arXiv subjects

Min-Young Lee

Publications and source records attributed to Min-Young Lee.

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.

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arXiv2026

ALOHA IRDCs Molecular Line Follow-up: I. Gas properties and kinematics

Infrared Dark Clouds are ideal sites for investigating the initial conditions of massive star and cluster formation. The A Lei Of the Habitat and Assembly of Infrared Dark Clouds (ALOHA IRDCs), a James Clerk Maxwell Telescope (JCMT) Large Program, has mapped nearby IRDCs with SCUBA-2. Complementary molecular line observations are needed to characterise the physical, kinematic, and chemical properties of the dense gas. We aim to determine the thermal, kinematic, and chemical properties of clumps identified in the ALOHA IRDCs, and to assess their evolutionary status and level of star-forming activity. We performed single-pointing K-band and W-band observations towards 56 ALOHA IRDCs clumps using the Effelsberg 100-m and Yebes 40-m telescopes, respectively. We derived NH3 kinetic temperatures using the hyperfine group ratio (HFGR) method and identified infall and shock signatures from HCO+, H13CO+, SiO, and HNCO profiles. Water masers and NH2D emission were used as complementary tracers of chemical evolution and star formation. The clumps exhibit kinetic temperatures of 15-29 K. We detect NH2D emission towards 18 sources, with NH2D centroid velocities consistent with NH3, indicating both species trace the same dense gas component. More than half of the clumps display blue-asymmetric HCO+ profiles, identifying them as infall candidates. Water masers are detected in 22 sources, with prominent velocity ranges and variability. Broad SiO emission (>~20 km/s) indicates strong shocks, while narrower extents (<~6km/s) likely trace large-scale interactions or low-velocity shocks. The widespread infall signatures, shock tracers, masers, and NH2D emission suggest that relatively quiescent, chemically young material can coexist with dynamically active gas affected by early protostellar feedback, providing insight into the coupled physical and chemical evolution of massive IRDC clumps.

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arXiv2026

The Multi-phase HI of the Milky Way and Nearby Galaxies

Atomic hydrogen (HI) is the dominant baryonic component of the interstellar medium (ISM) in Milky Way-like galaxies and the reservoir from which molecular clouds and stars ultimately form. The condensation of diffuse HI into cold structures is governed by a complex interplay between radiative cooling, turbulence, magnetic fields, stellar feedback, and galactic dynamics, acting over scales ranging from astronomical units to kiloparsecs. Understanding how these processes regulate the thermal structure of the HI, the formation of cold clouds, and the transfer of matter and energy across scales is essential for connecting the small-scale physics of the ISM to the evolution of galaxies. Recent advances from SKA precursors have transformed our view of the atomic ISM, revealing a highly structured and filamentary cold medium, increasing the density of HI absorption measurements by orders of magnitude, and enabling new approaches to infer the thermodynamic and magnetic properties of the gas from spectral-line datasets. SKA-mid will provide the first comprehensive characterization of HI as a multi-phase, turbulent, and magnetized medium across the Milky Way and nearby galaxies. Its combination of sensitivity, angular resolution, spectral resolution, and survey speed will enable matched emission-absorption studies, dense optical-depth grids, and detailed mapping of the atomic-to-molecular transition over a broad range of environments. Combined with polarization, Zeeman, recombination-line, and multi-wavelength observations, SKA-mid will establish a unified observational framework to study the evolution of diffuse matter in galaxies, in connection with star formation, from the Solar neighborhood to galactic scales.

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arXiv2026

CO-dark molecular gas traced by HCO$^+$ in the diffuse interstellar medium

A classic problem in the study of the interstellar medium (ISM) is the near-invisibility of molecular hydrogen (H$_2$) in cold environments. Observations of CO emission are typically used to indirectly trace H$_2$, but a significant fraction of H$_2$ in the diffuse ISM is not associated with any detectable CO emission (``CO-dark'' molecular gas). Meanwhile, observations of H$_2$ absorption trace nearly all of the H$_2$ in diffuse directions. In particular, a kinematically broad HCO$^+$ absorption signature traces extremely diffuse, CO-dark H$_2$. We have used sensitive observations of HCO$^+$, CO, and atomic hydrogen (HI) in absorption to constrain the properties of such diffuse molecular gas in five directions. The diffuse molecular gas revealed by broad HCO$^+$ absorption has a lower fraction of cold HI ($f_{\mathrm{CNM}} = 0.38^{+0.28}_{-0.27}$) and a lower fraction of hydrogen in H$_2$ ($f_{\mathrm{mol}}=0.09^{+0.06}_{-0.03}$) than gas traced by CO in the same directions. We detect almost no CO absorption from the gas traced by broad HCO$^+$ absorption. We constrain the CO abundance relative to H$_2$ to be $\lesssim10^{-6}$-$10^{-5}$ for gas traced by both broad and narrow HCO$^+$ absorption, consistent with chemical model predictions for the diffuse ISM. We further show that neither CO emission nor absorption is likely to be detected where $N(\mathrm{H_2})\lesssim\mathrm{few}\times10^{19}$ $\mathrm{cm^{-2}}$ - a result of both the low CO abundance and the low H$_2$ column - while HCO$^+$ absorption is readily detected for $N(\mathrm{H_2})\gtrsim\text{few}\times10^{18}$ $\mathrm{cm^{-2}}$. These results demonstrate that even modest amounts of cold HI can bear H$_2$, providing critical constraints on the HI-to-H$_2$ transition in the ISM.

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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.

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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.

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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$50 pc. This result indicates the presence of small-scale turbulent fluctuations in the SMC and quantifies its scale. In addition, we find strong correlations between slope values of the seven-point SFs and the stellar feedback-related quantities (e.g., H$\alpha$ 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.

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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.

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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.

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arXiv2025

A Nearby Dark Molecular Cloud in the Local Bubble Revealed via H$_2$ Fluorescence

A longstanding prediction in interstellar theory posits that significant quantities of molecular gas, crucial for star formation, may be undetected due to being ``dark" in commonly used molecular gas tracers, such as carbon monoxide. We report the discovery of Eos, the closest dark molecular cloud, located just 94 parsecs from the Sun. This cloud is the first molecular cloud ever to be identified using H$_2$ far ultra-violet (FUV) fluorescent line emission, which traces molecular gas at the boundary layers of star-forming and supernova remnant regions. The cloud edge is outlined along the high-latitude side of the North Polar Spur, a prominent x-ray/radio structure. Our distance estimate utilizes 3D dust maps, the absorption of the soft X-ray background, and hot gas tracers such as O\,{\sc vi}; these place the cloud at a distance consistent with the Local Bubble's surface. Using high-latitude CO maps we note a small amount (M$_{\rm{H}_2}\approx$20-40\,M$_\odot$) of CO-bright cold molecular gas, in contrast with the much larger estimate of the cloud's true molecular mass (M$_{\rm{H}_2}\approx3.4\times 10^3$\,M$_\odot$), indicating most of the cloud is CO-dark. Combining observational data with novel analytical models and simulations, we predict this cloud will photoevaporate in 5.7 million years, placing key constraints on the role of stellar feedback in shaping the closest star-forming regions to the Sun.

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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.

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arXiv2025

The Gaseous Blowout of the 30 Doradus Starburst Region in the LMC

Widespread galactic winds emanate from the Large Magellanic Cloud (LMC), with the 30 Doradus starburst region generating the fastest and most concentrated gas flows. We report on the gas distribution, kinematics, and ionization conditions of the near-side outflow along 8 down-the-barrel sightlines using UV absorption-line observations from the HST's ULLYSES program for this region along with H I 21-cm observations from the GASS and GASKAP surveys. We find that within 1.7 degrees from the center of 30 Doradus, the wind reaches maximum speeds of $100-150\,\text{km}\,\text{s}^{-1}$ from the LMC's disk. The total integrated column densities of low-ions (O I, Si II, and Fe II) in the blueshifted wind, up to $v_{\rm LSR}=150\,\text{km}\,\text{s}^{-1}$, are highest near the center and decline radially outward. We estimate an outflow mass of $M_{\rm outflow,\,Si II}\approx(5.7-8.6)\,\times 10^{5} M_{\odot}$, outflow rate of $\dot{M}_{\rm outflow}\gtrsim0.02 M_{\odot}\,\text{yr}^{-1}$, and mass loading factor of $η\gtrsim0.10$ within 0.52 degrees from the center of 30 Doradus. The observed ion ratios$-$together with photoionization modeling$-$reveal that this wind is roughly $40-97\%$ photoionized. The metallicities and dust depletion patterns of the high-velocity absorbers at $v_{\rm LSR}\approx+120\,\text{km}\,\text{s}^{-1}$ can be explained by either a foreground Milky Way (MW) halo cloud or an outflow from the LMC. For the high-ions, Si IV and C IV are broader and kinematically offset from the low-ions, suggesting turbulent mixing layers (TMLs) existing in the wind. Finally, our hydrodynamical simulations of the Magellanic Clouds (MCs) and MW system suggest that the Magellanic Corona can protect the LMC winds from the ram-pressure forces exerted by the MW's halo.

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arXiv2025

The Molecular Cloud Lifecycle I: Constraining H2 formation and dissociation rates with observations

Molecular clouds (MCs) are the birthplaces of new stars in galaxies. A key component of MCs are photodissociation regions (PDRs), where far-ultraviolet radiation plays a crucial role in determining the gas's physical and chemical state. Traditional PDR models assume chemical steady state (CSS), where the rates of H$_2$ formation and photodissociation are balanced. However, real MCs are dynamic and can be out of CSS. In this study, we demonstrate that combining H$_2$ emission lines observed in the far-ultraviolet or infrared with column density observations can be used to derive the rates of H$_2$ formation and photodissociation. We derive analytical formulae that relate these rates to observable quantities, which we validate using synthetic H$_2$ line emission maps derived from the SILCC-Zoom hydrodynamical simulation. Our method estimates integrated H$_2$ formation and dissociation rates with an accuracy $\approx 30$ % (on top of uncertainties in observed H$_2$ emission maps and column densities). Our simulations, valid for column densities $N \leq 2 \times 10^{22}$ cm$^{-2}$, cover a wide dynamic range in H$_2$ formation and photodissociation rates, showing significant deviations from CSS, with 74 % of the MC's mass deviating from CSS by a factor greater than 2. Our analytical formulae can effectively distinguish between regions in and out of CSS. When applied to actual H$_2$ line observations, our method can assess the chemical state of MCs, providing insights into their evolutionary stages and lifetimes. A NASA Small Explorer mission concept, Eos, will be proposed in 2025 and is specifically designed to conduct the types of observations outlined in this study.

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arXiv2025

ATLASGAL-selected high-mass clumps in the inner Galaxy: XI. Morphology and kinematics of warm inner envelopes

(Abridged) Massive stellar embryos are embedded in warm envelopes that provide mass reservoirs for the accretion process onto final stars. Feedback from star formation activities in return impacts the properties of the envelopes, offering us a unique opportunity to investigate star formation processes. We aim to characterise the warm envelopes of proto- or young stellar objects in different evolutionary stages based on the morphology and kinematics of the $^{13}$CO(6-5) emission and to examine their relations with star formation processes. Using the APEX telescope, we obtained maps of the mid-$J$ $^{13}$CO emission with an angular size of 80" x 80" towards 99 massive clumps from the ATLASGAL survey. Our maps are classified based on morphological complexities, and the radial structure of the emission is characterised for simple single-core sources. The velocity centroids of the emission are compared to small- and large-scale gas kinematics, aiming to shed light on the origin of envelope kinematics. $^{13}$CO(6-5) emission is detected towards sources in all stages of high-mass star formation, with a detection rate of 83% for the whole sample. The detection rate, line width, and line peak increase with evolution, and the line luminosity is strongly correlated with $L_{\mathrm{bol}}$ and $M_{\mathrm{clump}}$, indicating that the excitation of $^{13}$CO(6-5) emission is closely related to star formation processes. In addition, the radial distributions of the emission for single-core sources can be well fitted by power-law functions, suggesting a relatively simple envelope structure for many sources. As for the envelope kinematics, linear velocity gradients are common among the single-core sources. Our comparison of kinematics on different scales suggests that the origin of the linear velocity gradients in the warm envelopes is complex and unclear for many sources.

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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.

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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.

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arXiv2024

The Molecular Cloud Lifecycle II: Formation and Destruction of Molecular Clouds Diagnosed via H$_2$ Fluorescent Emission Emission

Molecular hydrogen (H$_2$) formation and dissociation are key processes that drive the gas lifecycle in galaxies. Using the SImulating the LifeCycle of Molecular Clouds (SILCC) zoom-in simulation suite, we explore the utility of future observations of H$_2$ dissociation and formation for tracking the lifecycle of molecular clouds. The simulations used in this work include non-equilibrium H$_2$ formation, stellar radiation, sink particles, and turbulence. We find that, at early times in the cloud evolution, H$_2$ formation rapidly outpaces dissociation and molecular clouds build their mass from the atomic reservoir in their environment. Rapid H$_2$ formation is also associated with a higher early star formation rate. For the clouds studied here, H$_2$ is strongly out of chemical equilibrium during the early stages of cloud formation but settles into a bursty chemical steady-state about 2 Myrs after the first stars form. At the latest stage of cloud evolution, dissociation outweighs formation and the clouds enter a dispersal phase. We discuss how theories for the molecular cloud lifecycle and the star formation efficiency may be distinguished with observational measurements of H$_2$ fluorescence with a space-based high-resolution FUV spectrometer, such as the proposed Hyperion and Eos NASA Explorer missions. Such missions would enable measurements of the H$_2$ dissociation and formation rates, which we demonstrate can be connected to different phases in a molecular cloud's star-forming life, including cloud building, rapidly star-forming, H$_2$ chemical equilibrium, and cloud destruction.

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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.

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