ISMGCC: Finding Gas Structures in Molecular Interstellar Medium Using Gaussian Decomposition and Graph Theory

We report the global properties recovered by an ongoing CO survey of the Milky Way Imaging Scroll Painting (MWISP) toward the Galactic outskirts. Our results are also compared to those extracted by a uniform decomposition method from the CfA 1.2 m CO survey and the FCRAO 14 m outer Galaxy survey (OGS). We find that more extended and unseen structures are present in the MWISP data. The total flux across the disk recovered by the MWISP survey is 1.6 times larger than those recovered by the CfA survey and OGS in the case of the same resolution. The discrepancies are scaling with distance. For example, in the outermost Outer Scutum–Centaurus arm, the flux ratios for MWISP to CfA and MWISP to OGS increase up to 43.8 and 7.4, respectively. Nonetheless, the census of molecular gas in our Galaxy is still far from complete by the MWISP, with flux completeness of <58%. The total mass ratios of the tabulated molecular clouds between different surveys are similar to the CO flux ratio. The application of these ratios to the total H2 mass of our Galaxy yields a correction factor of at least 1.4, meaning that the H2 mass of our Galaxy should be at least 40% more massive than previously determined. Including the completeness correction, an even more significant fraction of the matter should be contributed by baryonic matter. The mass spectrum in the outer Galactic plane is better described by a nontruncating power law with γ = −1.83 ± 0.05 and an upper mass of M 0 = (1.3 ± 0.5) × 106 M ☉.

The principle of the background-eliminated extinction-parallax (BEEP) method is examining the extinction difference between on- and off-cloud regions to reveal the extinction jump caused by molecular clouds, thereby revealing the distance in complex dust environments. The BEEP method requires high-quality images of molecular clouds and high-precision stellar parallaxes and extinction data, which can be provided by the Milky Way Imaging Scroll Painting (MWISP) CO survey and the Gaia DR2 catalog, as well as supplementary A V extinction data. In this work, the BEEP method is further improved (BEEP-II) to measure molecular cloud distances in a global search manner. Applying the BEEP-II method to three regions mapped by the MWISP CO survey, we collectively measured 238 distances for 234 molecular clouds. Compared with previous BEEP results, the BEEP-II method measures distances efficiently, particularly for those molecular clouds with large angular size or in complicated environments, making it suitable for distance measurements of molecular clouds in large samples.

We present an analysis of local molecular clouds ( km s−1, i.e., <1.5 kpc) in the first Galactic quadrant ( and ), a pilot region of the Milky Way Imaging Scroll Painting (MWISP) CO survey. Using the Spectral Clustering for Interstellar Molecular Emission Segmentation algorithm to divide large molecular clouds into moderate-size ones, we determined distances to 28 molecular clouds with the background-eliminated extinction-parallax method using the Gaia DR2 parallax measurements aided by A G and A V ; the distance ranges from 250 to about 1.5 kpc. These incomplete distance samples indicate a linear relationship between the distance and the radial velocity (V LSR) with a scatter of 0.16 kpc, and kinematic distances may be systematically larger for local molecular clouds. In order to investigate fundamental properties of molecular clouds, such as the total sample number, the line width, the brightness temperature, the physical area, and the mass, we decompose the spectral cube using the density-based spatial clustering of applications with noise (DBSCAN) algorithm. Post-selection criteria are imposed on DBSCAN clusters to remove the noise contamination, and we found that the separation of molecular cloud individuals is reliable based on a definition of independent consecutive structures in l–b–V space. The completeness of the local molecular cloud flux collected by the MWISP CO survey is about 80%. The physical area, A, shows a power-law distribution, dN/d , while the molecular cloud mass also follows a power-law distribution but is slightly flatter, dN/dM ∝ M −1.96 ± 0.11.

Context . The investigation of the interstellar gas surrounding supernova remnants (SNRs) is not only necessary to improve our knowledge of SNRs, but also to understand the nature of the progenitor systems. Aims . As part of the Milky Way Imaging Scroll Painting (MWISP) CO line survey, we studied the interstellar gas towards the Galactic historical SNRs. In this work, we present the CO observational results of the Crab Nebula. Methods . We performed large-field and high-sensitivity CO (1–0) molecular line observations towards the Crab Nebula using the 3×3 beam Superconducting Spectroscopic Array Receiver (SSAR) at the PMO 13.7 m millimeter telescope. The Gaia optical data were used to measure the distances of molecular clouds detected in this work, while the HI data from the GALFA-HI and the HI4PI surveys were used to compare the distributions of the molecular and atomic gas. Results . The CO observations show molecular clouds towards the Crab Nebula with velocities ranging from about 0 to 16 km s −1 . From the CO spectra, we find shocked signatures in the clouds with velocities of roughly [5, 11] km s −1 . These shocked molecular clouds, with an angular distance of ~0 ⋅ ∘ 4−0 ⋅ ∘ 5 towards the Crab Nebula, are located at the shell of a bubble discovered in the HI images in the same velocity range. The dimension of the bubble is roughly 2 ⋅ ∘ 3 × 2 ⋅ ∘ 6, and the expansion velocity is ~5 km s −1 . The kinetic energy released by the supernova is estimated to be about 3.5 × 10 51 erg. Conclusions . The kinetic energy inferred from the shocked molecular clouds, together with the HI bubble, supports the picture that the Crab Nebula belongs to a typical core-collapse SNR. Nevertheless, due to the significant uncertainty in the distance measurement, further observations are needed to verify the physical association between the shocked molecular clouds and the Crab Nebula.

ART2 is a 3D multiwavelength Monte Carlo radiative transfer (RT) code that couples continuum and emission lines to track the propagation of photons and their interactions with the interstellar medium (ISM). The original ART2 has been extensively applied to hydrodynamics simulations to study panchromatic properties of galaxies and ISM. Here, we describe new implementations of non-local thermodynamic equilibrium RT of molecular and atomic fine structure emission lines, and the parallelization of the code using a number of novel methods. The new ART2 can efficiently and self-consistently produce a full spectrum that includes both continuum and lines such as [C ii], [N ii], [O iii], Ly α, and CO. These essential features, together with the multiphase ISM model and the adaptive grid, make ART2 a multipurpose code to study multiwavelength properties of a wide range of astrophysical systems from planetary discs to large-scale structures. To demonstrate the capability of the new ART2, we applied it to two hydrodynamics simulations: the zoom-in Milky Way Simulation to obtain panchromatic properties of individual galaxies, and the large-scale IllustrisTNG100 Simulation to obtain global properties such as the line intensity mappings. These products are vital for a broad array of studies. By enabling direct comparison between numerical simulations and multiband observations, ART2 provides a crucial theoretical framework for the understanding of existing and future surveys, and the synergy between multiband galaxy surveys and line intensity mappings. Therefore, ART2 is a powerful and versatile tool to bridge the gap between theories and observations of cosmic structures.

We present ground-based measurements of 126 nearby galaxy centers in12CO and 92 in13CO in various low-Jtransitions. More than 60 galaxies were measured in at least four lines. The average relative intensities of the first fourJ12CO transitions are 1.00:0.92:0.70:0.57. In the first threeJtransitions, the average12CO-to-13CO intensity ratios are 13.0, 11.6, and 12.8, with individual values in any transition ranging from 5 to 25. The sizes of central CO concentrations are well defined in maps, but poorly determined by multi-aperture photometry. On average, theJ = 1−012CO fluxes increase linearly with the size of the observing beam. CO emission covers only a quarter of the HI galaxy disks. Using radiative transfer models (RADEX), we derived model gas parameters. The assumed carbon elemental abundances and carbon gas depletion onto dust are the main causes of uncertainty. The new CO data and published [CI] and [CII] data imply that CO, C°, and C+each represent about one-third of the gas-phase carbon in the molecular interstellar medium. The mean beam-averaged molecular hydrogen column density isN(H2) = (1.5 ± 0.2)×1021cm−2. Galaxy center CO-to-H2conversion factors are typically ten times lower than the “standard” Milky WayX°disk value, with a meanX(CO) = (1.9 ± 0.2)×1019cm−2/K km s−1and a dispersion 1.7. The corresponding [CI]-H2factor is five times higher thanX(CO), withX[CI] = (9 ± 2)×1019cm−2/K km s−1. No unique conversion factor can be determined for [CII]. The low molecular gas content of galaxy centers relative to their CO intensities is explained in roughly equal parts by high central gas-phase carbon abundances, elevated gas temperatures, and large gas velocity dispersions relative to the corresponding values in galaxy disks.

We derive a physical model for the observed relations between star formation rate (SFR) and molecular line (CO and HCN) emission in galaxies, and show how these observed relations are reflective of the underlying star formation law. We do this by combining 3D non-LTE radiative transfer calculations with hydrodynamic simulations of isolated disk galaxies and galaxy mergers. We demonstrate that the observed SFR-molecular line relations are driven by the relationship between molecular line emission and gas density, and anchored by the index of the underlying Schmidt law controlling the SFR in the galaxy. Lines with low critical densities (e.g. CO J=1-0) are typically thermalized and trace the gas density faithfully. In these cases, the SFR will be related to line luminosity with an index similar to the Schmidt law index. Lines with high critical densities greater than the mean density of most of the emitting clouds in a galaxy (e.g. CO J=3-2, HCN J=1-0) will have only a small amount of thermalized gas, and consequently a superlinear relationship between molecular line luminosity and mean gas density. This results in a SFR-line luminosity index less than the Schmidt index for high critical density tracers. One observational consequence of this is a significant redistribution of light from the small pockets of dense, thermalized gas to diffuse gas along the line of sight, and prodigious emission from subthermally excited gas. At the highest star formation rates, the SFR-Lmol slope tends to the Schmidt index, regardless of the molecular transition. The fundamental relation is the Kennicutt-Schmidt law, rather than the relation between SFR and molecular line luminosity. We use these results to make imminently testable predictions for the SFR-molecular line relations of unobserved transitions.

Context.The innermost regions of most galaxies are characterised by the presence of extremely dense nuclear star clusters. Nevertheless, these clusters are not the only stellar component present in galactic nuclei, where larger stellar structures known as nuclear stellar discs, have also been found. Understanding the relation between nuclear star clusters and nuclear stellar discs is challenging due to the large distance towards other galaxies which limits their analysis to integrated light. The Milky Way’s centre, at only ∼8 kpc, hosts a nuclear star cluster and a nuclear stellar disc, constituting a unique template to understand their relation and formation scenario.Aims.We aim to study the kinematics and stellar metallicity of stars from the Milky Way’s nuclear star cluster and disc to shed light on the relation between these two Galactic centre components.Methods.We used publicly available photometric, proper motions, and spectroscopic catalogues to analyse a region of ∼2.8′×4.9′ centred on the Milky Way’s nuclear star cluster. We built colour magnitude diagrams, and applied colour cuts to analyse the kinematic and metallicity distributions of Milky Way’s nuclear star cluster and disc stars with different extinction, along the line of sight.Results.We detect kinematic and metallicity gradients for the analysed stars along the line of sight towards the Milky Way’s nuclear star cluster, suggesting a smooth transition between the nuclear stellar disc and cluster. We also find a bi-modal metallicity distribution for all the analysed colour bins, which is compatible with previous work on the bulk population of the nuclear stellar disc and cluster. Our results suggest that these two Galactic centre components might be part of the same structure with the Milky Way’s nuclear stellar disc being the grown edge of the nuclear star cluster.

The current generation of (sub)mm-telescopes has allowed molecular line emission to become a major tool for studying the physical, kinematic, and chemical properties of extragalactic systems, yet exploiting these observations requires a detailed understanding of where emission lines originate within the Milky Way. In this paper, we present 60 arcsec (∼3 pc) resolution observations of many 3 mm band molecular lines across a large map of the W49 massive star-forming region (∼100 pc × 100 pc at 11 kpc), which were taken as part of the ‘LEGO’ IRAM-30m large project. We find that the spatial extent or brightness of the molecular line transitions are not well correlated with their critical densities, highlighting abundance and optical depth must be considered when estimating line emission characteristics. We explore how the total emission and emission efficiency (i.e. line brightness per H2 column density) of the line emission vary as a function of molecular hydrogen column density and dust temperature. We find that there is not a single region of this parameter space responsible for the brightest and most efficiently emitting gas for all species. For example, we find that the HCN transition shows high emission efficiency at high column density (1022 cm−2) and moderate temperatures (35 K), whilst e.g. N2H+ emits most efficiently towards lower temperatures (1022 cm−2; <20 K). We determine $X_{\mathrm{CO} (1-0)} \sim 0.3 \times 10^{20} \, \mathrm{cm^{-2}\, (K\, km\, s^{-1})^{-1}}$, and $\alpha _{\mathrm{HCN} (1-0)} \sim 30\, \mathrm{M_\odot \, (K\, km\, s^{-1}\, pc^2)^{-1}}$, which both differ significantly from the commonly adopted values. In all, these results suggest caution should be taken when interpreting molecular line emission.

Unveiling the 3D shape of the Milky Way's dark-matter halo is critical to understanding its formation history. We created an innovative dynamical model that makes minimal assumptions about the internal dynamical structures and accommodates a highly flexible triaxial DM halo. By applying the method to 6D phase-space data of K-giant stars from LAMOST + Gaia, we robustly determined the 3D dark-matter distribution of the Milky Way out to approximately 50 kpc. We discovered a triaxial, nearly oblate dark-matter halo with q_ ̊m DM = Z/X= 0.92 = Y/X= 0.8 > p_ p_ ̊m DM on average within 50 kpc, where the Z axis is defined perpendicular to the stellar disc. The axes ratio q_ ̊m DM ̊m DM is strongly preferred; the long-intermediate axis plane of the dark-matter halo is unexpectedly vertical to the Galactic disc, yet aligned with the `plane of satellites'. This striking configuration suggests that the Galactic disc (and the inner halo) has flipped, likely torqued by minor mergers, from an original alignment with the outer dark-matter halo and satellite plane, as is supported by Milky Way analogues from Auriga and TNG50. By allowing q_ ̊m DM (r) and p_ ̊m DM (r) to vary with radii, we find tentative evidence that the dark-matter halo is twisted. This agrees alignment with the disc in the inner regions and transitions to a vertical orientation at r≳ 20 kpc, supporting the disc flip scenario prediction. Such disc reorientation is non-trivial, yet its physical mechanism is straightforward to comprehend and naturally originates a vertical satellite plane. Our findings offer a unified framework that links dark-matter halo orientation, satellite alignment, and disc evolution, reinforcing the internal consistency of the Milky Way in the Λ cold dark matter model.

We compare molecular gas properties in the starbursting center of NGC 253 and the Milky Way Galactic center (GC) on scales of ∼1–100 pc using dendrograms and resolution-, area-, and noise-matched data sets in CO (1–0) and CO (3–2). We find that the size–line width relations in NGC 253 and the GC have similar slope, but NGC 253 has larger line widths by factors of ∼2–3. The σ 2/R dependency on column density shows that, in the GC, on scales of 10–100 pc the kinematics of gas over N > 3 × 1021 cm−2 are compatible with gravitationally bound structures. In NGC 253 this is only the case for column densities N > 3 × 1022 cm−2. The increased line widths in NGC 253 originate in the lower column density gas. This high velocity dispersion, not gravitationally self-bound gas, is likely in transient structures created by the combination of high average densities and feedback in the starburst. The high densities turn the gas molecular throughout the volume of the starburst, and the injection of energy and momentum by feedback significantly increases the velocity dispersion at a given spatial scale over what is observed in the GC.

We have complemented existing observations of H i absorption with new observations of HCO+, C2H, HCN, and HNC absorption from the Atacama Large Millimeter/submillimeter Array and the Northern Extended Millimeter Array in the directions of 20 background radio continuum sources with 4° ≤ ∣b∣ ≤ 81° to constrain the atomic gas conditions that are suitable for the formation of diffuse molecular gas. We find that these molecular species form along sightlines where A V ≳ 0.25, consistent with the threshold for the H i-to-H2 transition at solar metallicity. Moreover, we find that molecular gas is associated only with structures that have an H i optical depth >0.1, a spin temperature <80 K, and a turbulent Mach number ≳ 2. We also identify a broad, faint component to the HCO+ absorption in a majority of sightlines. Compared to the velocities where strong, narrow HCO+ absorption is observed, the H i at these velocities has a lower cold neutral medium fraction and negligible CO emission. The relative column densities and linewidths of the different molecular species observed here are similar to those observed in previous experiments over a range of Galactic latitudes, suggesting that gas in the solar neighborhood and gas in the Galactic plane are chemically similar. For a select sample of previously observed sightlines, we show that the absorption line profiles of HCO+, HCN, HNC, and C2H are stable over periods of ∼3 yr and ∼25 yr, likely indicating that molecular gas structures in these directions are at least ≳100 au in size.

We present the first localized detections of the cold neutral medium (CNM) in IC 10, offering a rare view of dense atomic gas in a low-metallicity ( Z / Z ⊙ ∼ 0.27) dwarf galaxy. As a low-metallicity starburst, IC 10’s interstellar medium conditions could reflect small scale physics conditions that mirror those of early galaxies, providing a unique window into the heating and cooling processes that shaped the interstellar medium in early-Universe environments. Leveraging the high angular (<5″ ∼ 15 pc) and spectral (0.4 km s −1 ) resolution of the Local Group L -band Survey, we searched for H I absorption against nine continuum radio sources and detected absorption along three sightlines corresponding to internal radio emission sources within IC 10. Using Gaussian decomposition and radiative transfer, we characterize the CNM, deriving spin temperatures of ∼30–55 K, column densities of (0.6–3.0) × 10 21 cm −2 , cold H I fractions of ∼21%–37%, and line widths of ∼5.6–13.6 km s −1 . For each individual detection of H I absorption, we find corresponding molecular emission from 12 CO ( J = 1–0), HCO + ( J = 1–0), and HCN ( J = 1–0) at similar velocities and with comparable line widths, indicating a well-mixed cold atomic and molecular medium. In IC 10, the CNM shows a clear kinematic connection to the high-density ISM, implying a stronger dynamical coupling with molecular gas than in the Milky Way, in line with expectations for low-metallicity environments. At the ∼15 pc scales probed by slightly extended H II regions in IC 10, unresolved CNM clouds likely contribute to line blending, so the observed broad H I line widths may partly reflect spatial and kinematic averaging.

Context. With data releases from the astrometric space mission Gaia, exploration of the structure of the Milky Way is now possible in unprecedented detail, and has unveiled many previously unknown structures in the Galactic disc and halo. One such feature is the Gaia phase spiral where the stars in the Galactic disc form a spiral density pattern in the Z − VZ plane. Many questions regarding the phase spiral remain, particularly how its amplitude and rotation change with position in the Galaxy. Aims. We aim to characterize the shape, rotation, amplitude, and metallicity of the phase spiral in the outer disc of the Milky Way. This will allow us to better understand which physical processes caused the phase spiral and may provide further clues as to the Milky Way’s past and the events that contributed to its current state. Methods. We use Gaia data release 3 (DR3) to get full position and velocity data on approximately 31.5 million stars, and metallicity for a subset of them. We then compute the angular momenta of the stars and develop a model to characterise the phase spiral in terms of amplitude and rotation at different locations in the disc. Results. We find that the rotation angle of the phase spiral changes with Galactic azimuth and galactocentric radius, making the phase spiral appear to rotate about 3° per degree in Galactic azimuth. Furthermore, we find that the phase spiral in the 2200 − 2400 kpc km s−1 range of angular momentum is particularly strong compared to the phase spiral that can be observed in the solar neighbourhood. The metallicity of the phase spiral appears to match that of the field stars of the Milky Way disc. Conclusions. We created a new model capable of fitting several key parameters of the Gaia phase spiral. We have been able to determine the rotation rate of the phase spiral to be about 3° per degree in Galactic azimuth. We find a maximum in the amplitude of the phase spiral at LZ ≈ 2300 km kpc s−1, which makes the phase spiral clearly visible. These results provide insights into the physical processes that led to the formation of the phase spiral and contribute to our understanding of the Milky Way’s past and present state.

Studies of the ages, abundances, and motions of individual stars in the Milky Way provide one of the best ways to study the evolution of disc galaxies over cosmic time. The formation of the Milky Way’s barred inner region in particular is a crucial piece of the puzzle of disc galaxy evolution. Using data from APOGEE and Gaia, we present maps of the kinematics, elemental abundances, and age of the Milky Way bulge and disc that show the barred structure of the inner Milky Way in unprecedented detail. The kinematic maps allow a direct, purely kinematic determination of the bar’s pattern speed of $41\pm 3\, \mathrm{km\, s}^{-1}\, \mathrm{kpc}^{-1}$ and of its shape and radial profile. We find the bar’s age, metallicity, and abundance ratios to be the same as those of the oldest stars in the disc that are formed in its turbulent beginnings, while stars in the bulge outside of the bar are younger and more metal-rich. This implies that the bar likely formed ${\approx}8\, \mathrm{Gyr}$ ago, when the decrease in turbulence in the gas disc allowed a thin disc to form that quickly became bar-unstable. The bar’s formation therefore stands as a crucial epoch in the evolution of the Milky Way, a picture that is in line with the evolutionary path that emerges from observations of the gas kinematics in external disc galaxies over the last ${\approx}10\, \mathrm{Gyr}$.