We use magnetohydrodynamical simulations of converging flows to investigate the process of molecular cloud formation and evolution out of the magnetised ISM. Here, we investigate whether the observed subcritical HI clouds can become supercritical and hence allow the formation of stars within them. To do so, we vary the turbulent Mach number of the flows, as well as the initial magnetic field strength. We show that dense cores are able to build up under all conditions, but that star formation in these cores is either heavily delayed or completely suppressed if the initial field strength is B>3 microGauss. To probe the effect of magnetic diffusion, we introduce a tilting angle between the flows and the uniform background magnetic field, which mimics non--ideal MHD effects. Even with highly inclined flows, the formed cores are devoid of star formation, because no magnetically supercritical regions are build up. Hence we conclude, that the problem of how supercritical cloud cores are generated still persists.
In this work, we investigate the observational and algorithmic effects on molecular cloud samples identified from position-position-velocity (PPV) space. By smoothing and cutting off the high quality data of the Milky Way Imaging Scroll Painting (MWISP) survey, we extract various molecular cloud samples from those altered data with the DBSCAN (density-based spatial clustering of applications with noise) algorithm. Those molecular cloud samples are subsequently used to gauge the significance of sensitivity, angular/velocity resolution, and DBSCAN parameters. Two additional surveys, the FCRAO Ou…
Optical stellar polarimetry in the Perseus molecular cloud direction is known to show a fully mixed bi-modal distribution of position angles across the cloud (Goodman et al. 1990). We study the Gaia trigonometric distances to each of these stars and reveal that the two components in position angles trace two different dust clouds along the line of sight. One component, which shows a polarization angle of -37.6 deg +/- 35.2 deg and a higher polarization fraction of 2.0 +/- 1.7%, primarily traces the Perseus molecular cloud at a distance of 300 pc. The other component, which shows a polarization…
Discovery of the Pigtail Molecular Cloud in the Galactic Center
本文报道了在银河系中央分子带（CMZ）中发现的螺旋分子云。这个“辫子”分子云出现在（l，b，V_LSR）~（-0.7deg，+0.0deg，-70 to -30 km/s）处，空间大小为~（20 pc）^2，质量为（2-6）10^5太阳质量。这是迄今为止在银河系中心区域发现的第三个螺旋状气体星云。线强度比表明，猪尾分子云具有略高的温度和/或密度比其他正常云在CMZ。我们还发现了一个高速机翼排放附近的足点的云。我们提出了一个猪尾状分子云的形成模型。它可能与一个扭曲盘绕的磁管有关，这是由于最内层x_1轨道上的云和最外层x_2轨道上的云之间的相互作用。
Molecular clouds toward three Spitzer bubbles S116, S117 and S118: Evidence for the cloud-cloud collision which formed the three \HII \ regions and a 10-pc scale molecular cavity
We report the first evidence for high-mass star formation triggered by collisions of molecular clouds in M33. Using the Atacama Large Millimeter/submillimeter Array, we spatially resolved filamentary structures of giant molecular cloud 37 in M33 using $^{12}$CO($J$ = 2-1), $^{13}$CO($J$ = 2-1), and C$^{18}$O($J$ = 2-1) line emission at a spatial resolution of $\sim$2 pc. There are two individual molecular clouds with a systematic velocity difference of $\sim$6 km s$^{-1}$. Three continuum sources representing up to $\sim$10 high-mass stars with the spectral types of B0V-O7.5V are embedded with…
Evolution of molecular clouds on galaxy-cloud scale revealed by gravitational network analysis : High-mass clouds may deplete nearby gas via accretion or merging
We present a study on the molecular gas towards a bright-rimmed cloud located to the north of the infrared dust bubble N30. Using the emission from the 12CO, 13CO, and C18O J=3-2 line, together with infrared and radio continuum data, we characterized the bubble and the related molecular cloud. In addition, we show an analysis of the behaviour of the abundance ratio 13CO/C18O towards the bright-rimmed cloud, and we search for clues on recent star-formation.
The origin of the molecular clouds orbiting SgrA* is one of the most debated questions about our Galactic Centre. Here, we present the hydrodynamic simulation of a molecular cloud infalling towards SgrA*, performed with the adaptive-mesh-refinement code RAMSES. Through such simulation, we propose that the circumnuclear disc and the +20 km/s cloud originated from the same tidal disruption episode, occurred less than 1 Myr ago. We also show that recent star formation is to be expected in the +20 km/s cloud, as also suggested by recent observations.
We investigate how the dynamical state of molecular clouds relates to host galaxy environment, and how this impacts the star formation efficiency in the Milky Way and seven nearby galaxies. We compile measurements of molecular cloud and host galaxy properties and determine mass-weighted mean cloud properties for entire galaxies and distinct subregions within. We find molecular clouds to be in ambient pressure-balanced virial equilibrium, where clouds in gas-rich, molecular-dominated, high-pressure regions are close to self-virialization, whereas clouds in gas-poor, atomic-dominated, low-pressu…
The determination of absolute and relative distances of molecular clouds along the line-of-sight towards the central molecular zone (CMZ) is crucial to infer its orbital structure, dynamics, and to understand star formation in the clouds. Recent results by Zoccali et al. 2021 suggest that the G0.253 + 0.016 cloud (the Brick) does not belong to the CMZ. This motivated us to cross check their results computing the absolute and relative distance to the Brick and also to other two molecular clouds (the 50 km/s, and the 20 km/s clouds), and discuss their CMZ membership. We used the colour magnitude…
We present new ${ }^{12}$CO(J=1-0) observations of Henize 2-10, a blue compact dwarf galaxy about 8.7 Mpc away, taken with the Atacama Large Millimeter Array. These are the highest spatial and spectral resolution observations, to date, of the molecular gas in this starburst galaxy. We measure a molecular mass of $1.2\times10^8 M_\odot$ in Henize 2-10, and most of the molecular gas is contained within a region having a size of about 310 pc. We use the CPROPS algorithm to identify 119 resolved giant molecular clouds distributed throughout the galaxy, and the molecular gas contained within these …
Microquasar SS 433 located at the geometric center of radio nebula W50 is a suitable source for investigating the physical process of how galactic jets affect the surrounding interstellar medium (ISM). Previous studies have searched for evidence of the interaction between the SS 433 jet and ISM, such as neutral hydrogen gas and molecular clouds; however, it is still unclear which ISM interacts with the jet. We looked for new molecular clouds that possibly interact at the terminal of the SS 433 eastern jet using the Nobeyama 45-m telescope and the Atacama Submillimeter Telescope Experiment (AST…
Recent large-area, deep CO surveys in the Galactic disk have revealed the formation of ~50 high-mass stars or clusters triggered by cloud-cloud collisions (CCCs). Although the Galactic Center (GC) -- which contains the highest volume density of molecular gas -- is the most favorable place for cloud collisions, systematic studies of CCCs in that region are still untouched. Here we report for the first time evidence of CCCs in the common foot point of molecular loops 1 and 2 in the GC. We have investigated the distribution of molecular gas toward the foot point by using a methodology for identif…
Molecular clouds are a fundamental ingredient of galaxies: they are the channels that transform the diffuse gas into stars. The detailed process of how they do it is not completely understood. We review the current knowledge of molecular clouds and their substructure from scales $\sim~$1~kpc down to the filament and core scale. We first review the mechanisms of cloud formation from the warm diffuse interstellar medium down to the cold and dense molecular clouds, the process of molecule formation and the role of the thermal and gravitational instabilities. We also discuss the main physical mech…
We study the physical drivers of slow molecular cloud mergers within a simulation of a Milky Way-like galaxy in the moving-mesh code Arepo, and determine the influence of these mergers on the mass distribution and star formation efficiency of the galactic cloud population. We find that 83 per cent of these mergers occur at a relative velocity below 5 km/s, and are associated with large-scale atomic gas flows, driven primarily by (1) expanding bubbles of hot, ionised gas caused by supernova explosions and (2) galactic rotation. The major effect of these mergers is to aggregate molecular mass in…
We use passive gas tracer particles in an Arepo simulation of a dwarf spiral galaxy to relate the Lagrangian evolution of star-forming gas parcels and their H2 molecules to the evolution of their host giant molecular clouds. We find that the median chemical lifetime of H2 is just 4 Myr, independent of the lifetime of its host molecular cloud, which may vary from 1 to 90 Myr, with a substantial portion of all star formation in the galaxy occurring in relatively long-lived clouds. The rapid ejection of gas from around young massive stars by early stellar feedback is responsible for this short H2…
$Context.$ The traditional approach to characterize the structure of molecular clouds is to map their line emission. $Aims.$ We aim to test and apply a stratified random sampling technique that can characterize the line emission from molecular clouds more efficiently than mapping. $Methods.$ We sampled the molecular emission from the Perseus cloud using the H2 column density as a proxy. We divided the cloud into ten logarithmically spaced column density bins, and we randomly selected ten positions from each bin. The resulting 100 cloud positions were observed with the IRAM 30m telescope, cover…
Examining Turbulence in Galactic Molecular Clouds. II. Turbulence Cascade Beyond the Scale of Giant Molecular Clouds
Purple Mountain Observatory and Key Laboratory of Radio Astronomy, Chinese Academy of Sciences（中国科学院紫金山天文台和射电天文重点实验室）；School of Astronomy and Space Science, University of Science and Technology of China（中国科学技术大学天文与空间科学学院）
We present the highest resolution (1") $^{12}$CO(1-0) observations of molecular gas in the dwarf starburst galaxy NGC 625, to date, obtained with ALMA. Molecular gas is distributed in discrete clouds within an area of $0.4$ kpc$^2$ and does not have well-ordered large-scale motions. We measure a total molecular mass in NGC 625 of $5.3\times 10^6 M_\odot$, assuming a Milky Way CO-to-H$_2$ conversion factor. We use the CPROPS package to identify molecular clouds and measure their properties. The 19 resolved CO clouds have a median radius of 20 pc, a median line width 2.5 km s$^{-1}$, and a media…
We present distances to 76 medium-sized molecular clouds and an extra large-scale one in the second Galactic quadrant ($104.75^\circ <l<150.25^\circ $ and $|b|<5.25^\circ$), 73 of which are accurately measured for the first time. Molecular cloud samples are drawn from $l$-$b$-$V$ space ($-95 < V_{\rm LSR}< 25$ \kms) with the density-based spatial clustering of applications with noise (DBSCAN) algorithm, and distances are measured with the background-eliminated extinction-parallax (BEEP) method using extinctions and Gaia DR2 parallaxes. The range of measured distances to 76 molecular clouds is …
We have obtained column density maps for an unbiased sample of 120 molecular clouds in the third quadrant of the Milky Way mid-plane (b$\le |5|^{\circ}$) within the galactic longitude range from 195$^{\circ}$ to 225$^{\circ}$, using the high sensitivity $^{12}$CO and $^{13}$CO ($J=1-0$) data from the Milky Way Imaging Scroll Painting (MWISP) project. The probability density functions of the molecular hydrogen column density of the clouds, N-PDFs, are fitted with both log-normal (LN) function and log-normal plus power-law (LN+PL) function. The molecular clouds are classified into three categori…
We present a large catalogue of molecular clouds with accurate distance estimates in the southern sky. Based on the three-dimensional (3D) dust extinction map and the best-fit extinction and distance information of over 17 million stars presented in Guo et al., we have identified 250 dust/molecular clouds in the southern sky using a hierarchical structure identification algorithm. Amongst them, 71 clouds locating at high Galactic latitudes ($b < -10^\circ$). We have estimated the distances to the clouds by fitting the extinction versus distance profiles of the lines of sight overlapping with t…
Simulations of molecular clouds often begin from highly idealised initial conditions, such as a uniform-density sphere with an artificially imposed turbulent velocity field. While the resulting structures may appear qualitatively similar to those detected in continuum and line observations, it is unclear whether they are genuinely representative of real molecular clouds. Recent observational work has discovered a tight, often close-to-linear relationship between the integrated intensity of molecular lines and the total column density of the cloud material. We combine magnetohydrodynamical simu…
