HI Self-absorption Features Research Articles

Atomic and molecular gas properties during cloud formation

Context.Molecular clouds, which harbor the birthplaces of stars, form out of the atomic phase of the interstellar medium (ISM). To understand this transition process, it is crucial to investigate the spatial and kinematic relationships between atomic and molecular gas.Aims.We aim to characterize the atomic and molecular phases of the ISM and set their physical properties into the context of cloud formation processes.Methods.We studied the cold neutral medium (CNM) by means of H Iself-absorption (HISA) toward the giant molecular filament GMF20.0-17.9 (distance = 3.5 kpc, length ~170 pc) and compared our results with molecular gas traced by13CO emission. We fitted baselines of HISA features to H Iemission spectra using first and second order polynomial functions.Results.The CNM identified by this method spatially correlates with the morphology of the molecular gas toward the western region. However, no spatial correlation between HISA and13CO is evident toward the eastern part of the filament. The distribution of HISA peak velocities and line widths agrees well with13CO within the whole filament. The column densities of the CNM probed by HISA are on the order of 1020cm−2while those of molecular hydrogen traced by13CO are an order of magnitude higher. The column density probability density functions (N-PDFs) of HISA (CNM) and H Iemission (tracing both the CNM and the warm neutral medium, WNM) have a log-normal shape for all parts of the filament, indicative of turbulent motions as the main driver for these structures. The H2N-PDFs show a broad log-normal distribution with a power-law tail suggesting the onset of gravitational contraction. The saturation of H Icolumn density is observed at ~25M⊙pc−2.Conclusions.We conjecture that different evolutionary stages are evident within the filament. In the eastern region, we witness the onset of molecular cloud formation out of the atomic gas reservoir while the western part is more evolved, as it reveals pronounced H2column density peaks and signs of active star formation.

Cloud formation in the atomic and molecular phase: H I self absorption (HISA) towards a giant molecular filament

Molecular clouds form from the atomic phase of the interstellar medium. However, characterizing the transition between the atomic and the molecular interstellar medium (ISM) is a complex observational task. Here we address cloud formation processes by combining H Iself absorption (HISA) with molecular line data. Column density probability density functions (N-PDFs) are a common tool for examining molecular clouds. One scenario proposed by numerical simulations is that the N-PDF evolves from a log-normal shape at early times to a power-law-like shape at later times. To date, investigations of N-PDFs have been mostly limited to the molecular component of the cloud. In this paper, we study the cold atomic component of the giant molecular filament GMF38.1-32.4a (GMF38a, distance = 3.4 kpc, length ~ 230 pc), calculate its N-PDFs, and study its kinematics. We identify an extended HISA feature, which is partly correlated with the13CO emission. The peak velocities of the HISA and13CO observations agree well on the eastern side of the filament, whereas a velocity offset of approximately 4 km s−1is found on the western side. The sonic Mach number we derive from the linewidth measurements shows that a large fraction of the HISA, which is ascribed to the cold neutral medium (CNM), is at subsonic and transonic velocities. The column density of the CNM part is on the order of 1020to 1021cm−2. The column density of molecular hydrogen, traced by13CO, is an order of magnitude higher. The N-PDFs from HISA (CNM), H Iemission (the warm and cold neutral medium), and13CO (molecular component) are well described by log-normal functions, which is in agreement with turbulent motions being the main driver of cloud dynamics. The N-PDF of the molecular component also shows a power law in the high column-density region, indicating self-gravity. We suggest that we are witnessing two different evolutionary stages within the filament. The eastern subregion seems to be forming a molecular cloud out of the atomic gas, whereas the western subregion already shows high column density peaks, active star formation, and evidence of related feedback processes.

Calibrating the HISA temperature: Measuring the temperature of the Riegel–Crutcher cloud

HI self absorption (HISA) clouds are clumps of cold neutral hydrogen (HI) visible in front of warm background gas, which makes them ideal places to study the properties of the cold atomic component of the interstellar medium (ISM). The Riegel-Crutcher (R-C) cloud is the most striking HISA feature in the Galaxy. It is one of the closest HISA clouds to us and is located in the direction of the Galactic Centre, which provides a bright background. High-resolution interferometric measurements have revealed the filamentary structure of this cloud, however it is difficult to accurately determine the temperature and the density of the gas without optical depth measurements. In this paper we present new HI absorption observations with the Australia Telescope Compact Array (ATCA) against 46 continuum sources behind the Riegel-Crutcher cloud to directly measure the optical depth of the cloud. We decompose the complex HI absorption spectra into Gaussian components using an automated machine learning algorithm. We find 300 Gaussian components, from which 67 are associated with the R-C cloud (0 < v_{LSR} < 10 kms, FWHM < 10 kms). Combining the new HI absorption data with HI emission data from previous surveys we calculate the spin temperature and find it to be between 20 and 80 K. Our measurements uncover a temperature gradient across the cloud with spin temperatures decreasing towards positive Galactic latitudes. We also find three new OH absorption lines associated with the cloud, which support the presence of molecular gas.

A Global View of Molecule-Forming Clouds in the Galaxy

AbstractWe have mapped cold atomic gas in 21cm line H i self-absorption (HISA) at arcminute resolution over more than 90% of the Milky Way's disk. To probe the formation of H2 clouds, we have compared our HISA distribution with CO J = 1-0 line emission. Few HISA features in the outer Galaxy have CO at the same position and velocity, while most inner-Galaxy HISA has overlapping CO. But many apparent inner-Galaxy HISA-CO associations can be explained as chance superpositions, so most inner-Galaxy HISA may also be CO-free. Since standard equilibrium cloud models cannot explain the very cold H i in many HISA features without molecules being present, these clouds may instead have significant CO-dark H2.

Narrow-line H i and cold structures in the interstellar medium

Context. In the HI line profiles in the Leiden-Argentina-Bonn (LAB) all-sky database, we have found a population of very cold HI clouds. So far, the role of these clouds in the interstellar medium (ISM) has remained unclear. Aims. In this paper, we attempt to confirm the existence of the narrow-line HI emission (NHIE) clouds by using the data from the Parkes Galactic all-sky survey (GASS) and try to find their place among other coldest constituents of the ISM. Methods. We repeat the search of NHIE with the GASS data and derive or compile some preliminary estimates for the distribution, temperatures, distances, linear sizes, column and number densities, masses, and the composition of NHIE clouds, and compare these data with corresponding estimates for HI self-absorption (HISA) features, the Planck cold clumps (CC), and infrared dark clouds (IRDC). Results. We demonstrate that from LAB and GASS we can separate comparable NHIE complexes, and the properties of the obtained NHIE clouds are very similar to those of HISA features, but both of these types of clouds are somewhat warmer and more extended and have lower densities than the cores in the Planck CC and IRDC. Conclusions. We conclude that NHIE may be the same type of clouds as HISA, but in different observing conditions, in the same way as the Planck CC and IRDC are most likely similar ISM structures in different observing conditions and probably in slightly different evolutionary stages. Both NHIE and HISA may be an intermediate phase between the diffuse cold neutral medium and star-forming molecular clumps represented by the Planck CC and IRDC.

The JCMT12CO(3–2) survey of the Cygnus X region

Cygnus X is one of the most complex areas in the sky. This complicates interpretation, but also creates the opportunity to investigate accretion into molecular clouds and many subsequent stages of star formation, all within one small field of view. Understanding large complexes like Cygnus X is the key to understanding the dominant role that massive star complexes play in galaxies across the Universe. The main goal of this study is to establish feasibility of a high-resolution CO survey of the entire Cygnus X region by observing part of it as a Pathfinder, and to evaluate the survey as a tool for investigating the star-formation process. A 2x4 degree area of the Cygnus X region has been mapped in the 12CO(3-2) line at an angular resolution of 15" and a velocity resolution of ~0.4km/s using HARP-B and ACSIS on the James Clerk Maxwell Telescope. The star formation process is heavily connected to the life-cycle of the molecular material in the interstellar medium. The high critical density of the 12CO(3-2) transition reveals clouds in key stages of molecule formation, and shows processes that turn a molecular cloud into a star. We observed ~15% of Cygnus X, and demonstrated that a full survey would be feasible and rewarding. We detected three distinct layers of 12CO(3-2) emission, related to the Cygnus Rift (500-800 pc), to W75N (1-1.8 kpc), and to DR21 (1.5-2.5 kpc). Within the Cygnus Rift, HI self-absorption features are tightly correlated with faint diffuse CO emission, while HISA features in the DR21 layer are mostly unrelated to any CO emission. 47 molecular outflows were detected in the Pathfinder, 27 of them previously unknown. Sequentially triggered star formation is a widespread phenomenon.

A synthetic 21-cm Galactic Plane Survey of a smoothed particle hydrodynamics galaxy simulation

We have created synthetic neutral hydrogen (HI) Galactic Plane Survey data cubes covering 90 degrees < l < 180 degrees, using a model spiral galaxy from SPH simulations and the radiative transfer code TORUS. The density, temperature and other physical parameters are fed from the SPH simulation into TORUS, where the HI emissivity and opacity are calculated before the 21-cm line emission profile is determined. Our main focus is the observation of Outer Galaxy `Perseus Arm' HI, with a view to tracing atomic gas as it encounters shock motions as it enters a spiral arm interface, an early step in the formation of molecular clouds. The observation of HI self-absorption features at these shock sites (in both real observations and our synthetic data) allows us to investigate further the connection between cold atomic gas and the onset of molecular cloud formation.

Dynamical Formation of Dark Molecular Hydrogen Clouds around Diffuse HiiRegions

We examine the triggering process of molecular cloud formation around diffuse HII regions. We calculate the time evolution of the shell as well as of the HII region in a two-phase neutral medium, solving the UV and FUV radiative transfer, the thermal and chemical processes in the time-dependent hydrodynamics code. In the cold neutral medium, the ambient gas is swept up in the cold (T \sim 30-40K) and dense (n \sim 10^3 cm^-3) shell around the HII region. In the shell, H_2 molecules are formed from the swept-up HI gas, but CO molecules are hardly formed. The reformation of H_2 molecules is more efficient with a higher-mass central star. The physical and chemical properties of gas in the shell are just intermediate between those of the neutral medium and molecular clouds observed by the CO emission. The dense shell with cold HI/H_2 gas easily becomes gravitationally unstable, and breaks up into small clouds. The cooling layer just behind the shock front also suffers from thermal instability, and will fragment into cloudlets with some translational motions. We suggest that the predicted cold ``dark'' HI/H_2 gas should be detected as the HI self-absorption (HISA) feature. We have sought such features in recent observational data, and found shell-like HISA features around the giant HII regions, W4 and W5. The shell-like HISA feature shows good spatial correlation with dust emission, but poor correlation with CO emission. Our quantitative analysis shows that the HISA cloud can be as cold as T \sim a few x 10K. (abridged)

CO in HiSelf‐absorbed Clouds in Perseus

We have observed 12CO J = 2 → 1 and J = 1 → 0 and 13CO J = 1 → 0 emission in two regions of H I self-absorption (HISA) in Perseus: a small, isolated HISA feature called the and a more extended HISA cloud called the complex. Using both large velocity gradient and Monte Carlo radiative transfer codes, we found that in the globule N(12CO) < 6.0 × 1015 cm-2, which, using photodissociation region (PDR) models, implies that N(H2) < 9.9 × 1020 cm-2. In the complex we found that the H2 column densities were in the range (1.2-2.2) × 1021 cm-2. By comparing the HISA and CO observations we were able to constrain the physical conditions and atomic gas fraction (f). In the globule, 8 K < Tspin < 22 K and 0.02 < f < 0.2, depending on whether the (unknown) gas density was 102, 103, or 104 cm-3. In the complex, 12 K < Tspin < 24 K and 0.02 < f < 0.05, and we were also able to constrain the gas density (100 cm-3 < n < 1200 cm-3). These results imply that the gas in the HISA clouds is colder and denser than that usually associated with the atomic ISM and, indeed, is similar to that seen in molecular clouds. The small atomic gas fractions also imply that there is a significant molecular component in these HISA clouds, even when little or no 12CO is detected. The level of 12CO detected and the visual extinction due to dust is consistent with the idea that these HISA clouds are undergoing a transition from the atomic to molecular phase.

The Physical Properties of a Cold HiFeature Observed Transitioning from Absorption to Emission

Large-scale surveys of neutral hydrogen (H I) 21 cm line emission from the Galactic plane at ~1' resolution have revealed an extensive population of H I self-absorption (HISA) features, i.e., presumably cold H I seen in absorption against warmer background H I. These observations provide previously unknown information regarding the morphology and distribution of cold H I gas in our Galaxy, but determining the physical properties of such features primarily depends on some sort of radiative transfer modeling that only loosely constrains various model parameters. We present here observations and analysis of one HISA feature that is seen to move from absorption into emission and show how observations made at the transition point allow the physical properties of the HISA feature to be tightly constrained. The derived physical properties are consistent with a diffuse, cold H I cloud with minimal molecular gas content. Identification of objects like this is currently uncommon in the interferometer-based Galactic plane surveys, which have necessarily focused on lower Galactic latitudes. As high-resolution Galactic surveys are extended to higher latitudes, we expect that many additional H I features will be seen transitioning from absorption to emission and will thus be amenable to the analysis techniques presented here.

A New View of Cold HiClouds in the Milky Way

We reveal cold Galactic clouds of neutral hydrogen in unprecedented detail. Our 21 cm synthesis maps, taken from the Canadian Galactic Plane Survey, show a numerous and diverse population of H I self-absorption (HISA) features in gas outside the solar circle. These objects vary in size, shape, and contrast against the background H I. All display a high level of angular and velocity structure, and most would appear significantly diluted, if not invisible, in lower resolution H I surveys. A number of Perseus arm features remain unresolved by the 1' beam of our survey, with apparent diameters less than 0.6 pc at 2 kpc distance. The majority of HISA features we detect have no obvious 12CO emission counterparts. This suggests that either HISA is not found predominantly in molecular clouds, as has often been presumed in the past, or that CO is not a good tracer of H2. Some HISA lacking CO shows far-infrared dust emission, though whether this arises from shielded molecular gas or from diffuse atomic clouds is not clear. Constraining the gas properties of HISA remains a difficult problem, but we introduce a new method that aids this process. Our approach relates a number of physical parameters via gas law and line integral relationships and should prove powerful if the input variables are sufficiently well known. We explore the current allowed parameter ranges for three sample features of very different appearance. We find spin temperatures ≲50 K and densities ≳102 cm-3.