Ji Hoon Kim

The central density relation (CDR) is one of the fundamental relations that dictate the physical properties of late-type galaxies; The dynamical mass surface density of a disk galaxy scales with its central surface brightness. While it may sound trivial, the dynamical surface density of late-type galaxies dominated by dark matter should not be related to luminous matter. In the standard galaxy formation picture based on LCDM, the distribution of late-type galaxy size at a given mass abides by the two following hypotheses or mixtures of them; SH (Same Halo) in which galaxy sizes depend on the spin of the parent dark matter halos, and DD (Density begets Density) in which the surface brightnesses depend on the density of the dark matter halos. However, these hypotheses cannot reproduce late-type galaxies that follow CDR and the baryonic Tully-Fisher relation at the same time. Using SPARC (Spitzer Photometry and Accurate Rotational Curves) and Cosmicflows-4 data, we are studying the correlations between the angular momentum, the spin parameter, and the central surface brightness of late-type galaxies, while small and large environments of galaxies affect these correlations. While we found that low surface brightness galaxies (LSBGs) have slightly higher specific angular momentum than high surface brightness galaxies (HSBGs), the difference is quite marginal. We also found that LSBGs have higher spin parameters than HSBGs, but have a large scatter. Moreover, we found a severe inconsistency across various proxies for the spin parameters. The effect of the environment is still unclear.

Photometric variability in young stellar objects (YSOs) provides critical insight into the mechanisms of mass accretion, disk evolution, and circumstellar extinction in early stellar evolution. We present an analysis of day-timescale optical variability in the Orion A central region using two-night 7-Dimensional Telescope (7DT) medium-band photometry obtained on 2024 March 23 and 24. The 7DT observations provide optical spectral sampling with 16 medium-band filters spanning 400–825 nm, enabling direct two-epoch comparisons. To remove satellite-trail contamination, we used an SSIM-based ResNet classifier (accuracy = 0.97, F1-score = 0.93) to exclude affected exposures. Subsequent photometry and two-epoch variability measurements yielded a working sample of 769 YSO candidates, among which we identified 110 variables (∼14%), including seven extreme cases with ∣Δ m _λ ∣ > 0.5 mag. To describe the wavelength dependence of the variability, we compared five simple templates: extinction-like changes ( R _V = 3.1 and 5.5), a gray (wavelength-independent) change, and two spot-like toy models (hot and cold) implemented as two-temperature surface mixtures. The best-fit results are dominated by spot-like templates (37 cold-spot and 22 hot-spot objects), with 37 sources best matched by extinction-like templates and 14 by the gray template. The m650 filter excess fraction is higher in the hot-spot and gray templates than in the others. This could be compatible with more frequent line-veiling-related contributions in those groups, although the m650 filter excess is not a direct accretion diagnostic.

We present a new constraint on the primordial helium abundance, Y _P , based on Subaru observations. A major source of uncertainty in previous Y _P determinations is the lack of extremely metal-poor galaxies (EMPGs; 0.01–0.1 Z _⊙ ), which have metallicities a few to ten times lower than the metal-poor galaxies (MPGs; 0.1–0.4 Z _⊙ ) predominantly used in earlier studies, requiring substantial extrapolation to zero metallicity. Here, we perform Subaru near-infrared spectroscopy of 29 galaxies, including 14 EMPGs. By incorporating existing optical spectra, we derive He/H for each galaxy using photoionization modeling of helium and hydrogen emission lines, including the He i 10830 Å line to break the density–temperature degeneracy. After carefully selecting galaxies with robust He/H determinations, and adding 58 galaxies from previous studies, we obtain Y P = 0.240 2 − 0.0040 + 0.0040 . This Y _P value is ∼1 σ lower than most of the previous estimates, but agrees with recent determinations using EMPGs and the cosmic microwave background constraint from the Atacama Cosmology Telescope experiment. Our result indicates N eff = 2.5 4 − 0.25 + 0.20 , showing a mild (∼2 σ ) tension with the Standard Model and Planck results. These tensions may suggest a nonzero lepton asymmetry ( ξ _e ≠ 0), which would alleviate the tension with ξ e = 0.0 5 − 0.03 + 0.02 . More observations of EMPGs and further assessments of systematic uncertainties are essential to test the potential tension more rigorously.

We develop a hybrid framework to identify kilonovae (KNe), using single-epoch, medium-band spectral energy distributions from the 7-Dimensional Telescope (7DT). The framework integrates an unsupervised anomaly classifier ( Isolation Forest ) to flag unusual events with a supervised multiclass classifier ( XGBoost ) that characterizes eight common transient types. Trained on realistically simulated 7DT photometry accounting for per-filter sensitivity, the classifier achieves macro F ₁ ∼ 0.80 (∼0.82) with 20 (40) filters across eight classes, Type Ia/Ibc/II SNe, SLSNe, TDEs, AGN, stellar variables, and asteroids. Without direct training, the anomaly detector recovers >90% of simulated and observed optically detectable KNe (AT 2017gfo) with a low contamination fraction, with a caveat of limitations of the training sample such as limited redshift range of SNe ( z < 0.15), and a relatively small number of early non-KNe spectra. A SHAP-based feature analysis reveals that only ∼40%–50% of the most informative filters are sufficient to retain near-baseline performance, while red-end filters contribute little. Combining the top-ranked half of the 40 7DT filters with a single LSST band reproduces the full-model accuracy within 1%–2%, suggesting practical follow-up strategies. These results demonstrate that 7DT’s medium-band system enables rapid, interpretable classifications and reliable anomaly alerts from single-epoch data—promising for gravitational-wave follow-up, Rubin alert stream filtering, and serendipitous transient discovery in the 7DT survey.

We present time series medium-band ( R ∼ 20–40) observations of the third interstellar object (ISO) 3I/ATLAS (C/2025 N1) obtained with the 7-Dimensional Telescope, which enabled spatially resolved monitoring of its gas and dust activity from 2025 July to September. The m400-band image ( λ _c = 400 nm, Δ λ ≈ 25 nm) reveals an emergence of a pronounced and spatially extended CN emission at a heliocentric distance of r _h < 3 au. This onset is consistently identified across multiple diagnostics, including a break in the light-curve evolution, excess reflectance, inward expansion of annular excess beyond 10,000–20,000 km, growth of the coma half-light radius from ∼11,000 to ∼19,000 km, and a rapid rise in the CN production rate Q CN relative to Afρ . We further separate the CN-emitting and dust-scattered components via two-dimensional surface brightness fitting into inner (dust) and outer (gas) components. We find that the outer component preserves a nearly constant profile shape, varying only in normalization, implying a relatively fast expansion of CN-bearing molecules. Together, these results reveal how a transition occurs in the optical from dust-dominated scattering at large heliocentric distances to volatile-driven, gas-dominated activity as 3I/ATLAS enters the inner solar system. The timing and characteristics of the CN activation resemble the volatile enhancement observed in 2I/Borisov, suggesting that both known active ISOs exhibit comparable activation behavior at heliocentric distances of ∼2–3 au.

The recently initiated SPHEREx and 7DS surveys will deliver low-resolution spectra ( R ∼ 20 − 130) for hundreds of millions of galaxies over the optical to near-infrared range (0.4 − 5.0 μm), covering a wide sky area without sample selection. These unique datasets will improve redshift estimation and provide a rich redshift catalog for the community. In this study, we forecast the performance of photometric redshift estimations using simulated SPHEREx and 7DS data. Four widely used template-fitting approaches and two machine-learning (ML) methods are used to derive photometric redshifts from low-resolution spectrophotometric data. We measured redshifts using mock catalogs based on the GAMA and COSMOS galaxy samples and achieved high precision for bright (13 < i < 18) galaxies, with σ _NMAD ≲ 0.005, bias ≲0.005, and a catastrophic failure rate ≲0.005 for all methods employed. We find that the combined SPHEREx + 7DS dataset significantly improves redshift estimation compared to using either the SPHEREx or 7DS datasets alone, highlighting the synergy between the two surveys. Moreover, we compare the redshift estimation performance across magnitude ranges for the different methods and examine the probability distribution functions (PDFs) produced by the template-fitting approaches. As a result, we identify some factors that can affect the redshift measurements, for example, treatments on dust extinction or inclusion of flux uncertainty in the ML model. We also show that the PDFs are relatively well calibrated, although the confidence intervals are generally underestimated, particularly for bright galaxies in the template-fitting methods. This study demonstrates the strong potential of SPHEREx and 7DS to deliver improved redshift measurements from low-resolution spectrophotometric data, underscoring the scientific value of jointly utilizing both datasets.

We report the results of a pilot study that searched for dual active galactic nuclei (AGN) in local ( z < 0.25) galaxies hosting double-peaked narrow emission lines in their optical spectra. We present high-resolution L -band (1.5 GHz or 18 cm) continuum images from the Very Long Baseline Array as well as WFC3/IR F160W images from the Hubble Space Telescope of two candidate dual AGN systems: J0948+6848 and J1223+5409. In both targets, we detected compact nonthermal radio emission that is approximately cospatial with the near-infrared AGN. Both systems host two high brightness temperature (>10 ⁸ K) radio sources that indicate the presence of either a parsec-scale-separation dual AGN ( d _sep ∼ 90 and ∼56 pc, respectively) or a radio jet. Matched-resolution multiband radio observations are necessary to further characterize the AGN activity in these systems.

We present time-series medium-band (R~20-40) observations of the third interstellar object 3I/ATLAS (C/2025 N1) obtained with the 7-Dimensional Telescope (7DT), enabling spatially resolved monitoring of its gas and dust activity from 2025 July to September. The m400-band image (lambda_c = 400 nm, Delta lambda approx 25 nm) reveals the emergence of pronounced and spatially extended CN emission at heliocentric distances r_h < 3 au. This onset is consistently identified across multiple diagnostics, including a break in the light-curve evolution, excess reflectance, inward expansion of annular excess beyond 10,000-20,000 km, growth of the coma half-light radius from ~11,000 to ~19,000 km, and a rapid rise in the CN production rate Q_CN relative to Af rho. We further separate the CN-emitting and dust-scattered components through two-dimensional surface-brightness fitting into inner (dust) and outer (gas) components. The outer component preserves a nearly constant profile shape, varying only in normalization, implying relatively fast expansion of CN-bearing molecules. Together, these results reveal a transition in the optical from dust-dominated scattering at large heliocentric distances to volatile-driven, gas-dominated activity as 3I/ATLAS enters the inner Solar System. The timing and characteristics of the CN activation resemble the volatile enhancement observed in 2I/Borisov, suggesting that both known active interstellar objects exhibit comparable activation behavior at heliocentric distances of ~2-3 au.

We investigate the expected accuracy of redshifts that can be obtained using low-resolution spectroscopic (medium-band) data from the 7-Dimensional Sky Survey (7DS). By leveraging 40 densely sampled filters with widths of FWHM = 25 nm, we create 7DS mock catalogs and estimate the redshift accuracy for three 7DS main surveys: the Wide-field Time-Domain Survey (WTS), the Intensive Monitoring Survey, and the Reference Imaging Survey. Using photometric redshifts calculated from EAZY , we find that the 5 yr WTS provides reliable photometric redshifts with a normalized median absolute deviation ( σ _NMAD ) ranging from 0.003 to 0.007 and a catastrophic failure fraction ( η ) from 0.8% to 8.1% at 19 ≤ m ₆₂₅ < 22. The spectral resolution R ∼ 50 of the medium-band data set effectively captures the 4000 Å break and various emission lines. We also explore the synergy with data obtained from the Pan-STARRS1, VIKING, and SPHEREx surveys. Combining the SPHEREx all-sky data with WTS significantly improves the accuracy of photometric redshift estimates, achieving η = 0.4% and σ _NMAD = 0.004 for fainter sources at higher redshifts. The additional near-IR information provided by SPHEREx and VIKING plays an essential role in resolving degeneracies between low and high redshifts. We also observe color excesses by subtracting adjacent broadband data, which improves the confinement of photometric redshifts and aids in the detection of strong emission line galaxies.