arXiv subjects

Hyeonguk Bahk

Publications and source records attributed to Hyeonguk Bahk.

8 recordsLinked to original sources

arXiv2026

Ongoing Morphological Transformation from Elliptical to Spiral Galaxies Induced by Gas Accretion from Neighboring Galaxies

We present results from spectroscopic observations of three interacting galaxy systems, each associated with a primary galaxy - VCC 1748, SDSS J1106, and 2MASX J0834 - which have been proposed as potential examples of reverse morphological transformation from elliptical to spiral types driven by external gas accretion. Using Gemini/GMOS longslit spectroscopy, we obtain spectra along longslits connecting the regions of interest between the two galaxies in each pair. We model the spatially binned spectra with the Penalized Pixel-Fitting (pPXF) method to derive velocity fields and radial profiles of gas-phase metallicity, stellar metallicity, and stellar population age. Among the three systems, only the VCC 1748 system provides spectra with sufficiently high signal-to-noise ratios (S/N) to reveal coherent velocity and metallicity structures extending between the companion spiral galaxy and its elliptical target. The velocity profile along the gas-bridge of the system is smooth and continuous from the companion spiral arm to the target galaxy, and the elliptical target, VCC 1748, exhibits younger stellar populations and a high star formation rate (SFR) in its newly formed disk regions compared to its central parts. These findings indicate in-situ star formation at the newly-formed spiral arm of the elliptical target, consistent with ongoing gas inflow from the spiral companion into the elliptical target. Altogether, the results capture an intermediate stage of morphological evolution in which an originally quiescent elliptical galaxy begins to assemble a rotationally supported disk through interaction-driven gas accretion.

astro-ph.GA↗

arXiv2026

Constructing a Mock Galaxy Catalog for the All-sky SPECtroscopic Survey of Nearby Galaxies (A-SPEC) Using the Machine-assisted Semi-Simulation Model

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.

astro-ph.CO↗

arXiv2026

The SPHEREx View of Galaxy Clusters: A Simulation-based Validation of the Forced Photometry Pipeline for Extended Sources

We present a simulation-driven assessment of the performance of the SPHEREx pipeline for galaxy cluster science, focusing on photometry, source blending, survey depth, and photometric redshift accuracy. To do that, we compile a sample of eight galaxy clusters spanning a wide redshift range ($z \approx 0.02$-$1.1$) and develop an end-to-end pipeline. We use the ancillary data from the DESI Legacy Survey and COSMOS survey, and generate realistic mock SPHEREx observations with the SPHEREx Sky Simulator. By performing forced photometry on these images with The Tractor, we quantify the characteristic biases and uncertainties relevant to cluster science. We find that the photometry is generally unbiased, but source blending is the primary driver of catastrophic outliers, particularly when the combined flux of neighbors is comparable to the flux of targets. Measuring the effective survey depth, we find that SPHEREx detects members down to $K_{s}\approx 20$ AB ($5σ$), 7-9 mag fainter than the brightest cluster galaxy (BCG) in nearby clusters but only 1-2 mag for clusters at $z \sim 1$, where the BCG itself has faded close to this depth. Despite these challenges, we demonstrate that SPHEREx can achieve a photometric redshift precision of $σ_{\mathrm{NMAD}}\approx 0.003$-$0.01$ for cluster galaxies with an appropriate sample selection based on brightness or signal-to-noise. Combining the redshifts of quality-selected members, we recover cluster redshifts with a bias of $|Δz|/(1+z) < 0.002$ and a scatter of $σ\approx 0.002$ at $z \lesssim 0.5$, meeting the precision required for cluster cosmology.

astro-ph.GA↗

arXiv2026

Characterization of the commercial spectrograph system for astronomical observations: PIXIS 1300BX Camera and IsoPlane 320A Spectrograph

We present the result from a comprehensive laboratory and on-sky characterization of the commercial spectrograph system consisting of a PIXIS 1300BX charge-coupled device (CCD) camera and an IsoPlane 320A spectrograph as part of the preparation of the forthcoming all-sky spectroscopic survey of nearby galaxies (A-SPEC). In the laboratory, we have quantified readout noise, dark current, gain, and full-well capacity via bias, dark, and photon transfer curve analysis at all acquisition modes. To do that, we have developed a gradient correction technique to address row-dependent signal gradients in the image, which are caused by the shutter-less condition of our CCD camera test setup. The technique successfully reproduces the values in the manufacturer specifications. We also have measured quantum efficiency exceeding 80% from 400--800 nm and $\gtrsim$ 90% between 450--750 nm, with sub-second persistence decay, making it ideal for rapid, multi-object spectroscopy. Using a set of diffraction gratings (150, 300, and 600 gr mm$^{-1}$), we have evaluated the spatial separability of multiple spectra and spectral resolution. We have conducted a test observation with this spectrograph system at the Seoul National University Astronomical Observatory (SAO) 1 m telescope and successfully demonstrated its capability of multi-object spectroscopy with moderate resolution of $R \approx 600 - 2600$. We release all Python codes for the test and recipes to facilitate further instrument evaluations.

astro-ph.IM↗

arXiv2026

A New Collision Avoidance Fiber Assignment Algorithm for Robotic Fiber Positioners in Multi-Object Spectroscopy

We present a new fiber assignment algorithm for a robotic fiber positioner system in multi-object spectroscopy. Modern fiber positioner systems typically have overlapping patrol regions, resulting in the number of observable targets being highly dependent on the fiber assignment scheme. To maximize observable targets without fiber collisions, the algorithm proceeds in three steps. First, it assigns the maximum number of targets for a given field of view without considering any collisions between fiber positioners. Then, the fibers in collision are grouped, and the algorithm finds the optimal solution resolving the collision problem within each group. We compare the results from this new algorithm with those from a simple algorithm that assigns targets in descending order of their rank by considering collisions. As a result, we could increase the overall completeness of target assignments by 10% with this new algorithm in comparison with the case using the simple algorithm in a field with 150 fibers. Our new algorithm is designed for the All-sky SPECtroscopic survey of nearby galaxies (A-SPEC) based on the K-SPEC spectrograph system, but can also be applied to similar fiber-based systems with heavily overlapping fiber positioners.

astro-ph.IM↗

arXiv2025

The Redshifts from 122 Bands: Comparative Redshift Forecast for Low-Resolution Spectra from SPHEREx and 7-Dimensional Sky Survey (7DS)

The recently initiated SPHEREx and 7DS surveys will deliver low-resolution spectra ($R\approx 30-130$) for hundreds of millions of galaxies over the optical to near-infrared range ($0.4-5.0μm$), 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 widely-used photometric redshift estimation methods using simulated SPHEREx and 7DS data. Four template-fitting approaches and two machine-learning (ML) methods are used to derive photometric redshifts from low-resolution spectrophotometric data. We measure redshifts using mock catalogs based on the GAMA and COSMOS galaxy samples and achieve high precision for bright (13 < i < 18) galaxies, with $σ_{NMAD}\lesssim 0.005$, bias $\lesssim 0.005$, and a catastrophic failure rate $\lesssim 0.005$ 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, like 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.

astro-ph.GA↗

arXiv2024

UPCluster-SZ: The Updated Catalog of Galaxy Clusters from the List of Planck Sunyaev-Zeldovich Sources

We present the updated galaxy cluster catalog of the second Planck catalog of Sunyaev-Zeldovich sources (PSZ2) through the compilation of the data for clusters and galaxies with spectroscopically measured redshifts in the literature. The original version of PSZ2 comprises 1653 SZ sources, of which 1203 have been validated as genuine galaxy clusters, while the remaining 450 sources are yet to be validated. To increase the number of genuine clusters in PSZ2, we first update the validations of the cluster candidates and their redshift information using the data compiled for the confirmed clusters and the member galaxies in the literature. We then use the galaxy redshift data in the fields of the remaining cluster candidates, by searching for possible member galaxies with measured spectroscopic redshifts around the Sunyaev-Zeldovich centroids. In this search process, we classify clusters as strong candidates if they contain more than nine galaxies within a 4500 km s$^{-1}$ velocity range and within 15 arcmin around the Sunyaev-Zeldovich centroids. This process results in the validation of 139 new genuine clusters, the update of redshift information on 399 clusters, and the identification of 10 strong candidates, which increases the number of validated clusters up to 1334 among the 1653 SZ sources. Our updated galaxy cluster catalog will be very useful for the studies of galaxy formation and cosmology through the combination with other all-sky surveys including WISE and SPHEREx.

astro-ph.CO↗

arXiv2021

Strong correlation between FeII/MgII ratio and Eddington ratio of type 1 Active galactic nuclei

The FeII/MgII line flux ratio has been used as an indicator of the Fe/Mg abundance ratio in the broad line region (BLR) of active galactic nuclei (AGNs). On the basis of archival rest-frame UV spectra obtained via the Hubble Space Telescope and the Sloan Digital Sky Survey, we investigate the FeII/MgII ratios of type 1 AGNs at z < 2. Over wide dynamic ranges of AGN properties (i.e., black hole mass, AGN luminosity, and Eddington ratio), we confirm that the FeII/MgII ratio strongly correlates with Eddington ratio but not with black hole mass, AGN luminosity, or redshift. Our results suggest that the metallicity in the BLR are physically related to the accretion activity of AGNs, but not to the global properties of galaxies (i.e., galaxy mass and luminosity). With regard to the relation between the BLR metallicity and the accretion rate of AGNs, we discuss that metal cooling may play an important role in enhancing the gas inflow into the central region of host galaxies, resulting in the high accretion rate of AGNs.

astro-ph.GA↗