2 < z < 4

Here we present the first systematic search for shocks in six gravitationally lensed galaxies at 2.37 < z < 3.625 with JWST/NIRSpec integral field spectroscopy from the LEnsing and Galaxy Growth: Observing Substructures (LEGGOS) survey. We employ diagnostics that utilize the fluxes and kinematics of shock-sensitive rest-frame optical emission lines H\b{eta}, [O iii] {\lambda}5008, [O i] {\lambda}6302, H{\alpha}, [N ii] {\lambda}6585, and [S ii] {\lambda}{\lambda}6718, 6733. We find that, on pixel, clump, and galaxy-integrated scales, the LEGGOS spectra show minimal if any evidence for shocks. The image plane pixels are < 8% within the shock regions of the rest-frame optical line ratio diagnostics for any individual galaxy, and the shock-identified pixels do not show a coherent spatial structure. We also leverage MAPPINGS V shock models to infer shock velocities from the observed emission lines, and find that the distributions of inferred shock velocities are inconsistent with those expected for shock-dominated gas. Altogether, none of these methods provide significant evidence for shocks in the six LEGGOS sources. We conclude by discussing the implications of the lack of evidence for shocks in the broader context of galaxy evolution at cosmic noon and earlier epochs.

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arXiv2026

LEGGOS: Direct abundances of N, O, Ne, S, and Ar in five lensed galaxies at Cosmic Noon

We present direct $T_e$ chemical abundances in five strongly lensed galaxies at Cosmic Noon ($2.481 \leq z \leq 3.625$) using JWST/NIRSpec data from the LEGGOS survey. We measure gas-phase abundances of N, O, S, and Ar in all five galaxies, and Ne in two galaxies. Three galaxies have electron temperature constraints from multiple different ionization zones, and we find that these are broadly consistent with temperature scaling relations observed in both local and high-$z$ galaxies. We find a range of oxygen abundances $8.04 \leq 12+\log(\text{O/H}) \leq 8.79$ (22 -- 126\% $Z_{\odot}$). The ratios of N/O and Ne/O are consistent with trends observed in local galaxies. We do not observe any evidence for significant N enhancement in our sample, though the youngest galaxy in our sample (SGAS-J1050) has a mildly elevated $\log(\text{N/O}) = 1.20 \pm 0.08$, which we suggest may be driven by a population of young massive stars. The ratios of S/O and Ar/O are generally sub-solar, similar to trends observed in other high-$z$ galaxies. We find that the S/O and Ar/O abundances are sub-solar, consistent with enrichment from core-collapse supernovae (CCSNe). Modeling of the star formation histories of the LEGGOS galaxies supports CCSNe enrichment, as each galaxy shows a recent period of star formation lasting $\lesssim 100$ Myr, indicating that type Ia supernovae would not yet have had enough time to contribute significantly to the gas-phase abundances of these galaxies.

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arXiv2026

LEGGOS I: The JWST LEGGOS Survey -- LEnsing and Galaxy Growth: Observing Substructures -- Unpacks the Nature of Clumpy Star Formation and Quenching in Gravitationally Lensed Galaxies beyond Cosmic Noon

We present first results from the JWST LEGGOS Survey (LEnsing and Galaxy Growth: Observing Substructures), aimed at studying the physics of clumpy star formation and quenching in eight lensed galaxies at $z\sim2$--4. LEGGOS combines multiple Cycle 2 JWST GO programs (GO 4125, GO 3843) and Cycle 1 archival data, and utilizes strong gravitational lensing with NIRCam imaging and NIRSpec integral-field spectroscopy. LEGGOS targets UV-bright, highly magnified systems to resolve $\sim$10--200 pc regions in both rest-frame optical continuum and nebular emission. This overview paper describes the survey design, data reduction and calibration strategy, and science-quality data products, and highlights early examples demonstrating how spectroscopy breaks key degeneracies inherent to photometry-only clump studies, including identifying recent quenching in previously-thought UV star forming galaxies. We introduce a uniform analysis framework that jointly models lensing reconstruction, multi-band photometry, and integral field spectroscopy to disentangle multiple stellar populations within individual clumps and their surrounding diffuse regions. Using maps of Balmer recombination lines and key emission line diagnostic ratios, we connect star formation histories, dust attenuation, and nebular conditions on sub-kpc scales -- LEGGOS galaxies range from uniform metallicities across the whole galaxy, to having higher clump metallicities and harder ionization conditions relative to diffuse regions. The full survey dataset, with simultaneous flux and morphology constraints on clumpy source-plane regions, and a flexible spectrophotometric SPS modeling approach, provides a direct bridge between parsec-scale star formation physics and galaxy assembly at and beyond cosmic noon, offering a robust and efficient means of resolving star formation in the first galaxies.

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