arXiv subjects

Min-Young Lee

Publications and source records attributed to Min-Young Lee.

At least 55 records · Page 3Linked to original sources

arXiv2019

Exploring the properties of warm and cold atomic hydrogen in the Taurus and Gemini regions

We report Arecibo 21 cm absorption-emission observations to characterise the physical properties of neutral hydrogen (HI) in the proximity of five giant molecular clouds (GMCs): Taurus, California, Rosette, Mon OB1, and NGC 2264. Strong HI absorption was detected toward all 79 background continuum sources in the ~60x20 square degree region. Gaussian decompositions were performed to estimate temperatures, optical depths and column densities of the cold and warm neutral medium (CNM, WNM). The properties of individual CNM components are similar to those previously observed along random Galactic sightlines and in the vicinity of GMCs, suggesting a universality of cold HI properties. The CNM spin temperature (Ts) histogram peaks at ~50K. The turbulent Mach numbers of CNM vary widely, with a typical value of ~4, indicating that their motions are supersonic. About 60% of the total HI gas is WNM, and nearly 40% of the WNM lies in thermally unstable regime 500-5000K. The observed CNM fraction is higher around GMCs than in diffuse regions, and increases with increasing column density (NHI) to a maximum of ~75%. On average, the optically thin approximation (N*(HI)) underestimates the total N(HI) by ~21%, but we find large regional differences in the relationship between N(HI) and the required correction factor, f=N(HI)/N*(HI). We examine two different methods (linear fit of f vs log10(N*(HI)) and uniform Ts) to correct for opacity effects using emission data from the GALFA-HI survey. We prefer the uniform Ts method, since the linear relationship does not produce convincing fits for all subregions.

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arXiv2019

Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud. II. 30 Doradus

With an aim of probing the physical conditions and excitation mechanisms of warm molecular gas in individual star-forming regions, we performed Herschel SPIRE FTS observations of 30 Doradus in the LMC. In our FTS observations, important FIR cooling lines in the ISM, including CO J=4-3 to 13-12, [CI] 370 micron, and [NII] 205 micron, were clearly detected. In combination with ground-based CO data, we then constructed CO spectral line energy distributions (SLEDs) on 10 pc scales over a 60 pc x 60 pc area and found that the shape of the observed CO SLEDs considerably changes across 30 Doradus, e.g., the peak transition varies from J=6-5 to 10-9, while the slope characterized by the high-to-intermediate J ratio ranges from 0.4 to 1.8. To examine the source(s) of these variations in CO transitions, we analyzed the CO observations, along with [CII] 158 micron, [CI] 370 micron, [OI] 145 micron, H2 0-0 S(3), and FIR luminosity data, using state-of-the-art models of PDRs and shocks. Our detailed modeling showed that the observed CO emission likely originates from highly-compressed (thermal pressure ~ 1e7-1e9 K cm-3) clumps on 0.7-2 pc scales, which could be produced by either UV photons (UV radiation field ~ 1e3-1e5 Mathis fields) or low-velocity C-type shocks (pre-shock medium density ~ 1e4-1e6 cm-3 and shock velocity ~ 5-10 km s-1). Considering the stellar content in 30 Doradus, however, we tentatively excluded the stellar origin of CO excitation and concluded that low-velocity shocks driven by kpc scale processes (e.g., interaction between the Milky Way and the Magellanic Clouds) are likely the dominant source of heating for CO. The shocked CO-bright medium was then found to be warm (temperature ~ 100-500 K) and surrounded by a UV-regulated low pressure component (a few (1e4-1e5) K cm-3) that is bright in [CII] 158 micron, [CI] 370 micron, [OI] 145 micron, and FIR dust continuum emission.

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arXiv2018

Optically thick HI does not dominate dark gas in the local ISM

The local interstellar medium (ISM) is suffused with "dark" gas, identified by excess infrared and gamma ray emission, yet undetected by standard ISM tracers such as neutral hydrogen (HI) or carbon monoxide emission. Based on observed dust properties from Planck, recent studies have argued that HI mixed with dust is strongly saturated and that dark gas is dominated by optically-thick HI. We test this hypothesis by reproducing this model using data from Planck and new 21 cm emission maps from GALFA-HI -- the first large-area 21cm emission survey with comparable angular resolution to Planck. We compare the results with those from a large sample of HI column densities based on direct observations of HI optical depth, and find that the inferred column density corrections are significantly lower than those inferred by the Planck-based model. Further, we rule out the hypothesis that the pencil-beam HI absorption sight lines preferentially miss opaque "blobs" with small covering fraction, as these structures require densities and pressures which are incompatible with ISM conditions. Our results support the picture that excess dust emission in the local ISM is not dominated by optically-thick HI, but is rather a combination of intrinsic changes in dust grain emissivities and H2 missed by CO observations.

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arXiv2018

The Multi-phase Turbulence Density Power Spectra in the Perseus Molecular Cloud

We derive two-dimensional spatial power spectra of four distinct interstellar medium tracers, HI, $^{12}$CO($J$=1--0), $^{13}$CO($J$=1--0), and dust, in the Perseus molecular cloud, covering linear scales ranging from $\sim$0.1 pc to $\sim$90 pc. Among the four tracers, we find the steepest slopes of $-3.23\pm0.05$ and $-3.22\pm0.05$ for the uncorrected and opacity-corrected HI column density images. This result suggests that the HI in and around Perseus traces a non-gravitating, transonic medium on average, with a negligible effect from opacity. On the other hand, we measure the shallowest slope of $-2.72\pm0.12$ for the 2MASS dust extinction data and interpret this as the signature of a self-gravitating, supersonic medium. Possible variations in the dust-to-gas ratio likely do not change our conclusion. Finally, we derive slopes of $-3.08\pm0.08$ and $-2.88\pm0.07$ for the ${}^{12}$CO(1--0) and ${}^{13}$CO(1--0) integrated intensity images. Based on theoretical predictions for an optically thick medium, we interpret these slopes of roughly $-3$ as implying that both CO lines are susceptible to the opacity effect. While simple tests for the impact of CO formation and depletion indicate that the measured slopes of ${}^{12}$CO(1--0) and ${}^{13}$CO(1--0) are not likely affected by these chemical effects, our results generally suggest that chemically more complex and/or fully optically thick media may not be a reliable observational tracer for characterizing turbulence.

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arXiv2016

Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud. I. N159W

We present Herschel SPIRE Fourier Transform Spectrometer (FTS) observations of N159W, an active star-forming region in the Large Magellanic Cloud (LMC). In our observations, a number of far-infrared cooling lines including CO(4-3) to CO(12-11), [CI] 609 and 370 micron, and [NII] 205 micron are clearly detected. With an aim of investigating the physical conditions and excitation processes of molecular gas, we first construct CO spectral line energy distributions (SLEDs) on 10 pc scales by combining the FTS CO transitions with ground-based low-J CO data and analyze the observed CO SLEDs using non-LTE radiative transfer models. We find that the CO-traced molecular gas in N159W is warm (kinetic temperature of 153-754 K) and moderately dense (H2 number density of (1.1-4.5)e3 cm-3). To assess the impact of the energetic processes in the interstellar medium on the physical conditions of the CO-emitting gas, we then compare the observed CO line intensities with the models of photodissociation regions (PDRs) and shocks. We first constrain the properties of PDRs by modelling Herschel observations of [OI] 145, [CII] 158, and [CI] 370 micron fine-structure lines and find that the constrained PDR components emit very weak CO emission. X-rays and cosmic-rays are also found to provide a negligible contribution to the CO emission, essentially ruling out ionizing sources (ultraviolet photons, X-rays, and cosmic-rays) as the dominant heating source for CO in N159W. On the other hand, mechanical heating by low-velocity C-type shocks with ~10 km/s appears sufficient enough to reproduce the observed warm CO.

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arXiv2015

HI-to-H2 transitions in the Perseus Molecular Cloud

We apply the Sternberg et al. (2014) theoretical model to analyze HI and H2 observations in the Perseus molecular cloud. We constrain the physical properties of the HI shielding envelopes and the nature of the HI-to-H2 transitions. Our analysis (Bialy et al. 2015) implies that in addition to cold neutral gas (CNM), less dense thermally-unstable gas (UNM) significantly contributes to the shielding of the H2 cores in Perseus.

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arXiv2015

The Lognormal Probability Distribution Function of the Perseus Molecular Cloud: A Comparison of HI and Dust

The shape of the probability distribution function (PDF) of molecular clouds is an important ingredient for modern theories of star formation and turbulence. Recently, several studies have pointed out observational difficulties with constraining the low column density (i.e. Av <1) PDF using dust tracers. In order to constrain the shape and properties of the low column density probability distribution function, we investigate the PDF of multiphase atomic gas in the Perseus molecular cloud using opacity-corrected GALFA-HI data and compare the PDF shape and properties to the total gas PDF and the N(H2) PDF. We find that the shape of the PDF in the atomic medium of Perseus is well described by a lognormal distribution, and not by a power-law or bimodal distribution. The peak of the atomic gas PDF in and around Perseus lies at the HI-H2 transition column density for this cloud, past which the N(H2) PDF takes on a powerlaw form. We find that the PDF of the atomic gas is narrow and at column densities larger than the HI-H2 transition the HI rapidly depletes, suggesting that the HI PDF may be used to find the HI-H2 transition column density. We also calculate the sonic Mach number of the atomic gas by using HI absorption line data, which yields a median value of Ms=4.0 for the CNM, while the HI emission PDF, which traces both the WNM and CNM, has a width more consistent with transonic turbulence.

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arXiv2015

HI-to-H2 Transitions in the Perseus Molecular Cloud

We use the Sternberg et al. (2014) theory for interstellar atomic to molecular (HI-to-H$_2$) conversion to analyze HI-to-H$_2$ transitions in five (low-mass) star-forming and dark regions in the Perseus molecular cloud, B1, B1E, B5, IC348, and NGC1333. The observed HI mass surface densities of 6.3 to 9.2 M$_{\odot}$ pc$^{-2}$ are consistent with HI-to-H$_2$ transitions dominated by HI-dust shielding in predominantly atomic envelopes. For each source, we constrain the dimensionless parameter $αG$, and the ratio $I_{\rm UV}/n$, of the FUV intensity to hydrogen gas density. We find $αG$ values from 5.0 to 26.1, implying characteristic atomic hydrogen densities 11.8 to 1.8 cm$^{-3}$, for $I_{\rm UV} \approx 1$ appropriate for Perseus. Our analysis implies that the dusty HI shielding layers are probably multiphased, with thermally unstable UNM gas in addition to cold CNM within the 21 cm kinematic radius.

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arXiv2015

Cold and Warm Atomic Gas around the Perseus Molecular Cloud. II. The Impact of High Optical Depth on the HI Column Density Distribution and Its Implication for the HI-to-H2 Transition

We investigate the impact of high optical depth on the HI saturation observed in the Perseus molecular cloud by using Arecibo HI emission and absorption measurements toward 26 radio continuum sources. The spin temperature and optical depth of individual HI components are derived along each line-of-sight, enabling us to estimate the correction for high optical depth. We examine two different methods for the correction, Gaussian decomposition and isothermal methods, and find that they are consistent (maximum correction factor ~ 1.2) likely due to the relatively low optical depth and insignificant contribution from the diffuse radio continuum emission for Perseus. We apply the correction to the optically thin HI column density on a pixel-by-pixel basis, and find that the total HI mass increases by ~10%. Using the corrected HI column density image and far-infrared data from the IRIS Survey, we then derive the H2 column density on ~0.4 pc scales. For five dark and star-forming sub-regions, the HI surface density is uniform with Sigma_HI ~ 7-9 solar mass/pc2, in agreement with the minimum HI surface density required for shielding H2 against photodissociation. As a result, Sigma_H2/Sigma_HI and Sigma_HI+Sigma_H2 show a tight relation. Our results are consistent with predictions for H2 formation in steady state and chemical equilibrium, and suggest that H2 formation is mainly responsible for the Sigma_HI saturation in Perseus. We also compare the optically thick HI with the observed "CO-dark" gas, and find that the optically thick HI only accounts for ~20% of the "CO-dark" gas in Perseus.

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arXiv2014

Spatially resolved physical conditions of molecular gas and potential star formation tracers in M83, revealed by the Herschel SPIRE FTS

Since the launch of the Herschel Space Observatory, our understanding about the photo-dissociation regions (PDR) has taken a step forward. In the bandwidth of the Fourier Transform Spectrometer (FTS) of the Spectral and Photometric Imaging REceiver (SPIRE) on board Herschel, ten CO rotational transitions, including J=4-3 to J=13-12, and three fine structure lines, including [CI] 609, [CI] 370, and [NII] 250 micron, are covered. In this paper, we present our findings from the FTS observations at the nuclear region of M83, based on the spatially resolved physical parameters derived from the CO spectral line energy distribution (SLED) map and the comparisons with the dust properties and star-formation tracers. We discuss (1) the potential of using [NII] 250 and [CI] 370 micron as star-formation tracers; (2) the reliability of tracing molecular gas with CO; (3) the excitation mechanisms of warm CO; (4) the possibility of studying stellar feedback by tracing the thermal pressure of molecular gas in the nuclear region of M83.

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arXiv2014

Dust and Gas in the Magellanic Clouds from the HERITAGE Herschel Key Project. II. Gas-to-Dust Ratio Variations across ISM Phases

The spatial variations of the gas-to-dust ratio (GDR) provide constraints on the chemical evolution and lifecycle of dust in galaxies. We examine the relation between dust and gas at 10-50 pc resolution in the Large and Small Magellanic Clouds (LMC and SMC) based on Herschel far-infrared (FIR), H I 21 cm, CO, and Halpha observations. In the diffuse atomic ISM, we derive the gas-to-dust ratio as the slope of the dust-gas relation and find gas-to-dust ratios of 380+250-130 in the LMC, and 1200+1600-420 in the SMC, not including helium. The atomic-to-molecular transition is located at dust surface densities of 0.05 Mo pc-2 in the LMC and 0.03 Mo pc-2 in the SMC, corresponding to AV ~ 0.4 and 0.2, respectively. We investigate the range of CO-to-H2 conversion factor to best account for all the molecular gas in the beam of the observations, and find upper limits on XCO to be 6x1020 cm-2 K-1 km-1 s in the LMC (Z=0.5Zo) at 15 pc resolution, and 4x 1021 cm-2 K-1 km-1 s in the SMC (Z=0.2Zo) at 45 pc resolution. In the LMC, the slope of the dust-gas relation in the dense ISM is lower than in the diffuse ISM by a factor ~2, even after accounting for the effects of CO-dark H2 in the translucent envelopes of molecular clouds. Coagulation of dust grains and the subsequent dust emissivity increase in molecular clouds, and/or accretion of gas-phase metals onto dust grains, and the subsequent dust abundance (dust-to-gas ratio) increase in molecular clouds could explain the observations. In the SMC, variations in the dust-gas slope caused by coagulation or accretion are degenerate with the effects of CO-dark H2. Within the expected 5--20 times Galactic XCO range, the dust-gas slope can be either constant or decrease by a factor of several across ISM phases. Further modeling and observations are required to break the degeneracy between dust grain coagulation, accretion, and CO-dark H2.

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arXiv2014

Cold and warm atomic gas around the Perseus molecular cloud I: Basic Properties

(Abridged) Using the Arecibo Observatory we have obtained neutral hydrogen (HI) absorption and emission spectral pairs in the direction of 26 background radio continuum sources in the vicinity of the Perseus molecular cloud. Strong absorption lines were detected in all cases allowing us to estimate spin temperature (T_s) and optical depth for 107 individual Gaussian components along these lines of sight. Basic properties of individual HI clouds (spin temperature, optical depth, and the column density of the cold and warm neutral medium, CNM and WNM) in and around Perseus are very similar to those found for random interstellar lines of sight sampled by the Millennium HI survey. This suggests that the neutral gas found in and around molecular clouds is not atypical. However, lines of sight in the vicinity of Perseus have on average a higher total HI column density and the CNM fraction, suggesting an enhanced amount of cold HI relative to an average interstellar field. Our estimated optical depth and spin temperature are in stark contrast with the recent attempt at using Planck data to estimate properties of the optically thick HI. Only ~15% of lines of sight in our study have a column density weighted average spin temperature lower than 50 K, in comparison with >85% of Planck's sky coverage. The observed CNM fraction is inversely proportional to the optical-depth weighted average spin temperature, in excellent agreement with the recent numerical simulations by Kim et al. While the CNM fraction is on average higher around Perseus relative to a random interstellar field, it is generally low, 10-50%. This suggests that extended WNM envelopes around molecular clouds, and/or significant mixing of CNM and WNM throughout molecular clouds, are present and should be considered in the models of molecule and star formation.

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arXiv2014

The CO-to-H2 Conversion Factor across the Perseus Molecular Cloud

We derive the CO-to-H2 conversion factor, X_CO = N(H2)/I_CO, across the Perseus molecular cloud on sub-parsec scales by combining the dust-based N(H2) data with the I_CO data from the COMPLETE Survey. We estimate an average X_CO ~ 3 x 10^19 cm^-2 K^-1 km^-1 s and find a factor of ~3 variations in X_CO between the five sub-regions in Perseus. Within the individual regions, X_CO varies by a factor of ~100, suggesting that X_CO strongly depends on local conditions in the interstellar medium. We find that X_CO sharply decreases at Av < 3 mag but gradually increases at Av > 3 mag, with the transition occurring at Av where I_CO becomes optically thick. We compare the N(HI), N(H2), I_CO, and X_CO distributions with two models of the formation of molecular gas, a one-dimensional photodissociation region (PDR) model and a three-dimensional magnetohydrodynamic (MHD) model tracking both the dynamical and chemical evolution of gas. The PDR model based on the steady state and equilibrium chemistry reproduces our data very well but requires a diffuse halo to match the observed N(HI) and I_CO distributions. The MHD model generally matches our data well, suggesting that time-dependent effects on H2 and CO formation are insignificant for an evolved molecular cloud like Perseus. However, we find interesting discrepancies, including a broader range of N(HI), likely underestimated I_CO, and a large scatter of I_CO at small Av. These discrepancies likely result from strong compressions/rarefactions and density fluctuations in the MHD model.

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arXiv2013

Characterizing the Turbulent Properties of the Starless Molecular Cloud MBM16

We investigate turbulent properties of the non-star-forming, translucent molecular cloud, MBM16 by applying the statistical technique of a two-dimensional spatial power spectrum (SPS) on the neutral hydrogen (HI) observations obtained by the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) survey. The SPS, calculated over the range of spatial scales from 0.1 to 17 pc, is well represented with a single power-law function, with a slope ranging from -3.3 to -3.7 and being consistent over the velocity range of MBM16 for a fixed velocity channel thickness. However, the slope varies significantly with the velocity slice thickness, suggesting that both velocity and density contribute to HI intensity fluctuations. By using this variation we estimate the slope of 3D density fluctuations in MBM16 to be -3.7\pm0.2. This is significantly steeper than what has been found for HI in the Milky Way plane, the Small Magellanic Cloud, or the Magellanic Bridge, suggesting that interstellar turbulence in MBM16 is driven on scales >17 pc and that the lack of stellar feedback could be responsible for the steep power spectrum.

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arXiv2012

A High Resolution Study of the HI-H2 Transition across the Perseus Molecular Cloud

To investigate the fundamental principles of H2 formation in a giant molecular cloud (GMC), we derive the HI and H2 surface density (Sigma_HI and Sigma_H2) images of the Perseus molecular cloud on sub-pc scales (~0.4 pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the HI data from the Galactic Arecibo L-band Feed Array HI Survey and an estimate of the local dust-to-gas ratio, we then derive the Sigma_H2 distribution across Perseus. We find a relatively uniform Sigma_HI ~ 6-8 Msun pc^-2 for both dark and star-forming regions, suggesting a minimum HI surface density required to shield H2 against photodissociation. As a result, a remarkably tight and consistent relation is found between Sigma_H2/Sigma_HI and Sigma_HI+Sigma_H2. The transition between the HI- and H2-dominated regions occurs at N(HI)+2N(H2) ~ (8-14) x 10^20 cm^-2. Our findings are consistent with predictions for H2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H2 formation. However, the importance of a warm neutral medium for H2 shielding, an internal radiation field, and the timescale of H2 formation still remain as open questions. We also compare H2 and CO distributions and estimate the fraction of "CO-dark" gas, f_DG ~ 0.3. While significant spatial variations of f_DG are found, we do not find a clear correlation with the mean V-band extinction.

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arXiv2011

The GALFA-HI Survey: Data Release 1

We present the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) survey, and its first full data release (DR1). GALFA-HI is a high resolution (~ 4'), large area (13000 deg^2), high spectral resolution (0.18 km/s), wide band (-700 < v_LSR < +700 km/s) survey of the Galactic interstellar medium in the 21-cm line hyperfine transition of neutral hydrogen conducted at Arecibo Observatory. Typical noise levels are 80 mK RMS in an integrated 1 km/s channel. GALFA-HI is a dramatic step forward in high-resolution, large-area Galactic HI surveys, and we compare GALFA-HI to past, present, and future Galactic HI surveys. We describe in detail new techniques we have developed to reduce these data in the presence of fixed pattern noise, gain variation, and inconsistent beam shapes, and we show how we have largely mitigated these effects. We present our first full data release, covering 7520 square degrees of sky and representing 3046 hours of integration time, and discuss the details of these data.

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arXiv2010

Galactic Small Scale Structure Revealed by the GALFA-HI Survey

The Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey is mapping the entire Arecibo sky at 21-cm, over a velocity range of -700 to +700 km/s (LSR), at a velocity resolution of 0.18 km/s and an angular resolution of 3.5 arcmin. The unprecedented resolution and sensitivity of the GALFA-HI survey have resulted in the detection of many isolated, very compact HI clouds at low Galactic velocities which are distinctly separated from the HI disk emission. In the limited area of ~4600 deg$^2$ searched so far, we have detected 96 such compact clouds. The detected clouds are cold with kinetic temperature less than 300 K. Moreover, they are quite compact and faint, with median values of 5 arcmin in angular size, 0.75 K in peak brightness temperature, and $5\times10^{18}$ cm$^{-2}$ in HI column density. From the modeling of spatial and velocity distributions of the whole compact cloud population, we find that the bulk of clouds are related to the Galactic disk, and are within a few kpc distance. We present properties of the compact clouds sample and discuss various possible scenarios for the origin of this clouds population and its role in the Galactic interstellar medium studies.

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arXiv2010

Compact HI clouds from the GALFA-HI survey

The Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey is mapping the entire Arecibo sky at 21-cm, over a velocity range of -700 to +700 km/s (LSR), at a velocity resolution of 0.18 km/s and a spatial resolution of 3.5 arcmin. The unprecedented resolution and sensitivity of the GALFA-HI survey have resulted in the detection of numerous isolated, very compact HI clouds at low Galactic velocities, which are distinctly separated from the HI disk emission. In the limited area of ~4600 deg$^2$ surveyed so far, we have detected 96 of such compact clouds. The detected clouds are cold with a median T$_{k,max}$ (the kinetic temperature in the case in which there is no non-thermal broadening) of 300 K. Moreover, these clouds are quite compact and faint, with median values of 5 arcmin in angular size, 0.75 K in peak brightness temperature, and $5 \times 10^{18}$ cm$^{-2}$ in HI column density. Most of the clouds deviate from Galactic rotation at the 20-30 km/s level, and a significant fraction show evidence for a multiphase medium and velocity gradients. No counterparts for these clouds were found in other wavebands. From the modeling of spatial and velocity distributions of the whole compact cloud population, we find that the bulk of the compact clouds are related to the Galactic disk, and their distances are likely to be in the range of 0.1 to a few kpc. We discuss various possible scenarios for the formation and maintenance of this cloud population and its significance for Galactic ISM studies.

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