Giant Molecular Filaments Research Articles

N2H+(1–0) as a tracer of dense gas in and between spiral arms

ABSTRACT Recent advances in identifying giant molecular filaments in Galactic surveys allow us to study the interstellar material and its dense, potentially star forming phase on scales comparable to resolved extragalactic clouds. Two large filaments detected in the 13CO/C18O(J = 3–2) Heterodyne Inner Milky Way Plane Survey (CHIMPS) survey, one in the Sagittarius-arm and one in an interarm region, were mapped with dense gas tracers inside a 0.06 square degrees area and with a spatial resolution of around 0.4 and 0.65 pc at the distance of the targets using the 30 m telescope of the Institut de Radioastronomie Millimétrique (IRAM) to investigate the environmental dependence of the dense gas fraction. The N2H+(1 − 0) transition, an excellent tracer of the dense gas, was detected in parsec-scale, elliptical clumps and with a filling factor of around 8.5 per cent in our maps. The N2H+-emitting areas appear to have higher dense gas fraction (e.g. the ratio of N2H+ and 13CO emission) in the interarm than in the arm which is opposite to the behaviour found by previous studies, using dust emission rather than N2H+ as a tracer of dense gas. However, the arm filament is brighter in 13CO and the infrared emission of dust, and the dense gas fraction determined as above is governed by the 13CO brightness. We caution that measurements regarding the distribution and fraction of dense gas on these scales may be influenced by many scale- and environment-dependent factors, as well as the chemistry and excitation of the particular tracers, then consider several scenarios that can reproduce the observed effect.

GMF G214.5-1.8 as traced by CO: I – cloud-scale CO freeze-out as a result of a low cosmic-ray ionization rate

ABSTRACT We present an analysis of the outer Galaxy giant molecular filament (GMF) G214.5-1.8 (G214.5) using IRAM 30m data of 12CO, 13CO, and C18O. We find that the 12CO (1-0) and (2-1) derived excitation temperatures are near identical and are very low, with a median of 8.2 K, showing that the gas is extremely cold across the whole cloud. Investigating the abundance of 13CO across G214.5, we find that there is a significantly lower abundance along the entire 13 pc spine of the filament, extending out to a radius of ∼0.8 pc, corresponding to Av ≳ 2 mag and Tdust ≲ 13.5 K. Due to this, we attribute the decrease in abundance to CO freeze-out, making G214.5 the largest scale example of freeze-out yet. We construct an axisymmetric model of G214.5’s 13CO volume density considering freeze-out and find that to reproduce the observed profile significant depletion is required beginning at low volume densities, n ≳ 2000 cm−3. Freeze-out at this low number density is possible only if the cosmic-ray ionization rate is ∼1.9 × 10−18 s−1, an order of magnitude below the typical value. Using time scale arguments, we posit that such a low ionization rate may lead to ambipolar diffusion being an important physical process along G214.5’s entire spine. We suggest that if low cosmic-ray ionization rates are more common in the outer Galaxy, and other quiescent regions, cloud-scale CO freeze-out occurring at low column and number densities may also be more prevalent, having consequences for CO observations and their interpretation.

Cold atomic gas identified by H I self-absorption

Context. Stars form in the dense interiors of molecular clouds. The dynamics and physical properties of the atomic interstellar medium (ISM) set the conditions under which molecular clouds and eventually stars form. It is, therefore, critical to investigate the relationship between the atomic and molecular gas phase to understand the global star formation process. Aims. Using the high angular resolution data from The H I/OH/Recombination (THOR) line survey of the Milky Way, we aim to constrain the kinematic and physical properties of the cold atomic hydrogen gas phase toward the inner Galactic plane. Methods. H I self-absorption (HISA) has proven to be a viable method to detect cold atomic hydrogen clouds in the Galactic plane. With the help of a newly developed self-absorption extraction routine (astroSABER), we built upon previous case studies to identify H I self-absorption toward a sample of giant molecular filaments (GMFs). Results. We find the cold atomic gas to be spatially correlated with the molecular gas on a global scale. The column densities of the cold atomic gas traced by HISA are usually on the order of 1020 cm−2 whereas those of molecular hydrogen traced by 13CO are at least an order of magnitude higher. The HISA column densities are attributed to a cold gas component that accounts for a fraction of ~5% of the total atomic gas budget within the clouds. The HISA column density distributions show pronounced log-normal shapes that are broader than those traced by H I emission. The cold atomic gas is found to be moderately supersonic with Mach numbers of approximately a few. In contrast, highly supersonic dynamics drive the molecular gas within most filaments. Conclusions. While H I self-absorption is likely to trace just a small fraction of the total cold neutral medium within a cloud, probing the cold atomic ISM by the means of self-absorption significantly improves our understanding of the dynamical and physical interaction between the atomic and molecular gas phase during cloud formation.

A Herschel study of G214.5−1.8: a young, cold, and quiescent giant molecular filament on the shell of a H i superbubble

ABSTRACT We present an analysis of the outer Galaxy giant molecular filament (GMF) G214.5−1.8 (G214.5) using Herschel data. We find that G214.5 has a mass of ∼16 000 M⊙, yet hosts only 15 potentially protostellar 70-$\mu$m sources, making it highly quiescent compared to equally massive clouds such as Serpens and Mon R2. We show that G214.5 has a unique morphology, consisting of a narrow ‘Main filament’ running north–south and a perpendicular ‘Head’ structure running east–west. We identify 33 distinct massive clumps from the column density maps, 8 of which are protostellar. However, the star formation activity is not evenly spread across G214.5 but rather predominantly located in the Main filament. Studying the Main filament in a manner similar to previous works, we find that G214.5 is most like a ‘Bone’ candidate GMF, highly elongated and massive, but it is colder and narrower than any such GMF. It also differs significantly due to its low fraction of high column density gas. Studying the radial profile, we discover that G214.5 is highly asymmetric and resembles filaments which are known to be compressed externally. Considering its environment, we find that G214.5 is co-incident, spatially and kinematically, with a H i superbubble. We discuss how a potential interaction between G214.5 and the superbubble may explain G214.5’s morphology, asymmetry and, paucity of dense gas and star formation activity, highlighting the intersection of a bubble-driven interstellar medium paradigm with that of a filament paradigm for star formation.

Simultaneous Evidence of Edge Collapse and Hub-filament Configurations: A Rare Case Study of a Giant Molecular Filament, G45.3+0.1

We study multiwavelength and multiscale data to investigate the kinematics of molecular gas associated with the star-forming complexes G045.49+00.04 (G45E) and G045.14+00.14 (G45W) in the Aquila constellation. An analysis of the FUGIN 13CO(1–0) line data unveils the presence of a giant molecular filament (GMF G45.3+0.1; length ∼75 pc, mass ∼1.1 × 106 M ⊙) having a coherent velocity structure at [53, 63] km s−1. The GMF G45.3+0.1 hosts G45E and G45W complexes at its opposite ends. We find large-scale velocity oscillations along GMF G45.3+0.1, which also reveals the linear velocity gradients of −0.064 and +0.032 km s−1 pc−1 at its edges. The photometric analysis of point-like sources shows the clustering of young stellar object (YSO) candidate sources at the filament’s edges where the presence of dense gas and H ii regions are also spatially observed. The Herschel continuum maps along with the CHIMPS 13CO(3–2) line data unravel the presence of parsec scale hub-filament systems (HFSs) in both sites, G45E and G45W. Our study suggests that the global collapse of GMF G45.3+0.1 is end dominated, with the addition to the signature of global nonisotropic collapse at the edges. Overall, GMF G45.3+0.1 is the first observational sample of filament where the edge-collapse and the hub-filament configurations are simultaneously investigated. These observations open the new possibility of massive star formation, including the formation of HFSs.

The “Maggie” filament: Physical properties of a giant atomic cloud

Context. The atomic phase of the interstellar medium plays a key role in the formation process of molecular clouds. Due to the line-of-sight confusion in the Galactic plane that is associated with its ubiquity, atomic hydrogen emission has been challenging to study. Aims. We investigate the physical properties of the “Maggie” filament, a large-scale filament identified in H I emission at line-of-sight velocities, vLSR ~−54 km s−1. Methods. Employing the high-angular resolution data from The H I/OH Recombination line survey of the inner Milky Way (THOR), we have been able to study H I emission features at negative vLSR velocities without any line-of-sight confusion due to the kinematic distance ambiguity in the first Galactic quadrant. In order to investigate the kinematic structure, we decomposed the emission spectra using the automated Gaussian fitting algorithm GAUSSPY+. Results. We identify one of the largest, coherent, mostly atomic H I filaments in the Milky Way. The giant atomic filament Maggie, with a total length of 1.2 ± 0.1 kpc, is not detected in most other tracers, and it does not show signs of active star formation. At a kinematic distance of 17 kpc, Maggie is situated below (by ≈500 pc), but parallel to, the Galactic H I disk and is trailing the predicted location of the Outer Arm by 5−10 km s−1 in longitude-velocity space. The centroid velocity exhibits a smooth gradient of less than ±3 km s−1 (10 pc)−1 and a coherent structure to within ±6 km s−1. The line widths of ~10 km s−1 along the spine of the filament are dominated by nonthermal effects. After correcting for optical depth effects, the mass of Maggie’s dense spine is estimated to be 7.2−1.9+2.5 × 105 M⊙. The mean number density of the filament is ~4 cm−3, which is best explained by the filament being a mix of cold and warm neutral gas. In contrast to molecular filaments, the turbulent Mach number and velocity structure function suggest that Maggie is driven by transonic to moderately supersonic velocities that are likely associated with the Galactic potential rather than being subject to the effects of self-gravity or stellar feedback. The probability density function of the column density displays a log-normal shape around a mean of ⟨NH I⟩ = 4.8 × 1020 cm−2, thus reflecting the absence of dominating effects of gravitational contraction. Conclusions. While Maggie’s origin remains unclear, we hypothesize that Maggie could be the first in a class of atomic clouds that are the precursors of giant molecular filaments.

Nobeyama 45 m Local Spur CO survey. I. Giant molecular filaments and cluster formation in the Vulpecula OB association

Abstract We have performed new large-scale 12CO, 13CO, and C18O J = 1–0 observations toward the Vulpecula OB association (l ∼ 60°) as part of the Nobeyama 45 m Local Spur CO survey project. Molecular clouds are distributed over ∼100 pc, with local peaks at the Sh 2-86, Sh 2-87, and Sh 2-88 high-mass star-forming regions in the Vulpecula complex. The molecular gas is associated with the Local Spur, which corresponds to the nearest inter-arm region located between the Local Arm and the Sagittarius Arm. We discovered new giant molecular filaments (GMFs) in Sh 2-86, with a length of ∼30 pc, width of ∼5 pc, and molecular mass of $\sim\!\! 4 \times 10^4\, M_{\odot }$. We also found that Sh 2-86 contains the three velocity components at 22, 27, and 33 km s−1. These clouds and GMFs are likely to be physically associated with Sh 2-86 because they have high 12CO J = 2–1 to J = 1–0 intensity ratios and coincide with the infrared dust emission. The open cluster NGC 6823 exists at the common intersection of these clouds. We argue that the multiple cloud interaction scenario, including GMFs, can explain cluster formation in the Vulpecula OB association.

A Kiloparsec-scale Molecular Wave in the Inner Galaxy: Feather of the Milky Way?

Abstract We report the discovery of a velocity coherent, kiloparsec-scale molecular structure toward the Galactic center region with an angular extent of 30° and an aspect ratio of 60:1. The kinematic distance of the CO structure ranges between 4.4 and 6.5 kpc. Analysis of the velocity data and comparison with the existing spiral arm models support that a major portion of this structure is either a subbranch of the Norma arm or an interarm giant molecular filament, likely to be a kiloparsec-scale feather (or spur) of the Milky Way, similar to those observed in nearby spiral galaxies. The filamentary cloud is at least 2.0 kpc in extent, considering the uncertainties in the kinematic distances, and it could be as long as 4 kpc. The vertical distribution of this highly elongated structure reveals a pattern similar to that of a sinusoidal wave. The exact mechanisms responsible for the origin of such a kiloparsec-scale filament and its wavy morphology remains unclear. The distinct wave-like shape and its peculiar orientation makes this cloud, named as the Gangotri wave, one of the largest and most intriguing structures identified in the Milky Way.

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.

A cold and diffuse giant molecular filament in the region of l = 41°, b = −1°

Data of 12CO/13CO/C18O J = 1 → 0 emission toward the Galactic plane region of l = 35° to 45° and b = –5° to +5° are available with the Milky Way Imaging Scroll Painting (MWISP) project. Using the data, we found a giant molecular filament (GMF) around l ≈ 38° ∼ 42°, b ≈ –3.5° ∼ 0°, VLSR ≈ 27 ∼ 40 km s−1, named the GMF MWISP G041–01. At a distance of 1.7 kpc, the GMF is about 160 pc long. With a median excitation temperature about 7.5 K and a median column density about 1021 cm−2, this GMF is very cold and very diffuse compared to known GMFs. Using the morphology in the data cube, the GMF is divided into four components among which three show filamentary structure. Masses of the components are 103 ∼ 104 M⊙, with a total mass for the whole filament being about 7 × 104 M⊙ from the local thermodynamic equilibrium method. 13CO cores inside each component are searched. Virial parameters are about 2.5 for these cores and have a power-law index of –0.34 against the mass. The mass fraction of dense cores traced by 13CO to the diffuse clouds traced by 12CO is about 7% for all components of the GMF. We found signatures of possible large scale filament-filament collision in the GMF.

Dense gas in a giant molecular filament

Context. Recent surveys of the Galactic plane in the dust continuum and CO emission lines reveal that large (≳50 pc) and massive (≳105 M⊙) filaments, know as giant molecular filaments (GMFs), may be linked to Galactic dynamics and trace the mid-plane of the gravitational potential in the Milky Way. Yet our physical understanding of GMFs is still poor. Aims. We investigate the dense gas properties of one GMF, with the ultimate goal of connecting these dense gas tracers with star formation processes in the GMF. Methods. We imaged one entire GMF located at l ~ 52–54° longitude, GMF54 (~68 pc long), in the empirical dense gas tracers using the HCN(1–0), HNC(1–0), and HCO+(1–0) lines, and their 13C isotopologue transitions, as well as the N2H+(1–0) line. We studied the dense gas distribution, the column density probability density functions (N-PDFs), and the line ratios within the GMF. Results. The dense gas molecular transitions follow the extended structure of the filament with area filling factors between 0.06 and 0.28 with respect to 13CO(1–0). We constructed the N-PDFs of H2 for each of the dense gas tracers based on their column densities and assumed uniform abundance. The N-PDFs of the dense gas tracers appear curved in log–log representation, and the HCO+ N-PDF has the flattest power-law slope index. Studying the N-PDFs for sub-regions of GMF54, we found an evolutionary trend in the N-PDFs that high-mass star-forming and photon-dominated regions have flatter power-law indices. The integrated intensity ratios of the molecular lines in GMF54 are comparable to those in nearby galaxies. In particular, the N2H+/13CO ratio, which traces the dense gas fraction, has similar values in GMF54 and all nearby galaxies except Ultraluminous Infrared Galaxies. Conclusions. As the largest coherent cold gaseous structure in our Milky Way, GMFs, are outstanding candidates for connecting studies of star formation on Galactic and extragalactic scales. By analyzing a complete map of the dense gas in a GMF we have found that: (1) the dense gas N-PDFs appear flatter in more evolved regions and steeper in younger regions, and (2) its integrated dense gas intensity ratios are similar to those of nearby galaxies.

ALMA Observations of Giant Molecular Clouds in M33. I. Resolving Star Formation Activities in the Giant Molecular Filaments Possibly Formed by a Spiral Shock

Abstract We report molecular line and continuum observations toward one of the most massive giant molecular clouds (GMCs), GMC-16, in M33 using ALMA with an angular resolution of 0.″44 × 0.″27 (∼2 pc × 1 pc). We have found that the GMC is composed of several filamentary structures in 12CO and 13CO(J = 2–1). The typical length, width, and total mass are ∼50–70 pc, ∼5–6 pc, and ∼105 M ⊙, respectively, which are consistent with those of giant molecular filaments (GMFs) as seen in the Galactic GMCs. The elongations of the GMFs are roughly perpendicular to the direction of the galaxy’s rotation, and several H ii regions are located at the downstream side relative to the filaments with an offset of ∼10–20 pc. These observational results indicate that the GMFs are considered to be produced by a galactic spiral shock. The 1.3 mm continuum and C18O(J = 2–1) observations detected a dense clump with the size of ∼2 pc at the intersection of several filamentary clouds, which is referred to as the “hub filament,” possibly formed by a cloud–cloud collision. A strong candidate for protostellar outflow in M33 has also been identified at the center of the clump. We have successfully resolved the parsec-scale local star formation activity in which the galactic scale kinematics may induce the formation of the parental filamentary clouds.

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.

Synthetic Large-scale Galactic Filaments: On Their Formation, Physical Properties, and Resemblance to Observations

Abstract Using a population of large-scale filaments extracted from an AREPO simulation of a Milky Way–like galaxy, we seek to understand the extent to which observed large-scale filament properties (with lengths ≳100 pc) can be explained by galactic dynamics alone. From an observer’s perspective in the disk of the galaxy, we identify filaments forming purely due to galactic dynamics, without the effects of feedback or local self-gravity. We find that large-scale galactic filaments are intrinsically rare, and we estimate that at maximum approximately one filament per kpc2 should be identified in projection, when viewed from the direction of our Sun in the Milky Way. In this idealized scenario, we find filaments in both the arm and interarm regions and hypothesize that the former may be due to gas compression in the spiral potential wells, with the latter due to differential rotation. Using the same analysis pipeline applied previously to observations, we analyze the physical properties of large-scale galactic filaments and quantify their sensitivity to projection effects and galactic environment (i.e., whether they lie in the arm or interarm regions). We find that observed “Giant Molecular Filaments” are consistent with being non-self-gravitating structures dominated by galactic dynamics. Straighter, narrower, and denser “Bone-like” filaments, like the paradigmatic Nessie filament, have similar column densities, velocity gradients, and galactic plane heights (z ≈ 0 pc) to those in our simple model, but additional physical effects (such as feedback and self-gravity) must be invoked to explain their lengths and widths.

Star-forming content of the giant molecular filaments in the Milky Way

Context.Through observations numerous giant molecular filaments (GMFs) have been discovered in the Milky Way. Their role in the Galactic star formation and Galaxy-scale evolution of dense gas is unknown.Aims.We investigate systematically the star-forming content of all currently known GMFs. This allows us to estimate the star formation rates (SFRs) of the GMFs and to establish relationships between the SFRs and the GMF properties.Methods.We identified and classified the young stellar object (YSO) population of each GMF using multiwavelength photometry from near- to far-infrared. We estimated the total SFRs assuming a universal and fully sampled initial mass function and luminosity function.Results.We uniformly estimate the physical properties of 57 GMFs. The GMFs show correlations between the13CO line width, mass, and size, similar to Larson’s relations. We identify 36 394 infrared excess sources in 57 GMFs and obtain SFRs for 46 GMFs. The median SFR surface density (ΣSFR) and star formation efficiency (SFE) of GMFs are 0.62M⊙Myr−1pc−2and 1%, similar to the nearby star-forming clouds. The star formation rate per free-fall time of GMFs is between 0.002−0.05 with the median value of 0.02. We also find a strong correlation between SFR and dense gas mass that is defined as gas mass above a visual extinction of 7 mag, which suggests that the SFRs of the GMFs scale similarly with dense gas as those of nearby molecular clouds. We also find a strong correlation between the mean SFR per unit length and dense gas mass per unit length. The origin of this scaling remains unknown, calling for further studies that can link the structure of GMFs to their SF activity and explore the differences between GMFs and other molecular clouds.

Physical Properties of Large-scale Galactic Filaments

Abstract The characterization of our Galaxy’s longest filamentary gas features has been the subject of several studies in recent years, producing not only a sizable sample of large-scale filaments, but also confusion as to whether all these features (e.g., “Bones,” “Giant Molecular Filaments”) are the same. They are not. We undertake the first standardized analysis of the physical properties (H2 column densities, dust temperatures, morphologies, radial column density profiles) and kinematics of large-scale filaments in the literature. We expand and improve upon prior analyses by using the same data sets, techniques, and spiral arm models to disentangle the filaments’ inherent properties from selection criteria and methodology. Our results suggest that the myriad filament-finding techniques are uncovering different physical structures, with length (11–269 pc), width (1–40 pc), mass ( ), aspect ratio (3:1–117:1), and high column density fraction (0.2%–100%) varying by over an order of magnitude across the sample of 45 filaments. We develop a radial profile-fitting code, RadFil, which is publicly available. We also perform a position–position–velocity (p–p–v) analysis on a subsample and find that while 60%–70% lie spatially in the plane of the Galaxy, only 30%–45% concurrently exhibit spatial and kinematic proximity to spiral arms. In a parameter space defined by aspect ratio, dust temperature, and column density, we broadly distinguish three filament categories, which could indicate different formation mechanisms or histories. Highly elongated “Bone-like” filaments show the most potential for tracing gross spiral structure (e.g., arms, spurs), while other categories could be large concentrations of molecular gas (giant molecular clouds, core complexes).

The evolution of giant molecular filaments

In recent years there has been a growing interest in studying giant molecular filaments (GMFs), which are extremely elongated (> 100pc in length) giant molecular clouds (GMCs). They are often seen as inter-arm features in external spiral galaxies, but have been tentatively associated with spiral arms when viewed in the Milky Way. In this paper, we study the time evolution of GMFs in a high-resolution section of a spiral galaxy simulation, and their link with spiral arm GMCs and star formation, over a period of 11Myrs. The GMFs generally survive the inter-arm passage, although they are subject to a number of processes (e.g. star formation, stellar feedback and differential rotation) which can break the giant filamentary structure into smaller sections. The GMFs are not gravitationally bound clouds as a whole, but are, to some extent, confined by external pressure. Once they reach the spiral arms, the GMFs tend to evolve into more substructured spiral arm GMCs, suggesting that GMFs may be precursors to arm GMCs. Here, they become incorporated into the more complex and almost continuum molecular medium that makes up the gaseous spiral arm. Instead of retaining a clear filamentary shape, their shapes are distorted both by their climb up the spiral potential and their interaction with the gas within the spiral arm. The GMFs do tend to become aligned with the spiral arms just before they enter them (when they reach the minimum of the spiral potential), which could account for the observations of GMFs in the Milky Way.

Giant molecular filaments in the Milky Way

Filamentary structures are common morphological features of the cold, molecular interstellar medium (ISM). Recent studies have discovered massive, hundred-parsec-scale filaments that may be connected to the large-scale, Galactic spiral arm structure. Addressing the nature of these Giant Molecular Filaments (GMFs) requires a census of their occurrence and properties. We perform a systematic search of GMFs in the fourth Galactic quadrant and determine their basic physical properties. We identify GMFs based on their dust extinction signatures in near- and mid-infrared and velocity structure probed by ^{13}CO line emission. We use the ^{13}CO line emission and ATLASGAL dust emission data to estimate the total and dense gas masses of the GMFs. We combine our sample with an earlier sample from literature and study the Galactic environment of the GMFs. We identify nine GMFs in the fourth Galactic quadrant; six are located in the Centaurus spiral arm and three in inter-arm regions. Combining this sample with an earlier study using the same identification criteria in the first Galactic quadrant results in 16 GMFs, nine of which are located within spiral arms. The GMFs have sizes of 80-160 pc and ^{13}CO-derived masses between 5-90 x 10^{4} Msun. Their dense gas mass fractions are between 1.5-37%, being higher in the GMFs connected to spiral arms. We also compare the different GMF-identification methods and find that emission and extinction based techniques overlap only partially, highlighting the need to use both to achieve a complete census.

Structures of GMC W 37

We carried out observations toward the giant molecular cloud W 37 with the J = 1 − 0 transitions of 12CO, 13CO and C18O using the 13.7m single-dish telescope at the Delingha station of Purple Mountain Observatory. Based on these CO lines, we calculated the column densities and cloud masses for molecular clouds with radial velocities around +20 km s−1. The gas mass of W 37, calculated from 13 CO emission, is 1.7 × 105 M⊙, above the criterion to be considered a giant molecular cloud. The dense ridge of W 37 is a dense filament, which is supercritical in terms of linear mass ratio. Dense clumps found by C18O emission are aligned along the dense ridge at regular intervals of about 2.8 pc, similar to the clump separation caused by large-scale ‘sausage instability’. We confirm the identification of the giant molecular filament (GMF) G 18.0-16.8 and find a new giant filament, G 16.5−15.8, located ∼ 0.7° to the west of G 18.0−16.8. Both GMFs are not gravitationally bound, as indicated by their low linear mass ratio (∼ 80 M⊙ pc−1). We compared the gas temperature map with the dust temperature map from Herschel images, and found similar structures. The spatial distributions of class I objects and the dense clumps are reminiscent of triggered star formation occurring in the northwestern part of W 37, which is close to NGC 6611.

What can simulated molecular clouds tell us about real molecular clouds?

We study the properties of giant molecular clouds (GMCs) from a smoothed particle hydrodynamics simulation of a portion of a spiral galaxy, modelled at high resolution, with robust representations of the physics of the interstellar medium. We examine the global molecular gas content of clouds, and investigate the effect of using CO or H2 densities to define the GMCs. We find that CO can reliably trace the high-density H2 gas, but misses less dense H2 clouds. We also investigate the effect of using 3D CO densities versus CO emission with an observer's perspective, and find that CO-emission clouds trace well the peaks of the actual GMCs in 3D, but can miss the lower density molecular gas between density peaks which is often CO-dark. Thus, the CO emission typically traces smaller clouds within larger GMC complexes. We also investigate the effect of the galactic environment (in particular the presence of spiral arms), on the distribution of GMC properties, and we find that the mean properties are similar between arm and inter-arm clouds, but the tails of some distributions are indicative of intrinsic differences in the environment. We find highly filamentary clouds (similar to the giant molecular filaments of our Galaxy) exclusively in the inter-arm region, formed by galactic shear. We also find that the most massive GMC complexes are located in the arm, and that as a consequence of more frequent cloud interactions/mergers in the arm, arm clouds are more sub-structured and have higher velocity dispersions than inter-arm clouds.