Minjung Park

The dust attenuation of galaxies is highly diverse and closely linked to stellar population properties and the star-dust geometry, yet its relationship to galaxy morphology remains poorly understood. We present a study of 141 galaxies ($9<\log (\rm M_{\star }/\rm M_{\odot })<11.5$) at 1.7 < z < 3.5 from the Blue Jay survey combining deep HST and JWST/NIRCam imaging, and R ∼ 1000 JWST/NIRSpec spectra. Using Prospector to perform a joint analysis of these data with non-parametric star-formation histories and a two-component dust model with flexible attenuation laws, we constrain stellar and nebular properties. We find that the shape and strength of the attenuation law vary systematically with optical dust attenuation (AV), stellar mass, and star formation rate (SFR). AV correlates strongly with stellar mass for starbursts, star-forming galaxies and quiescent galaxies. The inclusion of morphological information tightens these correlations: attenuation correlates more strongly with stellar mass and SFR surface densities than with the global quantities. The Balmer decrement-derived nebular attenuation for 67 of these galaxies shows consistent trends with the stellar continuum attenuation. We detect a wavelength-dependent size gradient: massive galaxies ($\rm M_{\star }\gtrsim 10^{10}~M_{\odot }$) appear ∼30% larger in the rest-optical than in the rest-NIR, driven by central dust attenuation that flattens optical light profiles. Lower-mass systems exhibit more diverse size ratios, consistent with either inside-out growth or central starbursts. These results demonstrate that dust attenuation significantly alters observed galaxy structure and highlight the necessity of flexible attenuation models for accurate physical and morphological inference at cosmic noon.

We present the Blue Jay survey, a cycle 1 JWST program aimed at studying the stellar and gas content of galaxies at cosmic noon. The survey consists of deep spectroscopy for 153 targets observed over two pointings in the COSMOS field using the NIRSpec micro-shutter assembly (MSA). We employed the three medium-resolution gratings, G140M, G235M, and G395M, with exposure times of 13 hours, 3.2 hours, and 1.6 hours, respectively. We thus obtained full coverage of the 1–5 µm range, corresponding to the entire rest-frame optical wavelength range, with a spectral resolution R ∼ 1000. Parallel observations provided deep multiband NIRCam imaging, which partially overlaps with the spectroscopic observations. The sample was carefully selected to provide a census of galaxies over the redshift range 1.7 < z < 3.5 above a redshift-dependent minimum stellar mass that ranges from 10 ^8.7 M _⊙ to 10 ^9.3 M _⊙ . The selection ensured that the Blue Jay sample is representative of the entire galaxy population at these redshifts, without strong biases in color, star formation rate, or other properties. The sizes of massive galaxies at these redshifts are comparable to or larger than the NIRSpec shutters, which requires custom strategies for designing and reducing the observations. Since the standard A − B nod subtraction leads to flux self-subtraction for large galaxies, we constructed a master background from empty shutters and subtracted it from each of the science spectra. This in turn allowed us to use shorter slitlets consisting of only two shutters per galaxy instead of the usual three, with a substantial increase in the multiplexing of the NIRSpec MSA. In order to obtain self-consistent fits of spectroscopic and photometric data, we measured multiband photometry using space-based observations from the exact area in the sky where the NIRSpec 1D spectrum is extracted. The spectra contain a wealth of emission and absorption lines, including hydrogen lines (Balmer and Paschen series), helium lines, metal emission lines from ionized gas, metal stellar absorption lines, and resonant absorption lines that can be used to study neutral gas (Na I D and Ca II H, K). Spectroscopic redshifts are unambiguously measured for 93% of the sample. The Blue Jay dataset, which we publicly release, represents the ideal sample for studying the stellar populations, neutral gas, and ionized gas in cosmic noon galaxies, including star formation histories, quenching, outflows, metallicity, and dust attenuation.

We present a spectro-photometric analysis of RUBIES-EGS-9809, a broad-line AGN-host galaxy at z=5.7 with a Balmer break and a dominant point-source component. The source is clearly dust reddened (AV≳3 mag), shows hot- and cold-dust emission, and is a Compton-thick X-ray emitter, with X-ray-to-bolometric ratio consistent with luminous AGN. The large AV agrees with a clear Na I absorption (EW=17±2 Å), possibly from a neutral-gas outflow (3-σ), while broad [O III] traces an ionized outflow, suggesting AGN feedback may be affecting the host. We estimate a stellar mass log(M⋆/M⊙)=10.7 ±0.3, Lbol∼4 × 10⁴⁶ erg s⁻¹, and a virial black-hole mass log(M•/M⊙)∼8.1 (subject to large systematics). The Balmer break is spatially resolved (0.1 arcsec, 0.6 kpc), consistent with a stellar origin, implying the galaxy is already old in stars while still heavily obscured. This shows that massive galaxies can host evolved stellar populations during their compact, dust-obscured phase, consistent with formation in an earlier dusty starburst. SED modelling suggests a declining SFR, but low-resolution spectroscopy alone cannot constrain the recent SFR, so we treat the source as a strong quenching candidate rather than a secure post-starburst system. Because quenching follows the decline in SFR, star-formation-driven outflows are disfavoured, leaving AGN feedback as a plausible driver. We speculate that such rapid feedback (∼100 Myr timescale) may precipitate quenching while the post-quenching phase stays dust-enshrouded, concealing the UV-bright quenched phase that would otherwise be easily detected, if dust free. By the time dust clears, the UV-luminous stars may have already dwindled, leaving a classic dust-free quiescent galaxy. This ‘dusty path’ to quiescence may explain the lack of intermediate-age quenched progenitors (ages 50–100 Myr) linking dusty starbursts to massive quiescent systems at z=3–7.

We update and expand the MESA Isochrones and Stellar Tracks (MIST) database to include variations in the α -capture elements, specifically [ α /Fe] = −0.2, 0, +0.2, +0.4, and +0.6 for the full range of −3 ≤ [Fe/H] ≤ +0.5 already covered by MIST. Variations in [ α /Fe] are included in a self-consistent manner from the stellar interior models to the synthetic spectra used to translate these models in the observational plane. We describe a number of updates to the physics utilized in these models as well as new information provided by the models. We validate the models with comparisons to other stellar evolution models including the previous generation of MIST and other models from the literature. MIST data products including stellar evolution tracks, isochrones, and bolometric correction tables can be obtained from the MIST project website, https://mist.science . All necessary files to reproduce MIST models are available from Zenodo.

Recent JWST observations of massive galaxies at z > 2 detect blueshifted absorption in Na I D and other resonant absorption lines, indicative of strong gas outflows in the neutral phase. However, the measured mass outflow rates are highly uncertain because JWST observations can only probe the column density of trace elements such as sodium, while most of the gas is in the form of hydrogen. The conversion between the column density of sodium and that of hydrogen is based on observations of gas clouds within the Milky Way, and has not been directly tested for massive galaxies at high redshift. To test this conversion, we studied a unique system consisting of a massive quiescent galaxy (J1439B) at z = 2.4189, located at a projected distance of 38 physical kiloparsecs from the bright background quasar QSO J1439+1117. The neutral outflow from the galaxy is observed as a sub-damped Lyman- α absorber (sub-DLA) in the spectrum of the background quasar, which enables a direct measurement of the hydrogen column density from Lyman transitions. We obtained new near-infrared spectroscopy with Magellan/FIRE and detect Na I D and other resonant absorption lines from Mg II, Mg I, and Fe II. This dataset allows us to derive new empirical calibrations between the column densities of trace elements and hydrogen, which can be used to estimate the mass and the rate of neutral gas outflows in other massive quiescent galaxies at high redshift. The calibration we derive for Na I is only 30% lower than the local relation that is typically assumed at high redshift, confirming that the neutral outflows observed with JWST at z > 2 can remove a large amount of gas and likely play a key role in galaxy quenching. However, using the local calibration for Mg II yields an order-of-magnitude discrepancy compared to the empirical calibration, possibly due to variations in dust depletion.

We present a substantial update to the MESA Isochrones and Stellar Tracks (MIST) library, extending the MIST model grids and isochrones down the white dwarf (WD) cooling sequence with realistic physics for WD cooling timescales. This work provides a large grid of MESA models for carbon–oxygen core WDs with hydrogen atmospheres (spectral type DA/DC), descended from full prior stellar evolution calculations. The model tracks, isochrones, and WD cooling timescale contours are available on the MIST project website and at doi: 10.5281/zenodo.15242047 . Our WD models provide a very large, publicly available grid with detailed physics for WD cooling timescales: realistic interior and envelope compositions, with element diffusion and heavy-element sedimentation, nuclear burning at the base of the WD hydrogen envelope, core crystallization, and C/O phase separation. As a large grid of open-source stellar evolution models, these WD models provide both out-of-the-box model tracks for comparison with observations and a framework for building further WD models to investigate variations in WD physics.

Using deep JWST/NIRSpec spectra from the Blue Jay survey, we performed the first systematic investigation of neutral gas content in massive galaxies at Cosmic Noon based on the Ca II H, K absorption lines. We analyzed a sample of nine galaxies at 1.8 < z < 2.8 with log M _* / M _⊙ > 10.6, for which we detected neutral gas absorption both in Ca II and in Na I. After removing the stellar continuum using the best-fit model obtained with Prospector , we fitted the excess absorption due to neutral gas in the Ca II H, K doublet and in the Na I D doublet, together with nearby emission lines produced by ionized gas. We measured covering fractions between 0.2 and 0.9 from the Ca II H and K lines, which are spectrally well resolved in the NIRSpec R ∼ 1000 observations, unlike the absorption lines in the Na I D doublet. We measured the velocity shift, velocity dispersion, and column density separately for Ca II and Na I. About half of the galaxies present blueshifted Ca II, indicative of an outflow of neutral gas and consistent with previous results based on Na I. The velocity shift and the column density measured from Ca II are correlated with those measured from Na I, implying that these absorption lines trace gas in similar physical conditions. However, the column densities are not in a 1:1 relation, meaning that the relative amount of Ca II and Na I atoms along the line of sight varies with the gas column density. After investigating possible reasons for this behavior, we derived an empirical relation between the column density of Ca II and the column density of Na I and, in a more indirect way, of neutral hydrogen H I. With this calibration, we measured the mass outflow rates in Blue Jay galaxies using the Ca II lines, finding values in the range ∼2.7 − 56 M _⊙ /yr, broadly consistent (within a factor of roughly five) with previous studies of the Na I lines. The employment of Ca II lines offers a new way to infer properties of neutral gas from current and future JWST observations of massive galaxies at Cosmic Noon and beyond.

We present new stellar population models, α -MC, self-consistently taking into account nonsolar [ α /Fe] abundances for both isochrones and stellar spectra. The α -MC models are based on α -enhanced MIST isochrones and C3K spectral libraries, which are publicly available in FSPS. Our new models cover a wide range of ages ( log ( age / yr ) = 5.0 - 10.3 ), metallicities ([Fe/H] = [−2.5, +0.5] in steps of 0.25, [ α /Fe] = −0.2, +0.0, +0.2, +0.4, +0.6), and wavelengths (0.1–2.5 μ m). We investigate the separate and combined effects of α -enhanced isochrones and stellar spectral libraries on simple stellar populations, including their broadband colors, spectral indices, and full spectra. We find that the primary effect of α -enhancement in isochrones is to lower the overall continuum levels and redden the continuum shapes, while α -enhancement in stellar spectra mainly affects individual spectral lines. At constant [Fe/H], α -enhancement has significant impacts on the broadband colors by ∼0.1–0.4 mag across all ages (0.01–10 Gyr). The effects of α -enhancement on colors at fixed [Z/H] are smaller, by ∼0.1–0.2 mag. The spectral indices involving α -elements, Ca4227 and Mg b , increase with [ α /Fe] (both at fixed [Fe/H] and fixed [Z/H]) due to enhanced α -abundances. At constant [Fe/H], α -enhancement weakens most Fe-sensitive and hydrogen Balmer lines. Our new self-consistent α -enhanced models will be essential in deriving accurate physical properties of high-redshift galaxies, where α -enhancement is expected to be common.

The complex physics governing nebular emission in galaxies, particularly in the early Universe, often defy simple low-dimensional models. This has proven to be a significant barrier in understanding the (often diverse) ionizing sources powering this emission. We present Cue,^{10 10} Cue v0.1 is available online at GitHub, https://github.com/yi-jia-li/cue with a copy deposited to Zenodo (Y. Li et al. 2024). a highly flexible tool for interpreting nebular emission across a wide range of abundances and ionizing conditions of galaxies at different redshifts. Unlike typical nebular models used to interpret extragalactic nebular emission, our model does not require a specific ionizing spectrum as a source, instead approximating the ionizing spectrum with a four-part piecewise power law. We train a neural net emulator based on the CLOUDY photoionization modeling code and make self-consistent nebular continuum and line emission predictions. Along with the flexible ionizing spectra, we allow freedom in [O/H], [N/O], [C/O], gas density, and total ionizing photon budget. This flexibility allows us to either marginalize over or directly measure the incident ionizing radiation, thereby directly interrogating the source of the ionizing photons in distant galaxies via their nebular emission. Our emulator demonstrates a high accuracy, with ∼1% uncertainty in predicting the nebular continuum and ∼5% uncertainty in the emission lines. Mock tests suggest Cue is well calibrated and produces useful constraints on the ionizing spectra when the signal-to-noise ratio of H _α is ≳10, and furthermore capable of distinguishing between the ionizing spectra predicted by single and binary stellar models. The computational efficiency of neural networks facilitates future applications of Cue for rapid modeling of the nebular emission in large samples and Monte Carlo sampling techniques.

Gas-versus-star counter-rotating galaxies are characterized by the presence of a disk of stars and a disk of gas that are co-spatial but rotating in opposite directions. Using the N EW H ORIZON simulation, we identified and studied ten such galaxies in field environments with a stellar mass of M _* ∼ [1–5] × 10 ¹⁰ M _⊙ . For all of them, the retrograde accretion of gas either from gas stripping from a nearby companion or from the circumgalactic medium is the starting point of the formation process. This is followed by the coexistence of two distinct disks of gas (or components) rotating in opposite directions, with the pre-existing disk in the inner parts of the galaxy and the accreted gas in the outer parts. The latter progressively replaces the former, leading to the final gas-star kinetic misalignment configuration. During the process, star formation is first enhanced and then progressively decreases. We roughly estimate that a higher fraction of the pre-existing gas is converted into stars rather than being expelled. We also found that the black hole (BH) activity tends to be enhanced during the removal of the pre-existing gas. Furthermore, our analysis suggests that the formation of a counter-rotating gas component is always accompanied with the formation of counter-rotating stellar disks. These stellar disks can have diverse properties, but in general, they host a younger and more metal rich population of stars with respect to the main disk, depending on the star formation history and BH activity. The central part of counter-rotating disks also tend to be characterized by a younger population, an enhanced star formation rate, and a higher metallicity than their outer parts. The high metallicity comes from the progressive metal enrichment of the accreted gas through mixing with the pre-existing gas and by supernovae activity as the accreted gas sinks toward the center of the galaxy. In case of major mergers, a large amount of accreted stars from the companion would be distributed at large distances from the remnant center due to conservation of the initial orbital angular momentum. This process might favor the observation of two distinct counter-rotating stellar disks, particularly in observed projected velocity fields from integral field spectroscopy surveys, as well as stellar streams characterized by specific age-metallicity properties.