arXiv subjects

S. Oh

Publications and source records attributed to S. Oh.

At least 19 recordsLinked to original sources

arXiv2026

MRI-guided cardiac radiotherapy using a 1.5 T MR-linac: surveying emerging patterns of care

Background: Cardiac tumours are rare and treated by surgical resections, which are complex, invasive and carry procedural risks. MRI-guided radiotherapy (MRgRT) noninvasively facilitates conformal dose deliveries using soft-tissue imaging and adaptive radiotherapy techniques. Aim: To assess the application of MRgRT for cardiac tumours, we conducted a patterns-of-care analysis with users of the 1.5T MR-linac. Materials & methods: A survey was distributed to users of the 1.5T MR-linac that treated patients with cardiac tumours. The survey included 30 questions concerning the patient cohort, imaging, treatment planning/simulation, radiotherapy treatment and treatment outcome. Results: Users from six international institutes completed the survey reporting twelve cardiac MRgRT treatments between 2021-2024. The median age[range] of the patients was 59[16-81] years with 50% of the cases concerning the treatment of primary tumours. The prescribed dose ranged between 30-60 Gy, with 30 Gy being prescribed the most (59%). In 75% of the cases, the treatment plans were delivered in five fractions with 6-8 Gy per fraction. Daily images were acquired with T1- and T2-weighted MRI and respiratory motion monitoring was performed in 92% of the cases using cine imaging. A single case was treated with intrafraction motion management using a novel vendor-provided gating solution on the MR-linac. Treatment outcomes were reported for 50% of the cases. All, but one case, attained local control using MRgRT without serious adverse events (grade$\geq$3). Conclusion: This study provides real-world insights into the feasibility and early outcomes to aid the development of cardiac MRgRT treatment recommendations and protocol harmonization.

physics.med-ph↗

arXiv2025

Hector Galaxy Survey: Data Processing, Quality Control and Early Science

The Hector Galaxy Survey is a new optical integral field spectroscopy (IFS) survey currently using the AAT to observe up to 15,000 galaxies at low redshift ($z < 0.1$). The Hector instrument employs 21 optical fibre bundles feeding into two double-beam spectrographs to enable wide-field multi-object IFS observations of galaxies. To efficiently process the survey data, we adopt the data reduction pipeline developed for the SAMI Galaxy Survey, with significant updates to accommodate Hector's dual-spectrograph system. These enhancements address key differences in spectral resolution and other instrumental characteristics relative to SAMI, and are specifically optimised for Hector's unique configuration. We introduce a two-dimensional arc fitting approach that reduces the RMS velocity scatter by a factor of 1.2--3.4 compared to fitting arc lines independently for each fibre. The pipeline also incorporates detailed modelling of chromatic optical distortion in the wide-field corrector, to account for wavelength-dependent spatial shifts across the focal plane. We assess data quality through a series of validation tests, including wavelength solution accuracy, spectral resolution, throughput characterisation, astrometric precision, sky subtraction residuals, and flux calibration stability (4\% systematic offset when compared to Legacy Survey fluxes). We demonstrate that Hector delivers high-fidelity, science-ready datasets, supporting robust measurements of galaxy kinematics, stellar populations, and emission-line properties, and provide examples. Additionally, we address systematic uncertainties identified during the data processing and propose future improvements to enhance the precision and reliability of upcoming data releases. This work establishes a robust data reduction framework for Hector, delivering high-quality data products that support a broad range of extragalactic studies.

astro-ph.GA↗

arXiv2023

Measurement of the transverse single-spin asymmetry for forward neutron production in a wide $p_T$ range in polarized $p+p$ collisions at $\sqrt{s} = 510$ GeV

Transverse single-spin asymmetries $A_{\textrm{N}}$ of forward neutrons at pseudorapidities larger than 6 had only been studied in the transverse momentum range of $p_{\textrm{T}} < 0.4$ GeV/$c$. The RHICf Collaboration has extended the previous measurements up to 1.0 GeV/$c$ in polarized $p+p$ collisions at $\sqrt{s}~=~510$GeV, using an electromagnetic calorimeter installed in the zero-degree area of the STAR detector at the Relativistic Heavy Ion Collider. The resulting $A_{\textrm{N}}$s increase in magnitude with $p_{\textrm{T}}$ in the high longitudinal momentum fraction $x_{\textrm{F}}$ range, but reach a plateau at lower $p_{\textrm{T}}$ for lower $x_{\textrm{F}}$. For low transverse momenta the $A_{\textrm{N}}$s show little $x_{\textrm{F}}$ dependence and level off from intermediate values. For higher transverse momenta the $A_{\textrm{N}}$s show also an indication to reach a plateau at increased magnitudes. The results are consistent with previous measurements at lower collision energies, suggesting no $\sqrt{s}$ dependence of the neutron asymmetries. A theoretical model based on the interference of $\pi$ and $a_1$ exchange between two protons could partially reproduce the current results, however an additional mechanism is necessary to describe the neutron $A_{\textrm{N}}$s over the whole kinematic region measured.

nucl-ex↗

arXiv2023

WALLABY Pilot Survey: the Potential Polar Ring Galaxies NGC~4632 and NGC~6156

We report on the discovery of two potential polar ring galaxies (PRGs) in the WALLABY Pilot Data Release 1 (PDR1). These untargetted detections, cross-matched to NGC 4632 and NGC 6156, are some of the first galaxies where the Hi observations show two distinct components. We used the iDaVIE virtual reality software to separate the anomalous gas from the galactic gas and find that the anomalous gas comprises ~ 50% of the total H i content of both systems. We have generated plausible 3D kinematic models for each galaxy assuming that the rings are circular and inclined at 90 degrees to the galaxy bodies. These models show that the data are consistent with PRGs, but do not definitively prove that the galaxies are PRGs. By projecting these models at different combinations of main disk inclinations, ring orientations, and angular resolutions in mock datacubes, we have further investigated the detectability of similar PRGs in WALLABY. Assuming that these galaxies are indeed PRGs, the detectability fraction, combined with the size distribution of WALLABY PDR1 galaxies, implies an incidence rate of ~ 1% - 3%. If this rate holds true, the WALLABY survey will detect hundreds of new polar ring galaxies.

astro-ph.GA↗

arXiv2023

Estimating the selection function of Gaia DR3 sub-samples

Understanding which sources are present in an astronomical catalogue and which are not is crucial for the accurate interpretation of astronomical data. In particular, for the multidimensional Gaia data, filters and cuts on different parameters or measurements introduces a selection function that may unintentionally alter scientific conclusions in subtle ways. We aim to develop a methodology to estimate the selection function for different sub-samples of stars in the Gaia catalogue. Comparing the number of stars in a given sub-sample to those in the overall Gaia catalogue, provides an estimate of the sub-sample membership probability, as a function of sky position, magnitude and colour. This estimate must differentiate the stochastic absence of sub-sample stars from selection effects. When multiplied with the overall Gaia catalogue selection function this provides the total selection function of the sub-sample. We present the method by estimating the selection function of the sources in Gaia DR3 with heliocentric radial velocity measurements. We also compute the selection function for the stars in the Gaia-Sausage/Enceladus sample, confirming that the apparent asymmetry of its debris across the sky is merely caused by selection effects. The developed method estimates the selection function of the stars present in a sub-sample of Gaia data, given that the sub-sample is completely contained in the Gaia parent catalogue (for which the selection function is known). This tool is made available in a GaiaUnlimited Python package.

astro-ph.GA↗

arXiv2022

WALLABY Pilot Survey: Public release of HI kinematic models for more than 100 galaxies from phase 1 of ASKAP pilot observations

We present the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY) Pilot Phase I HI kinematic models. This first data release consists of HI observations of three fields in the direction of the Hydra and Norma clusters, and the NGC 4636 galaxy group. In this paper, we describe how we generate and publicly release flat-disk tilted-ring kinematic models for 109/592 unique HI detections in these fields. The modelling method adopted here - which we call the WALLABY Kinematic Analysis Proto-Pipeline (WKAPP) and for which the corresponding scripts are also publicly available - consists of combining results from the homogeneous application of the FAT and 3DBAROLO algorithms to the subset of 209 detections with sufficient resolution and S/N in order to generate optimized model parameters and uncertainties. The 109 models presented here tend to be gas rich detections resolved by at least 3-4 synthesized beams across their major axes, but there is no obvious environmental bias in the modelling. The data release described here is the first step towards the derivation of similar products for thousands of spatially-resolved WALLABY detections via a dedicated kinematic pipeline. Such a large publicly available and homogeneously analyzed dataset will be a powerful legacy product that that will enable a wide range of scientific studies.

astro-ph.GA↗

arXiv2022

Measurement of forward photon production cross-section in pp collisions at $\sqrt{s}$ = 510 GeV with RHICf detector

This study reported the inclusive differential production cross-section of photons in six pseudorapidity regions: 6.1 < $\eta$ < 6.5, 6.5 < $\eta$ < 7.0, 7.0 < $\eta$ <7.5, 7.5 < $\eta$ <8.0, 8.0 < $\eta$ < 8.5, and $\eta$ > 8.5, measured through the RHICf experiment with pp collisions at $\sqrt{s}$ = 510 GeV conducted in June 2017. In addition, the cross-sections in the three regions of the $x_F$-$p_T$ phase space coverage that are same as those of the LHCf results at $\sqrt{s}$ = 7 and 13 TeV were obtained and compared. Considering the uncertainties, the results were observed to be consistent with both the Feynman scaling law and the model predictions of EPOS-LHC, QGSJET-II-04, Sibyll 2.3d, and DPMjet-III 2019.1, although certain models exhibited weak collision energy dependencies.

hep-ex↗

arXiv2020

The colors of bulges and disks in the core and outskirts of galaxy clusters

The role of the environment on the formation of S0 galaxies is still not well understood, specifically in the outskirts of galaxy clusters. We study eight low-redshift clusters, analyzing galaxy members up to cluster-centric distances $\sim2.5\,R_{200}$. We perform 2D photometric bulge-disk decomposition in the $g$-, $r$- and $i$-bands from which we identify 469 double-component galaxies. We analyze separately the colors of the bulges and the disks and their dependence on the projected cluster-centric distance and on the local galaxy density. For our sample of cluster S0 galaxies, we find that bulges are redder than their surrounding disks, show a significant color-magnitude trend, and have colors that do not correlate with environment metrics. On the other hand, the disks associated with our cluster S0s become significantly bluer with increasing cluster-centric radius, but show no evidence for a color-magnitude relation. The disk color-radius relation is mainly driven by galaxies in the cluster core at $0\leq R/ R_{200}<0.5$. No significant difference is found for the disk colors of backsplash and infalling galaxies in the projected phase space. Beyond $R_{200}$, the disk colors do not change with the local galaxy density, indicating that the colors of double-component galaxies are not affected by pre-processing. A significant color-density relation is observed for single-component disk-dominated galaxies beyond $R_{200}$. We conclude that the formation of cluster S0 galaxies is primarily driven by cluster core processes acting on the disks, while evidence of pre-processing is found for single-component disk-dominated galaxies. We publicly release the data from the bulge-disk decomposition.

astro-ph.GA↗

arXiv2020

The MAGPI Survey -- science goals, design, observing strategy, early results and theoretical framework

We present an overview of the Middle Ages Galaxy Properties with Integral Field Spectroscopy (MAGPI) survey, a Large Program on ESO/VLT. MAGPI is designed to study the physical drivers of galaxy transformation at a lookback time of 3-4 Gyr, during which the dynamical, morphological, and chemical properties of galaxies are predicted to evolve significantly. The survey uses new medium-deep adaptive optics aided MUSE observations of fields selected from the GAMA survey, providing a wealth of publicly available ancillary multi-wavelength data. With these data, MAGPI will map the kinematic and chemical properties of stars and ionised gas for a sample of 60 massive (> 7 x 10^10 M_Sun) central galaxies at 0.25 < z <0.35 in a representative range of environments (isolated, groups and clusters). The spatial resolution delivered by MUSE with Ground Layer Adaptive Optics (GLAO, 0.6-0.8 arcsec FWHM) will facilitate a direct comparison with Integral Field Spectroscopy surveys of the nearby Universe, such as SAMI and MaNGA, and at higher redshifts using adaptive optics, e.g. SINS. In addition to the primary (central) galaxy sample, MAGPI will deliver resolved and unresolved spectra for as many as 150 satellite galaxies at 0.25 < z <0.35, as well as hundreds of emission-line sources at z < 6. This paper outlines the science goals, survey design, and observing strategy of MAGPI. We also present a first look at the MAGPI data, and the theoretical framework to which MAGPI data will be compared using the current generation of cosmological hydrodynamical simulations including EAGLE, Magneticum, HORIZON-AGN, and Illustris-TNG. Our results show that cosmological hydrodynamical simulations make discrepant predictions in the spatially resolved properties of galaxies at z ~ 0.3. MAGPI observations will place new constraints and allow for tangible improvements in galaxy formation theory.

astro-ph.GA↗

arXiv2020

The SAMI Galaxy Survey: bulge and disk stellar population properties in cluster galaxies

We explore stellar population properties separately in the bulge and the disk of double-component cluster galaxies to shed light on the formation of lenticular galaxies in dense environments. We study eight low-redshift clusters from the Sydney-AAO Multi-object Integral field (SAMI) Galaxy Survey, using 2D photometric bulge-disk decomposition in the $g$, $r$ and $i$-bands to characterize galaxies. For 192 double-component galaxies with $M_{*}>10^{10~}M_{\odot}$ we estimate the color, age and metallicity of the bulge and the disk. The analysis of the $g-i$ colors reveals that bulges are redder than their surrounding disks with a median offset of 0.12$\pm$0.02 mag, consistent with previous results. To measure mass-weighted age and metallicity we investigate three methods: (i) one based on galaxy stellar mass weights for the two components, (ii) one based on flux weights and (iii) one based on radial separation. The three methods agree in finding 62% of galaxies having bulges that are 2-3 times more metal-rich than the disks. Of the remaining galaxies, 7% have bulges that are more metal-poor than the disks, while for 31% the bulge and disk metallicities are not significantly different. We observe 23% of galaxies being characterized by bulges older and 34% by bulges younger with respect to the disks. The remaining 43% of galaxies have bulges and disks with statistically indistinguishable ages. Redder bulges tend to be more metal-rich than the disks, suggesting that the redder color in bulges is due to their enhanced metallicity relative to the disks instead of differences in stellar population age.

astro-ph.GA↗

arXiv2019

Gate tunable quantum Hall effects in defect-suppressed Bi2Se3 films

Despite many years of efforts, attempts to reach the quantum regime of topological surface states (TSS) on an electrically tunable topological insulator (TI) platform have so far failed on binary TI compounds such as Bi2Se3 due to high density of interfacial defects. Here, utilizing an optimal buffer layer on a gatable substrate, we demonstrate the first electrically tunable quantum Hall effects (QHE) on TSS of Bi2Se3. On the n-side, well-defined QHE shows up, but it diminishes near the charge neutrality point (CNP) and completely disappears on the p-side. Furthermore, around the CNP the system transitions from a metallic to a highly resistive state as the magnetic field is increased, whose temperature dependence indicates presence of an insulating ground state at high magnetic fields.

cond-mat.mtrl-sci↗

arXiv2017

The status of KAGRA underground cryogenic gravitational wave telescope

KAGRA is a 3-km interferometric gravitational wave telescope located in the Kamioka mine in Japan. It is the first km-class gravitational wave telescope constructed underground to reduce seismic noise, and the first km-class telescope to use cryogenic cooling of test masses to reduce thermal noise. The construction of the infrastructure to house the interferometer in the tunnel, and the initial phase operation of the interferometer with a simple 3-km Michelson configuration have been completed. The first cryogenic operation is expected in 2018, and the observing runs with a full interferometer are expected in 2020s. The basic interferometer configuration and the current status of KAGRA are described.

gr-qc↗

arXiv2017

Coexistence of bulk and surface states probed by Shubnikov-de Haas oscillations in Bi$_2$Se$_3$ with high charge-carrier density

Topological insulators are ideally represented as having an insulating bulk with topologically protected, spin-textured surface states. However, it is increasingly becoming clear that these surface transport channels can be accompanied by a finite conducting bulk, as well as additional topologically trivial surface states. To investigate these parallel conduction transport channels, we studied Shubnikov-de Haas oscillations in Bi$_2$Se$_3$ thin films, in high magnetic fields up to 30 T so as to access channels with a lower mobility. We identify a clear Zeeman-split bulk contribution to the oscillations from a comparison between the charge-carrier densities extracted from the magnetoresistance and the oscillations. Furthermore, our analyses indicate the presence of a two-dimensional state and signatures of additional states the origin of which cannot be conclusively determined. Our findings underpin the necessity of theoretical studies on the origin of and the interplay between these parallel conduction channels for a careful analysis of the material's performance.

cond-mat.mes-hall↗

arXiv2017

Nonvolatile Solid-State Charged-Polymer Gating of Topological Insulators into the Topological Insulating Regime

We demonstrate the ability to reduce the carrier concentration of thin films of the topological insulator (TI) Bi2Se3 by utilizing a novel approach, namely non-volatile electrostatic gating via corona charging of electret polymers. Sufficient electric field can be imparted to a polymer-TI bilayer to result in significant electron density depletion, even without the continuous connection of a gate electrode or the chemical modification of the TI. We show that the Fermi level of Bi2Se3 is shifted towards the Dirac point with this method. Using THz spectroscopy, we find that the surface chemical potential is lowered into the bulk band gap (~ 50 meV above the Dirac point and 170 meV below the conduction band minimum) and it is stabilized in the intrinsic regime while enhancing electron mobility. This represents the first use of a charged polymer gate for modulating TI charge density. The mobility of surface state electrons is enhanced to a value as high as ~1600 cm^2/Vs at 5K.

cond-mat.mes-hall↗

arXiv2016

Surface vibrational modes of the topological insulator Bi$_2$Se$_3$ observed by Raman spectroscopy

We present polarization resolved Raman scattering study of surface vibration modes in the topological insulator Bi$_2$Se$_3$ single crystal and thick films. Besides the four Raman active bulk phonons, we observed four additional modes with much weaker intensity and slightly lower energy than the bulk counterparts. Using symmetry analysis, we assigned these additional modes to out-of-plane surface phonons. Comparing with first principle calculations, we conclude that the appearance of these modes is due to $c$-axis lattice distortion and van der Waals gap expansion near the crystal surface. Two of the surface modes at 60 and 173 cm$^{-1}$ are associated with Raman active $A_{1g}$ bulk phonon modes, the other two at 136 and 158 cm$^{-1}$ are associated with infrared active bulk phonons with $A_{2u}$ symmetry. The latter become Raman allowed due to reduction of crystalline symmetry from $D_{3d}$ in the bulk to $C_{3v}$ on the crystal surface. In particular, the 158 cm$^{-1}$ surface phonon mode shows a Fano lineshape under resonant excitation, suggesting interference in the presence of electron-phonon coupling of the surface excitations.

cond-mat.mtrl-sci↗

arXiv2016

Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator

Topological insulators have been proposed to be best characterized as bulk magnetoelectric materials that show response functions quantized in terms of fundamental physical constants. Here we lower the chemical potential of three-dimensional (3D) Bi$_2$Se$_3$ films to $\sim$ 30 meV above the Dirac point, and probe their low-energy electrodynamic response in the presence of magnetic fields with high-precision time-domain terahertz polarimetry. For fields higher than 5 T, we observed quantized Faraday and Kerr rotations, whereas the DC transport is still semi-classical. A non-trivial Berry phase offset to these values gives evidence for axion electrodynamics and the topological magnetoelectric effect. The time structure used in these measurements allows a direct measure of the fine structure constant based on a topological invariant of a solid-state system.

cond-mat.mes-hall↗

arXiv2016

Tuning and Stabilizing Topological Insulator Bi2Se3 in the Intrinsic Regime by Charge Extraction with Organic Overlayers

In this work, we use charge extraction via organic overlayer deposition to lower the chemical potential of topological insulator Bi2Se3 thin films into the intrinsic (bulk-insulating) regime. We demonstrate the tuning and stabilization of intrinsic topological insulators at high mobility with low-cost organic films. With the protection of the organic charge extraction layers tetrafluorotetracyanoquinodimethane(F4TCNQ) or tris(acetylacetonato)cobalt(III) (Co(acac)3), the sample is stable in the atmosphere with chemical potential ~135 meV above the Dirac point (85 meV below the conduction band minimum, well within the topological insulator regime) after four months, which is an extraordinary level of environmental stability. The Co complex represents the first use of an organometallic for modulating TI charge density. The mobility of surface state electrons is enhanced as high as ~2000 cm2/Vs. Even at room temperature, a true topologically insulating state is realized and stabilized for months' exposure to the atmosphere.

cond-mat.mes-hall↗

arXiv2015

Towards the understanding of the origin of charge-current-induced spin voltage signals in the topological insulator Bi$_2$Se$_3$

Topological insulators provide a new platform for spintronics due to the spin texture of the surface states that are topologically robust against elastic backscattering. Here, we report on an investigation of the measured voltage obtained from efforts to electrically probe spin-momentum locking in the topological insulator Bi$_2$Se$_3$ using ferromagnetic contacts. Upon inverting the magnetization of the ferromagnetic contacts, we find a reversal of the measured voltage. Extensive analysis of the bias and temperature dependence of this voltage was done, considering the orientation of the magnetization relative to the current. Our findings indicate that the measured voltage can arise due to fringe-field-induced Hall voltages, different from current-induced spin polarization of the surface state charge carriers, as reported recently. Understanding the nontrivial origin of the measured voltage is important for realizing spintronic devices with topological insulators.

cond-mat.mes-hall↗