We investigated the kinematics and dynamics of gas structures on galaxy-cloud scales in two spiral galaxies NGC5236 (M83) and NGC4321 (M100) using CO (2$-$1) line. We utilised the FILFINDER algorithm on integrated intensity maps for the identification of filaments in two galaxies. Clear fluctuations in velocity and density were observed along these filaments, enabling the fitting of velocity gradients around intensity peaks. The variations in velocity gradient across different scales suggest a gradual and consistent increase in velocity gradient from large to small scales, indicative of gravitational collapse, something also revealed by the correlation between velocity dispersion and column density of gas structures. Gas structures at different scales in the galaxy may be organised into hierarchical systems through gravitational coupling. All the features of gas kinematics on galaxy-cloud scale are very similar to that on cloud-clump and clump-core scales studied in previous works. Thus, the interstellar medium from galaxy to dense core scales presents multi-scale/hierarchical hub-filament structures. Like dense core as the hub in clump, clump as the hub in molecular cloud, now we verify that cloud or cloud complex can be the hub in spiral galaxies. Although the scaling relations and the measured velocity gradients support the gravitational collapse of gas structures on galaxy-cloud scales, the collapse is much slower than a pure free-fall gravitational collapse.

We present observations of the Mopra carbon monoxide (CO) survey of the Southern Galactic Plane, covering Galactic longitudes spanning $l = 250^{\circ}$ ($-110^{\circ}$) to $l = 355^{\circ}$ ($-5^{\circ}$), with a latitudinal coverage of at least $|b|<1^\circ$, totalling an area of $>$210 deg$^{2}$. These data have been taken at 0.6 arcmin spatial resolution and 0.1 km s$^{-1}$ spectral resolution, providing an unprecedented view of the molecular gas clouds of the Southern Galactic Plane in the 109–115 GHz $J = 1-0$ transitions of $^{12}$CO, $^{13}$CO, C$^{18}$O, and C$^{17}$O.

Context: Theoretical scenarios describe the phenomenon of mass accumulation by high-mass young stellar objects (HMYSOs) through disk accretion. Aims: To find out whether the rotation of the core around hyper-compact (HC) HII regions is common Methods: The molecular core G301.1364-00.2249A was selected as the subject of investigation. Observations were carried out on CH3CN, SO2, H29α radio recombination line and continuum emission. Analysis involved the “Moment 0, 1, 2" and “Population-diagram” methods. Result: The structures of G301.1364-00.2249A exhibited clear definition. CH3CN moment 0 images revealed multiple enhanced emission regions and an absorption area. The moment 1 image depicted a velocity gradient from southeast to northwest. Conclusion: The molecular gas has a rotational motion in the direction from the southeast to northwest. The rotational temperature was measured at 293 K.

In the last 20 years a small group of 6.7GHz methanol maser sources displaying periodic variability have been identified. This variability is thought to reflect local processes linked to star formation. A number of models have been proposed e.g. colliding wind binary, protostellar pulsation, accretion on binary system. Recent studies of known sources as well as non-periodic flaring masers suggest an episodic accretion as a driving mechanism. We present the results of VLBI observation program aimed at studying known periodic methanol maser sources. High resolution maps of emission, source morphology and evolution in time will be discussed. Those results will help us fully understand the nature of maser periodicity in star forming regions.

A pilot project has been proceeded to map $1\, \textrm{deg}^2$ on the Galactic plane for radio recombination lines (RRLs) using the Five-hundred-metre Aperture Spherical Telescope (FAST). The motivation is to verify the techniques and reliabilities for a large-scale Galactic plane RRL survey with FAST aiming to investigate the ionised environment in the Galaxy. The data shows that the bandpass of the FAST 19 beam L-band is severely affected by radio frequency interferences and standing wave ripples, which can hardly be corrected by traditional low order polynomials. In this paper, we investigate a series of penalised least square (PLS) based baseline correction methods for radio astronomical spectra that usually contain weak signals with high level of noise. Three promising penalised least squares based methods, AsLS, arPLS, and asPLS are evaluated. Adopting their advantages, a modified method named rrlPLS is developed to optimise the baseline fitting to our RRL spectra. To check their effectiveness, the four methods are tested by simulations and further verified using observed data sets. It turns out that the rrlPLS method, with optimised parameter $\lambda=2\times10^8$ , reveals the most sensitive and reliable emission features in the RRL map. By injecting artificial line profiles into the real data cube, a further evaluation of profile distortion is conducted for rrlPLS. Comparing to simulated signals, the processed lines with low signal-to-noise ratio are less affected, of which the uncertainties are mainly caused by the rms noise. The rrlPLS method will be applied for baseline correction in future data processing pipeline of FAST RRL survey. Configured with proper parameters, the rrlPLS technique verified in this work may also be used for other spectroscopy projects.

The processes of star formation and feedback take place on the scales of giant molecular clouds (GMCs; ~ 100 pc) within galaxies and play a major role in governing galaxy evolution. By applying a robust statistical method to PHANGS observations, we systematically measure the evolutionary timeline from molecular clouds to exposed young stellar regions, across an unprecedented sample of 54 galaxies. These timescales depend on galaxy environment, revealing the role of galactic-scale dynamical processes in the small-scale GMC evolution. Furthermore, in the 5 nearest galaxies of our sample, we have refined the GMC timeline further and established the duration of the heavily obscured phase, using 24 μm emission. These results represent a major first step towards a comprehensive picture of cloud assembly and feedback, which will be extended to 19 more galaxies with our ongoing JWST Large Program.

Star formation, from cold giant molecular clouds to diverse population of stars, is a complex process involving a wide variety of physical processes. In this work, we constrain the link between the gas-star formation cycle and several secular and environmental probe of galaxies. Specifically, we quantify the spatial correlation between molecular gas and star-forming regions for 49 nearby galaxies using the ALMA and narrowband-Hα imaging from the PHANGS survey. At the resolution (150 pc) at which the individual molecular clouds and star-forming regions can be identified, we find that molecular clouds and star-forming regions do not necessarily coexist. The decoupled molecular clouds and star-forming regions are a signature of evolutionary cycling and feedback of the star formation process. Therefore, the impact of galactic-scale conditions and environments must be considered for a complete understanding of how stars form in galaxies and how this process influences the evolution of the host galaxies.

The depletion of CO molecules is observed in infrared dark clouds. However, only few exsamples are found in pc-scale. An NH3 emission is one of good counter parts of C18O because of similar effective critical density. Our NH3 observations of a molecular filament associated with CMa OB1 or KAG 71, which is a target of Kagoshima Galactic Object survey with Nobeyama 45-m telescope by Mapping in Ammonia lines (KAGONMA) project. Although NH3 data shows similarity in morphology with infrared data suggesting no depletion, C18O in the clumps 4 and 6 are weaker than expected based on NH3 data. After examining the dissipation of the high-density gas, photodissociation, and depletion, we concluded that CO is depleted at least in the clump 4. It is a new example of depletion in pc-scale.

We studied the probability distribution function of the column density (N-PDF) of molecular clouds based on a fit with a multi-log-normal function using the Nobeyama 45-m Cygnus X CO survey data. We identified 124 molecular clouds in 13CO data using the DENDROGRAM and SCIMES algorithms. The N-PDF was constructed for 11 extended (≥ 0.4 deg2) molecular clouds of these identified clouds. We found that every N-PDF is well-fitted with one or two log-normal (LN) distributions. We investigated the distributions of the column density, C18O dense cores, and radio continuum source in each cloud and found that the N-PDF was less correlated with the star-forming activity. The LN N-PDF parameters showed two impressive features. First, the LN distribution at the low-density part had the same mean column density (∼1021.5 cm−2 ) for almost all the molecular clouds. Second, the wider LN distribution tended to show the lower mean density of the structures.

We have carried out ALMA observations toward the environments of G333.0162+00.7615 which was considered as a candidate of high-mass young stellar object (HMYSO) in previous studies. Our dust continuum, molecular line emission and radio recombination line emission observations show that this source is not HMYSO associated with hypercompact (HC) HII regions. Instead, we discovered two new hot cores associate with earliest stages of high mass star formation region. We estimated the rotational temperatures of these cores about 270 K from J=14→13 rotational transition of CH3CN ladder. The moment maps show velocity gradients confirming that this cores are rotating.

We employ the Feedback In Realistic Environments (FIRE-2) physics model to study how the properties of giant molecular clouds (GMCs) evolve during galaxy mergers. Due to the rarity of mergers in the local Universe, samples of nearby merging galaxies suitable for studies of individual GMCs are limited. Idealized simulations provide us with a new window to study GMC evolution during a merger, and assist in interpreting observations. We conduct a pixel-by-pixel analysis of the simulated molecular gas properties in both undisturbed control galaxies and galaxy mergers. The simulated GMC-pixels follow a similar trend in a diagram of velocity dispersion (σv) versus gas surface density (Σmol) as observed in normal spiral galaxies in the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey. For simulated mergers, we see a significant increase in both the Σmol and σv for GMC-pixels by a factor of 5 – 10, which put these pixels to be above the trend of PHANGS galaxies in the σv vs Σmol diagram. This deviation indicates that GMCs in the simulated merger are more gravitationally unbound and have higher virial parameter (αvir) of 10 – 100, which is much larger than that of simulated control galaxies. Furthermore, we find that the increase in αvir generally happens at the same time as the increase in global star formation rate (SFR), which suggests feedback is playing a role in dispersing the gas. The correspondence between high αvir and SFR also suggests some other physical mechanisms besides self-gravity are helping the GMCs in starburst mergers to collapse and form stars.

To understand the physical properties of the interstellar medium (ISM) in various scales, we should investigate it with pc-scale resolution over kpc scale coverage. Here, we report the sub-kpc scale Gas Density Histogram (GDH) of the Milky Way. GDH is a histogram of averaged density and corresponds to the probability density distribution (PDF) of gas volume density. We use galactic plain survey data (l =10∘− 50∘) at 12CO and 13CO (J = 1 − 0) obtained as a part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45m telescope (FUGIN). With this method and data, we are free from spatial structure and molecular cloud identification. GDH can be well fitted with single or double log-normal distribution; which we call as the low-density log-normal (L-LN) and high-density log-normal (H-LN) components. We found both the H-LN fraction (fH) and L-LN width (σL) along the gas density axis show a coherent structure on the longitude-velocity diagram. It suggests that there is a relationship between the ISM property and kpc scale structure in the Milky Way.

We examine the physical conditions required for the formation of H2 in the solar neighborhood by comparing H i emission and absorption spectra toward 58 lines of sight at b < −5∘ to CO(1–0) and dust data. Our analysis of CO-associated cold and warm neutral medium (CNM and WNM) shows that the formation of CO-traced molecular gas is favored in regions with high column densities where the CNM becomes colder and more abundant. In addition, our comparison to the one-dimensional steady-state H i-to-H2 transition model of Bialy et al. (2016) suggests that only a small fraction of the clumpy CNM participates in the formation of CO-traced molecular gas. Another possible interpretation would be that missing physical and chemical processes in the model could play an important role in H2 formation.

The Galactic TeV ${\gamma}$ -ray source ${\mathrm{HESS\,J}1804{-}216}$ is currently an unidentified source. In an attempt to unveil its origin, we present here the most detailed study of interstellar gas using data from the Mopra Southern Galactic Plane CO Survey, 7- and 12-mm wavelength Mopra surveys and Southern Galactic Plane Survey of HI. Several components of atomic and molecular gas are found to overlap ${\mathrm{HESS\,J}1804{-}216}$ at various velocities along the line of sight. The CS(1-0) emission clumps confirm the presence of dense gas. Both correlation and anti-correlation between the gas and TeV ${\gamma}$ -ray emission have been identified in various gas tracers, enabling several origin scenarios for the TeV ${\gamma}$ -ray emission from ${\mathrm{HESS\,J}1804{-}216}$ . For a hadronic scenario, ${\mathrm{SNR\,G}8.7{-}0.1}$ and the progenitor supernova remnant (SNR) of ${\mathrm{PSR\,J}1803{-}2137}$ require cosmic ray (CR) enhancement factors of ${\mathord{\sim} 50}$ times the solar neighbour CR flux value to produce the TeV ${\gamma}$ -ray emission. Assuming an isotropic diffusion model, CRs from both these SNRs require a slow diffusion coefficient, as found for other TeV SNRs associated with adjacent ISM gas. The morphology of gas located at 3.8 kpc (the dispersion measure distance to ${\mathrm{PSR\,J}1803{-}2137}$ ) tends to anti-correlate with features of the TeV emission from ${\mathrm{HESS\,J}1804{-}216}$ , making the leptonic scenario possible. Both pure hadronic and pure leptonic scenarios thus remain plausible.

We compare the properties of shocked gas in Sgr B2 with maps obtained from 3D simulations of a collision between two fractal clouds. In agreement with 13CO(1-0) observations, our simulations show that a cloud-cloud collision produces a region with a highly turbulent density substructure with an average . Similarly, our numerical multi-channel shock study shows that colliding clouds are efficient at producing internal shocks with velocities of 5 − 50 km s−1 and Mach numbers of ∼ 4 − 40, which are needed to explain the ∼ 10−9 SiO abundances inferred from our SiO(2-1) IRAM observations of Sgr B2. Overall, we find that both the density structure and the shocked gas morphology in Sgr B2 are consistent with a Myr-old cloud-cloud collision. High-velocity shocks are produced during the early stages of the collision and can ignite star formation, while moderate- and low-velocity shocks are important over longer time-scales and can explain the extended SiO emission in Sgr B2.

Stars form in clusters, while planets form in gaseous disks around young stars. Cluster dissolution occurs on longer time scales than disk dispersal. Planet formation thus typically takes place while the host star is still inside the cluster. We explore how the presence of other stars affects the evolution of circumstellar disks. Our numerical approach requires multi-scale and multi-physics simulations where the relevant components and their interactions are resolved. The simulations start with the collapse of a turbulent cloud, from which stars with disks form, which are able to influence each other. We focus on the effect of extinction due to residual cloud gas on the early evolution of circumstellar disks. We find that this extinction protects circumstellar disks against external photoevaporation, but these disks then become vulnerable to dynamic truncation by passing stars. We conclude that circumstellar disk evolution is heavily affected by the early evolution of the cluster.

To form stars in hydrodynamical simulations, we introduce the grouped star formation prescription to convert the grouped sink particles into stars that follow the IMF. We show that this method is robust in different physical scales. Such methods to form stars are likely to become more important as galactic or even cosmological scale simulations begin to probe sub-parsec scales.

Star formation provides insight into the physical processes that govern the transformation of gas into stars. A key missing piece in a predictive theory of star formation is the link between scales of individual stars and star clusters up to entire galaxies. LEGUS is now providing the information to test the overall organization and spatial evolution of star formation. We present our latest findings of using star clusters from LEGUS combined with ALMA CO observations to investigate the transition from molecular gas to star formation in local galaxies. This work paves the way for future JWST observations of the embedded phase of star formation, the last missing ingredient to connect young star clusters and their relation with gas reservoirs. Multi-wavelength studies of local galaxies and their stellar and gas components will help shed light on early phases of galaxy evolution and properties of the ISM at high-z.

Thanks to the remarkable ALMA capabilities and the unique configuration of the Cosmic Snake galaxy behind a massive galaxy cluster, we could resolve molecular clouds down to 30 pc linear physical scales in a typical Milky Way progenitor at z = 1.036, through CO(4–3) observations performed at the ∼ 0.2″ angular resolution. We identified 17 individual giant molecular clouds. These high-redshift molecular clouds are clearly different from their local analogues, with 10–100 times higher masses, densities, and internal turbulence. They are offset from the Larson scaling relations. We argue that the molecular cloud physical properties are dependent on the ambient interstellar conditions particular to the host galaxy. We find these high-redshift clouds in virial equilibrium, and derive, for the first time, the CO-to-H2 conversion factor from the kinematics of independent molecular clouds at z = 1. The measured large clouds gas masses demonstrate the existence of parent gas clouds with masses high enough to allow the in-situ formation of similarly massive stellar clumps seen in the Cosmic Snake galaxy in comparable numbers. Our results support the formation of molecular clouds by fragmentation of turbulent galactic gas disks, which then become the stellar clumps observed in distant galaxies.

We investigate the interstellar medium towards seven TeV gamma-ray sources thought to be pulsar wind nebulae using Mopra molecular line observations at 7 mm [CS(1–0), SiO(1–0, v = 0)], Nanten CO(1–0) data and the Southern Galactic Plane Survey/GASS Hi survey. We have discovered several dense molecular clouds co-located to these TeV gamma-ray sources, which allows us to search for cosmic rays coming from progenitor SNRs or, potentially, from pulsar wind nebulae. We notably found SiO(1–0, v = 0) emission towards HESS J1809–193, highlighting possible interaction between the adjacent supernova remnant SNR G011.0–0.0 and the molecular cloud at d ∼ 3.7 kpc. Using morphological features, and comparative studies of our column densities with those obtained from X-ray measurements, we claim a distance d ∼ 8.6 − 9.7kpc for SNR G292.2–00.5, d ∼ 3.5 − 5.6 kpc for PSR J1418–6058 and d ∼ 1.5 kpc for the new SNR candidate found towards HESS J1303–631. From our mass and density estimates of selected molecular clouds, we discuss signatures of hadronic/leptonic components from pulsar wind nebulae and their progenitor SNRs. Interestingly, the molecular gas, which overlaps HESS J1026–582 at d ∼ 5 kpc, may support a hadronic origin. We find however that this scenario requires an undetected cosmic-ray accelerator to be located at d < 10 pc from the molecular cloud. For HESS J1809–193, the cosmic rays which have escaped SNR G011.0–0.0 could contribute to the TeV gamma-ray emission. Finally, from the hypothesis that at most 20% the pulsar spin down power could be converted into CRs, we find that among the studied pulsar wind nebulae, only those from PSR J1809–1917 could potentially contribute to the TeV emission.