arXiv subjects

Hiep Nguyen

Publications and source records attributed to Hiep Nguyen.

At least 19 recordsLinked to original sources

arXiv2026

The GASKAP-HI Survey towards the Magellanic Clouds: Cold Atomic Gas Survival and Evolution in the Large Magellanic Cloud

We use atomic hydrogen (HI) absorption detections from the GASKAP-HI survey to investigate the properties of cold atomic gas in the Large Magellanic Cloud (LMC). Using the radiative transfer method, we decompose 155 sightlines into 330 cold neutral medium (CNM), 2 thermally unstable neutral medium (UNM), and 310 warm neutral medium (WNM) components. We find that the CNM in the LMC exhibits higher optical depths (median 0.46), lower spin temperatures (median $\sim$37 K), broader linewidths (median $\sim$4.9 km s$^{-1}$), and slightly lower CNM fractions (median $\sim$23%) than in the Milky Way. We examine the connection between the CNM, molecular gas, and star formation, finding that CNM correlates more closely with molecular gas than WNM, while molecular gas shows a tighter relation with star formation. Molecular hydrogen (H$_2$) formation begins near $N{_\mathrm{HI,CNM}}\sim10^{20}~\mathrm{cm^{-2}}$, and nearly all sightlines with $N_{\mathrm{HI,CNM}}>10^{21}~\mathrm{cm^{-2}}$ contain molecular gas. The CNM fraction increases with visual extinction ($A_V$), and the LMC maintains CNM fractions comparable to those in the Milky Way at substantially lower $A_V$, likely due to higher local densities and a longer line-of-sight depth. Sightlines near expanding shells tend to show higher CNM fractions, although this is partly driven by higher total HI column densities. Finally, the CNM kinematics generally follow the HI disk rotation, with about 7% of components showing velocity offsets greater than $25~\mathrm{km~s^{-1}}$, likely tracing inflows or outflows driven by stellar feedback or large-scale interactions within the Magellanic System.

astro-ph.GA↗

arXiv2026

Revealing the cold skeleton of the Magellanic Clouds and the Magellanic Bridge with ASKAP

We present the GASKAP-HI pilot absorption survey of neutral hydrogen (HI) in the Magellanic system. This survey provides 3219 sightlines across the Large (LMC) and Small Magellanic Clouds (SMC) and the Magellanic Bridge (MB) towards 1.4-GHz continuum sources, representing a 15-fold increase on pre--GASKAP-HI sampling of the Magellanic System. We find 344 candidate detections of cold gas at Magellanic velocities (vLSRK >= 90 km s-1), with signal-to-noise ratio > 3 detection rates of 44% (LMC; 192 of 438), 73% (SMC; 85 of 117) and 4% (MB; 35 of 793). We examine the candidate detections within the MB, Gaussian decompose these and examine the cold gas across the MB. Here we find that the majority of cold gas detections are found closer to the SMC. We also find potential evidence of the recent formation of cold gas on the outskirts of a shell within the MB. We find a mean cold gas fraction of fCNM = 0.12 +- 0.08 for the MB, which is very similar to the SMC and lower than the LMC value of 0.14. Overall, we reveal cold gas distributed extensively across the Magellanic system, including within the MB, and surmise that the cold gas in the MB is either pulled from the SMC as part of the formation of the MB, or formed in the turbulence of those same interactions.

astro-ph.GA↗

arXiv2026

High-fidelity stellar extinction with Gaia and APOGEE -- I. The method and a new extinction curve

The scarcity of high-fidelity extinction measurements remains a bottleneck in deriving accurate stellar properties from Gaia parallaxes. In this work, we aim to derive precision extinction estimates for APOGEE DR19 stars, establishing a new benchmark for Galactic stellar population studies. We first determine reddening by comparing observed colorsr, etrieved from photometric surveys or standardized synthetic magnitudes from Gaia BP/RP spectra, to intrinsic colors predicted via an XGBoost model. The model is trained on minimally reddened stars to infer intrinsic colors and their associated uncertainties, using APOGEE stellar parameters (Teff, logg, [Fe/H], and [alpha/Fe]). The derived reddening values are then converted into extinctions using an anchor ratio of A_BP / A_RP = 1.694 +/- 0.004, derived from red-clump-like stars. Here, we provide extinction measurements in 39 filters across 10 photometric systems and introduce a new empirical extinction curve optimized for broadband passbands. Our extinction estimates (Av) outperform existing results (Bayestar19, StarHorse, SEDEX), achieving a typical precision of 0.03 mag in Av. Notably, we identify systematic deviations of up to 30% between monochromatic and passband-integrated extinction ratios at wavelengths greater than 700 nm. This result highlights the necessity of adopting passband-specific coefficients when correcting extinction to derive stellar parameters. The derived extinction and reddening data are available to the community for download through Zenodo.

astro-ph.SR↗

arXiv2025

A High-resolution Study of the Cold Neutral Medium in and around 30 Doradus

With the aim of evaluating the roles of the cold neutral medium (CNM) in the cloud-scale baryon cycle, we perform a high-resolution study of the CNM in and around the extreme star-forming region 30 Doradus (30 Dor). For our study, we use Galactic Australian Square Kilometre Array Pathfinder H I Survey data and produce H I emission and absorption cubes on 7 pc scales. To examine the CNM structures toward 30 Dor, we decompose the H I absorption cube into 862 Gaussian components and find that these components are distributed at four velocity ranges (B1, B2, B3, and B4, respectively): 200$-$230 km s$^{-1}$, 230$-$260 km s$^{-1}$, 260$-$277 km s$^{-1}$, and 277$-$300 km s$^{-1}$. We derive line-of-sight average spin temperatures and opacity-corrected total H I column densities and show that the B1$-$B4 structures have systematically different properties, indicating that they are physically distinct. As for the nature of the observed CNM structures, we find that B2 is associated with the main dense structure where ionized, atomic, and molecular gases are concentrated. B3 and B4 trace inflows whose combined mass flux rate of 0.14 $M_{\odot}$ yr$^{-1}$ is comparable to the current star formation rate, while B1 probes outflows with a much lower mass flux rate of 0.007 $M_{\odot}$ yr$^{-1}$. Interestingly, the H I column densities in B1$-$B4 are nearly uniform with a factor of two spatial variations, implying the presence of H I shielding layers for H$_{2}$ formation.

astro-ph.GA↗

arXiv2025

Finetuning LLMs for Automatic Form Interaction on Web-Browser in Selenium Testing Framework

Automated web application testing is a critical component of modern software development, with frameworks like Selenium widely adopted for validating functionality through browser automation. Among the essential aspects of such testing is the ability to interact with and validate web forms, a task that requires syntactically correct, executable scripts with high coverage of input fields. Despite its importance, this task remains underexplored in the context of large language models (LLMs), and no public benchmark or dataset exists to evaluate LLMs on form interaction generation systematically. This paper introduces a novel method for training LLMs to generate high-quality test cases in Selenium, specifically targeting form interaction testing. We curate both synthetic and human-annotated datasets for training and evaluation, covering diverse real-world forms and testing scenarios. We define clear metrics for syntax correctness, script executability, and input field coverage. Our empirical study demonstrates that our approach significantly outperforms strong baselines, including GPT-4o and other popular LLMs, across all evaluation metrics. Our work lays the groundwork for future research on LLM-based web testing and provides resources to support ongoing progress in this area.

cs.SE↗

arXiv2025

VLSP 2025 MLQA-TSR Challenge: Vietnamese Multimodal Legal Question Answering on Traffic Sign Regulation

This paper presents the VLSP 2025 MLQA-TSR - the multimodal legal question answering on traffic sign regulation shared task at VLSP 2025. VLSP 2025 MLQA-TSR comprises two subtasks: multimodal legal retrieval and multimodal question answering. The goal is to advance research on Vietnamese multimodal legal text processing and to provide a benchmark dataset for building and evaluating intelligent systems in multimodal legal domains, with a focus on traffic sign regulation in Vietnam. The best-reported results on VLSP 2025 MLQA-TSR are an F2 score of 64.55% for multimodal legal retrieval and an accuracy of 86.30% for multimodal question answering.

cs.CL↗

arXiv2025

Study of HI Turbulence in the SMC Using Multi-point Structure Functions

Turbulence in the interstellar medium (ISM) plays an important role in many physical processes, including forming stars and shaping complex ISM structures. In this work, we investigate the HI turbulent properties of the Small Magellanic Cloud (SMC) to reveal what physical mechanisms drive the turbulence and at what scales. Using the high-resolution HI data of the Galactic ASKAP (GASKAP) survey and multi-point structure functions (SF), we perform a statistical analysis of HI turbulence in 34 subregions of the SMC. Two-point SFs tend to show a linear trend, and their slope values are relatively uniform across the SMC, suggesting that large-scale structures exist and are dominant in the two-point SFs. On the other hand, seven-point SF enables us to probe small-scale turbulence by removing large-scale fluctuations, which is difficult to achieve with the two-point SFs. In the seven-point SFs, we find break features at scales of 34-84 pc, with a median scale of $\sim$50 pc. This result indicates the presence of small-scale turbulent fluctuations in the SMC and quantifies its scale. In addition, we find strong correlations between slope values of the seven-point SFs and the stellar feedback-related quantities (e.g., H$α$ intensities, the number of young stellar objects, and the number of HI shells), suggesting that stellar feedback may affect the small-scale turbulent properties of the HI gas in the SMC. Lastly, estimated sonic Mach numbers across the SMC are subsonic, which is consistent with the fact that the HI gas of the SMC primarily consists of the warm neutral medium.

astro-ph.GA↗

arXiv2025

Multi-wavelength probes of the Milky Way's Cold Interstellar Medium: Radio HI and Optical KI Absorption with GASKAP and GALAH

We present a comparative analysis of interstellar hydrogen (HI) and potassium (KI) absorption from the radio and optical surveys, GASKAP and GALAH, to study the physical and kinematic properties of the cold interstellar medium (ISM) in the Milky Way foreground towards the Magellanic Clouds. By comparing GASKAP HI absorption with interstellar KI absorption detected in GALAH spectra of nearby stars (within 12 arcmin angular distance or a spatial separation of ~0.75 pc), we reveal a strong kinematic correlation between these two tracers of the cold neutral ISM. The velocity offsets between matched HI and KI absorption components are small, with a mean (median) offset of -1.3 (-1.2) km s-1 and a standard deviation of 2.3 km s-1. The high degree of kinematic consistency suggests a close spatial association between Ki and cold HI gas. Correlation analyses reveal a moderate positive relationship between HI and KI line-of-sight properties, such as KI column density with HI column density or HI brightness temperature. We observe a ~63% overlap in the detection of both species towards 290 (out of 462) GASKAP HI absorption lines of sight, and estimate a median KI/HI abundance ratio of ~2.3 x 10^(-10), in excellent agreement with previous findings. Our work opens up an exciting avenue of Galactic research that uses large-scale surveys in the radio and optical wavelengths to probe the neutral interstellar medium through its diverse tracers.

astro-ph.GA↗

arXiv2025

New Interpretation for the Orientation of the LMC's Gaseous Arms B and E using ULLYSES

The Large Magellanic Cloud (LMC) experiences disruption from tidal and ram-pressure forces as it travels through the halo of the Milky Way. In this project, we combine radio emission-line observations from the GASS and GASKAP surveys with UV absorption-line observations from the HST Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) program to trace the material in front of the LMC. Along our 8 stellar sightlines near 30 Doradus, we observe gaseous structures likely associated with two arm-like features flowing in and around the LMC's disk. We detect the nearside gas in neutral, low, and medium ionization species. The lower-ionization species likely undergo both thermal and non-thermal broadening while the moderately-ionized phase is influenced by more non-thermal processes. The total integrated column density of AlIII decreases with increasing angular offset from 30 Doradus, with sightlines within 0.25 degrees containing more moderately ionized gas. We demonstrate from a Gaussian decomposition technique on the HI emission that both arms likely trace an additional 1.0 degree in Galactic longitude toward the 30 Doradus region than previously predicted. We constrain the orientation of the arms by suggesting that they likely converge around (l,b) =(280.5$^\circ$, -31.2$^\circ$) and at least partially cross in front of the LMC. Our observations are consistent with two competing origins of the arms: 1) outflowing material is swept back by tidal and ram-pressure forces or 2) tidally stripped inflows fuel the ongoing stellar activity inside the LMC. Future studies are needed to distinguish between these scenarios.

astro-ph.GA↗

arXiv2025

Automated Web Application Testing: End-to-End Test Case Generation with Large Language Models and Screen Transition Graphs

Web applications are critical to modern software ecosystems, yet ensuring their reliability remains challenging due to the complexity and dynamic nature of web interfaces. Recent advances in large language models (LLMs) have shown promise in automating complex tasks, but limitations persist in handling dynamic navigation flows and complex form interactions. This paper presents an automated system for generating test cases for two key aspects of web application testing: site navigation and form filling. For site navigation, the system employs screen transition graphs and LLMs to model navigation flows and generate test scenarios. For form filling, it uses state graphs to handle conditional forms and automates Selenium script generation. Key contributions include: (1) a novel integration of graph structures and LLMs for site navigation testing, (2) a state graph-based approach for automating form-filling test cases, and (3) a comprehensive dataset for evaluating form-interaction testing. Experimental results demonstrate the system's effectiveness in improving test coverage and robustness, advancing the state of web application testing.

cs.SE↗

arXiv2025

A neutral hydrogen absorption study of cold gas in the outskirts of the Magellanic Clouds using the GASKAP-HI survey

Cold neutral hydrogen (HI) is a crucial precursor for molecular gas formation and can be studied via HI absorption. This study investigates HI absorption in low column density regions of the Small and Large Magellanic Clouds (SMC and LMC) using the Galactic-ASKAP HI (GASKAP-HI) survey, conducted by the Australian Square Kilometer Array Pathfinder (ASKAP). We select 10 SMC directions in the outer regions and 18 LMC directions, with 4 in the outskirts and 14 within the main disk. Using the radiative transfer method, we decompose the emission and absorption spectra into individual cold neutral medium (CNM) and warm neutral medium (WNM) components. In the SMC, we find HI peak optical depths of 0.09-1.16, spin temperatures of ~20-50 K, and CNM fractions of 1-11%. In the LMC, optical depths range from 0.03 to 3.55, spin temperatures from ~10 to 100 K, and CNM fractions from 1% to 100%. The SMC's low CNM fractions likely result from its low metallicity and large line-of-sight depth. Additionally, the SMC's outskirts show lower CNM fractions than the main body, potentially due to increased CNM evaporation influenced by the hot Magellanic Corona. Shell motions dominate the kinematics of the majority of CNM clouds in this study and likely supply cold HI to the Magellanic Stream. In the LMC, high CNM fraction clouds are found near supergiant shells, where thermal instability induced by stellar feedback promotes WNM-to-CNM transition. Although no carbon monoxide (CO) has been detected, enhanced dust shielding in these areas helps maintain the cold HI.

astro-ph.GA↗

arXiv2025

Considerations with stacking absorption spectra: cold HI gas in cirrus region of the Milky Way

We use the Milky Way neutral hydrogen (HI) absorption and emission spectra from the Galactic Australian Square Kilometre Array Pathfinder (GASKAP) Phase II Pilot survey along with toy models to investigate the effects of stacking multicomponent spectra on measurements of peak optical depth and spin temperature. Shifting spectra by the peak in emission, 'primary' components shifted to 0 km s$^{-1}$ are correctly averaged. Additional components on individual sightlines are averaged with non-centred velocities, producing a broader and shallower 'secondary' component in the resulting stack. Peak optical depths and brightness temperatures of the secondary components from stacks are lower limits of their true average values due to the velocity offset of each component. The spin temperature however is well correlated with the truth since the velocity offset of components affects the emission and absorption spectra equally. Stacking 462 GASKAP absorption-emission spectral pairs, we detect a component with a spin temperature of 1320 $\pm$ 263 K, consistent with gas from the unstable neutral medium and higher than any previous GASKAP detection in this region. We also stack 2240 pilot survey spectra containing no Milky Way absorption, revealing a primary narrow and secondary broad component, with spin temperatures belonging to the cold neutral medium (CNM). Spatially binning and stacking the non-detections across the plane-of-sky by their distance from CNM absorption detections, the primary component's optical depth decreases with distance from known locations of cold gas. The spin temperature however remains stable in both components, over an approximate physical plane-of-sky distance of $\sim$ 100 pc.

astro-ph.GA↗

arXiv2024

TPCNet: Representation learning for HI mapping

We introduce TPCNet, a neural network predictor that combines Convolutional and Transformer architectures with Positional encodings, for neutral atomic hydrogen (HI) spectral analysis. Trained on synthetic datasets, our models predict cold neutral gas fraction ($f_\text{CNM}$) and HI opacity correction factor ($R_\text{HI}$) from emission spectra based on the learned relationships between the desired output parameters and observables (optically-thin column density and peak brightness). As a follow-up to Murray et al. (2020)'s shallow Convolutional Neural Network (CNN), we construct deep CNN models and compare them to TPCNet models. TPCNet outperforms deep CNNs, achieving a 10% average increase in testing accuracy, algorithmic (training) stability, and convergence speed. Our findings highlight the robustness of the proposed model with sinusoidal positional encoding applied directly to the spectral input, addressing perturbations in training dataset shuffling and convolutional network weight initializations. Higher spectral resolutions with increased spectral channels offer advantages, albeit with increased training time. Diverse synthetic datasets enhance model performance and generalization, as demonstrated by producing $f_\text{CNM}$ and $R_\text{HI}$ values consistent with evaluation ground truths. Applications of TPCNet to observed emission data reveal strong agreement between the predictions and Gaussian decomposition-based estimates (from emission and absorption surveys), emphasizing its potential in HI spectral analysis.

astro-ph.GA↗

arXiv2024

ZeFaV: Boosting Large Language Models for Zero-shot Fact Verification

In this paper, we propose ZeFaV - a zero-shot based fact-checking verification framework to enhance the performance on fact verification task of large language models by leveraging the in-context learning ability of large language models to extract the relations among the entities within a claim, re-organized the information from the evidence in a relationally logical form, and combine the above information with the original evidence to generate the context from which our fact-checking model provide verdicts for the input claims. We conducted empirical experiments to evaluate our approach on two multi-hop fact-checking datasets including HoVer and FEVEROUS, and achieved potential results results comparable to other state-of-the-art fact verification task methods.

cs.CL↗

arXiv2024

Local HI Absorption towards the Magellanic Cloud foreground using ASKAP

We present the largest Galactic neutral hydrogen HI absorption survey to date, utilizing the Australian SKA Pathfinder Telescope at an unprecedented spatial resolution of 30''. This survey, GASKAP-HI, unbiasedly targets 2,714 continuum background sources over 250 square degrees in the direction of the Magellanic Clouds, a significant increase compared to a total of 373 sources observed by previous Galactic absorption surveys across the entire Milky Way. We aim to investigate the physical properties of cold (CNM) and warm (WNM) neutral atomic gas in the Milky Way foreground, characterized by two prominent filaments at high Galactic latitudes (between $-45^{\circ}$ and $-25^{\circ}$). We detected strong HI absorption along 462 lines of sight above the 3$σ$ threshold, achieving an absorption detection rate of 17%. GASKAP-HI's unprecedented angular resolution allows for simultaneous absorption and emission measurements to sample almost the same gas clouds along a line of sight. A joint Gaussian decomposition is then applied to absorption-emission spectra to provide direct estimates of HI optical depths, temperatures, and column densities for the CNM and WNM components. The thermal properties of CNM components are consistent with those previously observed along a wide range of Solar neighborhood environments, indicating that cold HI properties are widely prevalent throughout the local interstellar medium. Across our region of interest, CNM accounts for ~30% of the total HI gas, with the CNM fraction increasing with column density toward the two filaments. Our analysis reveals an anti-correlation between CNM temperature and its optical depth, which implies that CNM with lower optical depth leads to a higher temperature.

astro-ph.GA↗

arXiv2023

A Galactic Eclipse: The Small Magellanic Cloud is Forming Stars in Two, Superimposed Systems

The structure and dynamics of the star-forming disk of the Small Magellanic Cloud (SMC) have long confounded us. The SMC is widely used as a prototype for galactic physics at low metallicity, and yet we fundamentally lack an understanding of the structure of its interstellar medium (ISM). In this work, we present a new model for the SMC by comparing the kinematics of young, massive stars with the structure of the ISM traced by high-resolution observations of neutral atomic hydrogen (HI) from the Galactic Australian Square Kilometer Array Pathfinder survey (GASKAP-HI). Specifically, we identify thousands of young, massive stars with precise radial velocity constraints from the Gaia and APOGEE surveys and match these stars to the ISM structures in which they likely formed. By comparing the average dust extinction towards these stars, we find evidence that the SMC is composed of two structures with distinct stellar and gaseous chemical compositions. We construct a simple model that successfully reproduces the observations and shows that the ISM of the SMC is arranged into two, superimposed, star-forming systems with similar gas mass separated by ~5 kpc along the line of sight.

astro-ph.GA↗

arXiv2023

Probing the Conditions for the Hı-to-H$_{2}$ Transition in the Interstellar Medium

In this paper, we investigate the conditions for the HI-to-H$_{2}$ transition in the solar neighborhood by analyzing HI emission and absorption measurements toward 58 Galactic lines of sight (LOSs) along with $^{12}$CO(1$-$0) (CO) and dust data. Based on the accurate column densities of the cold and warm neutral medium (CNM and WNM), we first perform a decomposition of gas into atomic and molecular phases and show that the observed LOSs are mostly HI-dominated. In addition, we find that the CO-dark H$_{2}$, not the optically thick HI, is a major ingredient of the dark gas in the solar neighborhood. To examine the conditions for the formation of CO-bright molecular gas, we analyze the kinematic association between HI and CO and find that the CNM is kinematically more closely associated with CO than the WNM. When CNM components within CO line widths are isolated, we find the following characteristics: spin temperature $<$ 200 K, peak optical depth $>$ 0.1, CNM fraction of $\sim$0.6, and $V$-band dust extinction $>$ 0.5 mag. These results suggest that CO-bright molecular gas preferentially forms in environments with high column densities where the CNM becomes colder and more abundant. Finally, we confront the observed CNM properties with the steady-state H$_{2}$ formation model of Sternberg et al. and infer that the CNM must be clumpy with a small volume filling factor. Another possibility would be that missing processes in the model, such as cosmic-rays and gas dynamics, play an important role in the HI-to-H$_{2}$ transition.

astro-ph.GA↗

arXiv2023

GNOMES II: Analysis of the Galactic diffuse molecular ISM in all four ground state hydroxyl transitions using Amoeba

We present observations of the four 2 Pi 3/2 J = 3/2 ground-rotational state transitions of the hydroxyl molecule (OH) along 107 lines of sight both in and out of the Galactic plane: 92 sets of observations from the Arecibo telescope and 15 sets of observations from the Australia Telescope Compact Array (ATCA). Our Arecibo observations included off-source pointings, allowing us to measure excitation temperature (Tex) and optical depth, while our ATCA observations give optical depth only. We perform Gaussian decomposition using the Automated Molecular Excitation Bayesian line-fitting Algorithm 'AMOEBA' (Petzler, Dawson, and Wardle 2021) fitting all four transitions simultaneously with shared centroid velocity and width. We identify 109 features across 38 sightlines (including 58 detections along 27 sightlines with excitation temperature measurements). While the main lines at 1665 and 1667 MHz tend to have similar excitation temperatures (median Tex(main) difference = 0.6 K, 84% show Tex(main) difference < 2 K), large differences in the 1612 and 1720 MHz satellite line excitation temperatures show that the gas is generally not in LTE. For a selection of sightlines we compare our OH features to associated (on-sky and in velocity) HI cold gas components (CNM) identified by Nguyen et al. (2019) and find no strong correlations. We speculate that this may indicate an effective decoupling of the molecular gas from the CNM once it accumulates.

astro-ph.GA↗