Jaehyun Lee

Context. Submillimeter galaxies (SMGs) are among the most intriguing and luminous objects in the high-redshift universe. These highly dusty star-forming systems provide valuable insights into galaxy formation and evolution and are likely affected by their cosmic environment. However, it is challenging to obtain large-area surveys with resolved SMGs and their spectroscopic redshifts. Aims. We quantify the environmental dependence of SMGs from z = 4 to z = 1 by assessing their preferential association with cosmic web structures (cluster-halo, filaments, and voids/walls) and evolution using the FLAMINGO simulation. Methods. We used the cosmic web finder DisPerSE to identify filamentary structures at redshifts z = 4, 3, 2, 1.5, and 1 in the highest-resolution FLAMINGO simulation using galaxies with M _∗ ≥ 10 ⁹ M _⊙ as tracers of large-scale structure. We defined five cosmic environments (inner cluster-halo, outer cluster-halo, inner filament, outer filament, and void/wall) at each redshift considering mass evolution of cluster-halos and density evolution of filaments. Results. The fraction of SMGs in the inner cluster-halo environment declines from ∼30% at z = 4 to ∼3% by z = 1, and similar trends are observed in other environments. The abundance of SMGs within a cluster-halo increases with halo mass and mirrors the increase in the total galaxy population. Consequently, the ratio of the SMG halo occupation to that of all galaxies is largely insensitive to halo mass, but varies with redshift. In contrast, the ratio of the halo occupation of non-SMGs to that of all galaxies declines with halo mass and shows little redshift evolution because star formation is suppressed in massive halos. We further show that SMGs tend to contribute less to the massive end of the stellar mass function in low-density environments, with central and satellite SMGs forming two distinct populations in the inner cluster-halos. SMGs occupy the metal-rich side of the star-forming gas metallicity distribution, but rarely attain the very highest metallicities because ongoing enrichment is limited by gas depletion. Interestingly, the brightest SMGs (S ₈₅₀ > 10 mJy) are found exclusively in inner cluster-halos, highlighting a strong connection between SMG luminosity and environmental density. Our results show that SMGs dominate star formation in dense environments, contributing up to 80% of the star formation rate in inner cluster-halos at z = 4, but less than 50% in low-density regions. Taken together, these results demonstrate that SMGs dominate the buildup of stellar mass and metals in dense environments, while their relative importance declines toward lower-redshift and lower-density regions.

We analyze the rest-frame optical (∼8000 Å) morphologies and star formation activity of Ly α emitters (LAEs) at redshifts 2.4, 3.1, and 4.5, identified in the One-hundred-deg ² DECam Imaging in Narrowband (ODIN) survey. To compare their physical properties with those of other galaxies, we construct a comparison sample of typical star-forming galaxies (SFGs) at similar redshifts from the COSMOS2025 catalog. Using James Webb Space Telescope NIRCam images from the COSMOS-Web survey, we measure the rest-frame optical sizes and Sérsic indices. We first examine their size–mass relations and find that LAEs at all three redshifts have smaller sizes than typical SFGs, with the size difference decreasing at higher redshifts. We also find that LAEs tend to have larger Sérsic indices at z = 2.4 and 3.1 than typical SFGs, but the difference becomes weaker at z = 4.5. These trends are qualitatively reproduced in the Horizon Run 5 cosmological hydrodynamical simulation. We then investigate star formation activity and find that LAEs exhibit higher star formation rates (SFRs) than typical SFGs at all redshifts considered. Finally, we examine the connection between Ly α emission and galaxy structure, finding that the rest-frame equivalent width (REW) of the Ly α emission line is negatively and positively correlated with size and Sérsic index, respectively. In addition, we find a strong positive correlation between Ly α REW and the ratio of the instantaneous SFR to that averaged over the last 100 Myr (i.e., SFR _inst /SFR _100Myr ). These results suggest the compact and starbursting nature of LAEs, and provide important constraints on the physical mechanisms responsible for Ly α photon escape from galaxies.

We present a detailed study of the 3D morphology of two extended associations of multiple protoclusters at z = 3.1. These protocluster “complexes,” designated COSMOS-z3.1-A and COSMOS-z3.1-C, are the most prominent overdensities of z = 3.1 Ly α emitters (LAEs) identified in the COSMOS field by the One-hundred-deg ² DECam Imaging in Narrowbands survey. These protocluster complexes have been followed up with extensive spectroscopy from Keck, Gemini, and DESI. Using a probabilistic method that combines photometrically selected and spectroscopically confirmed LAEs, we reconstruct the 3D structure of these complexes on scales of ≈50 cMpc. We validate our reconstruction method using the IllustrisTNG300-1 cosmological hydrodynamical simulation and show that it consistently outperforms approaches relying solely on spectroscopic data. The resulting 3D maps reveal that both complexes are irregular and elongated along a single axis, emphasizing the impact of sightline on our perception of structure morphology. The complexes consist of multiple density peaks, 10 in COSMOS-z3.1-A and 4 in COSMOS-z3.1-C. The former is confirmed to be a proto-supercluster, similar to Hyperion at z = 2.4 but observed at an even earlier epoch. Multiple “tails” connected to the cores of the density peaks are seen, likely representing cosmic filaments feeding into these extremely overdense regions. The 3D reconstructions further provide strong evidence that Ly α blobs preferentially reside in the outskirts of the highest density regions. Descendant mass estimates of the density peaks suggest that COSMOS-z3.1-A and COSMOS-z3.1-C will evolve to become ultramassive structures by z = 0, with total masses log ( M / M ⊙ ) ≳ 15.3 , exceeding that of Coma.

In this pilot study, we investigate whether intracluster light (ICL) can serve as an observational discriminator of dark matter physics. The self-interacting dark matter (SIDM) model has gained increasing attention as a possible resolution to small-scale discrepancies between collisionless cold dark matter (CDM) simulations and observations, predicting distinct tidal interaction histories within galaxy clusters. We analyze Cluster-EAGLE zoom-in galaxy clusters resimulated from identical initial conditions in both CDM and SIDM frameworks. The morphological similarity between dark matter and multiple baryonic tracers (gas, all stars, galaxies, and the combined brightest cluster galaxy plus ICL, BCG+ICL) is quantified using the weighted overlap coefficient, a contour-overlap statistic. We find that dark matter is traced most accurately by BCG+ICL, followed by gas, all stars, and galaxies. Although the differences are modest, they are systematic: The collisionless tracers (all stars, galaxies, and BCG+ICL) show better correspondence with the dark matter distribution in CDM, while the relative advantage of BCG+ICL over gas is reduced in SIDM. This reduction is driven primarily by weaker BCG+ICL–dark matter correspondence in SIDM. Qualitatively, this behavior is consistent with the expectation that collisionless stellar components more naturally trace collisionless dark matter in CDM, whereas self-interactions in SIDM introduce an effective viscosity that weakens the dominance of BCG+ICL as a dark matter tracer relative to gas. Our results demonstrate the potential of ICL as a novel observational probe of dark matter physics and provide a first step toward using diffuse cluster light to constrain the nature of dark matter.

Protoclusters represent sites of accelerated galaxy formation and extreme astrophysical activity characteristic of dense environments. Identifying massive protoclusters and mapping their spatial structures are therefore crucial for understanding how large-scale environment influences galaxy evolution. We combine wide-field Ly α imaging from the One-hundred-deg ² DECam Imaging in Narrowbands survey with extensive Dark Energy Spectroscopic Instrument and ancillary spectroscopy across the extended COSMOS and XMM Large Scale Structure (LSS) fields (≈14 deg ² ) to search for massive protoclusters. We confirm six systems at z ≈ 2.4 and ≈ 3.1, including three newly identified structures and three which overlap with previously known structures and/or systems detected using other tracers. We reconstruct their three-dimensional structures, estimate descendant halo masses, and for one structure at z ≈ 3.12, demonstrate that overlapping narrowband filters (NB497 and N501) provide accurate redshift tomography for emission-line galaxies. One protocluster at z ≈ 2.45 overlaps with one of the LATIS tomographic fields, enabling direct comparison between galaxy and H i overdensities traced by Ly α forest absorption. Another at z ≈ 3.12 hosts a massive quiescent galaxy ( M _* ≈ 1.2 × 10 ¹¹ M _⊙ ), suggesting that overdense environments may play a role in accelerating galaxy assembly and quenching. Comparing Ly α emission properties across environments, we find that protocluster galaxies exhibit higher median line fluxes and a deficit of faint emitters relative to the field. The effect is strongest when combining 2D and 3D density information, indicating that galaxies in the densest protocluster cores are most affected by environmental processes. This effect is stronger at z ≈ 3.1 than at z ≈ 2.4, suggesting possible redshift evolution.

We present the first observational hints of the severe removal of both molecular and H i gas from the dwarf galaxy VCC 1249. This extreme stripping event is thought to be driven by the combined effects of tidal interaction and ram pressure. Using deep CO (2−1) observations from the James Clerk Maxwell Telescope, we obtained marginal CO detections in three regions within the stripped H i tail, with molecular masses of ∼10 ⁵ –10 ⁶ M _⊙ , comparable to typical masses of giant molecular clouds. In contrast, we did not find CO emission within the stellar disk of VCC 1249. This indicates the severe removal of cold gas, which likely caused the sudden cessation of star formation in the galaxy. This identifies VCC 1249 as a unique laboratory for witnessing the rapid, environmentally driven quenching of a dwarf galaxy. Our findings provide a critical observational link between gas removal mechanisms and the dramatic phase transition of cluster dwarfs from star-forming to quiescent systems.

We present a methodology for constructing a mock galaxy catalog for the All-sky SPECtroscopic survey of nearby galaxies (A-SPEC) using the Machine-assisted Semi-Simulation Model. The model is trained on the cosmological magnetohydrodynamical simulation IllustrisTNG to predict baryonic properties of subhalos from dark-matter-only features and is applied to our own N -body simulation tailored to satisfy the requirements of A-SPEC. We have improved the model's accuracy by introducing additional features such as subhalo anisotropy parameters and modified definitions of the subhalo environment, which result in the coefficient of determination R ² = 0.96, 0.90, 0.70, 0.79 for stellar mass, gas mass, star formation rate, and gas metallicity, respectively. The resulting mock galaxies reproduce the luminosity-dependent clustering of the target galaxies when tuned to match the number density. We discuss avenues for further improvement, including the role of environment in the predictions. We release the mock galaxy catalog with the baryonic properties predicted from the model.

Spatially extended Ly α nebulae, known as Ly α blobs (LABs), are a rare population at z > 2 that are thought to trace protogroups or the progenitors of massive galaxies in the present-day Universe. However, their dark matter halo properties (e.g., halo mass) are still uncertain, due to their rarity and strong field-to-field variation. The One-hundred-deg ² DECam Imaging in Narrowbands (ODIN) survey has discovered 103 and 112 LABs in the extended (∼9 deg ² ) COSMOS field at z ∼ 2.4 and 3.1, respectively, enabling the estimation of their bias and host halo masses through clustering analysis. We measure the angular autocorrelation functions (ACFs) of the LABs and derive galaxy bias factors of b = 4.0 ± 0.8 and 3.8 ± 0.7, corresponding to minimum halo masses of 2 . 8 − 1.8 + 3.0 and 0 . 7 − 0.5 + 0.8 × 1 0 12 M _⊙ and median halo masses of 4 . 2 − 2.5 + 3.8 and 1 . 1 − 0.7 + 1.1 × 1 0 12 M _⊙ at z ∼ 2.4 and 3.1, respectively. LABs occupy ∼11 − 8 + 39 % and ∼3 − 2 + 9 % of all dark matter halos above these minimum halo masses. These findings suggest that LABs inhabit massive dark matter halos, likely tracing protogroup environments that evolve into present-day massive halos (∼10 ¹³ M _⊙ ), where massive elliptical galaxies or galaxy groups reside, by z = 0.

We investigated Lyman-continuum (LyC) emission from Lyman-alpha emitters (LAEs) at z = 4.5, identified in the One-hundred-deg ² DECam Imaging in Narrowbands (ODIN) survey. Of the 7498 LAEs (4101 in COSMOS and 3397 in XMM-LSS), we excluded LAEs that are either likely low-z objects or contaminated by neighboring sources. Additional background modeling process with thorough quality assessments leaves a final sample of 851 galaxies. We then performed forced photometry on u / u *-band images from the CFHT large area u -band deep survey to measure their LyC fluxes. This represents the largest sample of z = 4.5 LAEs searched for such a purpose. Within this sample, we identified 12 “gold” and 39 “silver” LyC-emitting candidates, with LyC fluxes detected of >3 σ and between 2 σ and 3 σ , respectively, in the range of 5.16–55.29 nJy. No LyC signal is detected in the weighted mean stack of the final sample (0.20 ± 0.37 nJy). Given the UVC magnitudes of LAEs in our sample, the expected LyC emission is likely below the detection limit even when stacking the full sample of ODIN LAEs. Nevertheless, having a large sample of LAEs remains valuable for identifying individual LyC leaker candidates. Among the gold and silver candidates, the LyC flux appears to correlate positively with UVC flux and negatively with Ly α equivalent width, although the correlations are weak. A larger sample of LyC leakers will allow a more robust confirmation of these trends and provide better insights into their physical origins.

Ly α blobs (LABs) are large, spatially extended Ly α -emitting objects whose nature remains unclear. Their statistical properties, such as number densities and luminosity functions, are still uncertain because of small sample sizes and large cosmic variance. The One-hundred-deg ² DECam Imaging in Narrowbands (ODIN) survey, with its large volume, offers an opportunity to overcome these limitations. We describe our LAB selection method and present 112 new LABs in the 9 deg ² E-COSMOS field. We begin with the conventional LAB selection approach, crossmatching Ly α -emitting galaxies with extended Ly α sources, yielding 89 LAB candidates. To obtain a more complete LAB sample, we introduce a new selection pipeline that models all galaxies detected in deep broadband imaging, subtracts them from the narrowband image, and then directly detects extended Ly α emission. This method successfully identifies 23 additional low-SB LABs, which could otherwise be missed by the conventional method. The number density of ODIN LABs near an ODIN protocluster ( n = 7.5 × 10 ⁻⁵ cMpc ⁻³ ) is comparable to that found in the SSA22 protocluster and is 4 times higher than the average across the field. The cumulative Ly α luminosity function within the protocluster regions is similar to that measured for the LABs in the SSA22 protocluster, suggesting a large excess of luminous LABs relative to the average field. These findings suggest the Ly α luminosities and number densities of LABs are environment-dependent. ODIN will provide an expansive LAB and protocluster samples across six additional fields and two more redshifts, allowing us to investigate the nature of LABs in relation to their environments.