We investigate data from the Galactic Effelsberg--Bonn HI Survey (EBHIS), supplemented with data from the third release of the Galactic All Sky Survey (GASS III) observed at Parkes. We explore the all sky distribution of the local Galactic HI gas with $|v_{\rm LSR}| < 25 $ kms$^{-1}$ on angular scales of 11' to 16'. Unsharp masking (USM) is applied to extract small scale features. We find cold filaments that are aligned with polarized dust emission and conclude that the cold neutral medium (CNM) is mostly organized in sheets that are, because of projection effects, observed as filaments. These filaments are associated with dust ridges, aligned with the magnetic field measured on the structures by Planck at 353 GHz. The CNM above latitudes $|b|>20^\circ$ is described by a log-normal distribution, with a median Doppler temperature $T_{\rm D} = 223$ K, derived from observed line widths that include turbulent contributions. The median neutral hydrogen (HI) column density is $N_{\rm HI} \simeq 10^{19.1}\,{\rm cm^{-2}}$. These CNM structures are embedded within a warm neutral medium (WNM) with $N_{\rm HI} \simeq 10^{20} {\rm cm^{-2}}$. Assuming an average distance of 100 pc, we derive for the CNM sheets a thickness of $< 0.3$ pc. Adopting a magnetic field strength of $B_{\rm tot} = (6.0 \pm 1.8)μ$G, proposed by Heiles & Troland 2005, and assuming that the CNM filaments are confined by magnetic pressure, we estimate a thickness of 0.09 pc. Correspondingly the median volume density is in the range $ 14 < n < 47 {\rm cm^{-3}}$.
The interstellar medium (ISM) is an inseparable part of the Milky Way ecosystem whose evolutionary history remains a challenging question. We trace the evolution of the molecular ISM using a sample of Young Stellar Objects (YSO) association --molecular cloud complex (YSO-MC complex). We derive their three-dimensional (3D) velocities by combining the Gaia astrometric measurements of the YSO associations and the CO observations of the associated molecular clouds. Based on the 3D velocities, we simulate the motions of the YSO-MC complexes in the Galactic potential and forecast the ISM evolution b…
The Milky Way atlas for linear filaments II. clump rotation versus filament orientation
Chongqing University（重庆大学）；Peking University（北京大学）；S. N. Bose National Centre for Basic Sciences（S.N.玻色基础科学国家中心）；Tsinghua University（清华大学）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）
We present a statistical study of the filamentary structure orientation in the CO emission observations obtained in the Milky Way Imaging Scroll Painting (MWISP) survey in the range $25.8°< l < 49.7°$, $|b| \leq 1.25°$, and $-100 < v_{\rm LSR} < 135$ km/s. We found that most of the filamentary structures in the $^{12}$CO and $^{13}$CO emission do not show a global preferential orientation either parallel or perpendicular to the Galactic plane. However, we found ranges in Galactic longitude and radial velocity where the $^{12}$CO and $^{13}$CO filamentary structures are parallel to the Galactic…
The Milky Way Atlas for Linear Filaments III: Giant filaments and magnetic fields as evidence of a bubbly Galactic disk
Kavli Institute for Astronomy and Astrophysics, Peking University（北京大学科维理天文与天体物理研究所）；Department of Physics, Graduate School of Science, Nagoya University（名古屋大学理学研究科物理学系）；Xinjiang Astronomical Observatory, Chinese Academy of Sciences（中国科学院新疆天文台）；University of the Chinese Academy of Sciences（中国科学院大学）
The advent of more precise measurements of distance estimates for some objects (bayesian estimates for filaments, trigonometric estimates for masers) permits a better comparison of their relative locations, and a comparison with a most recent spiral arm model fitted to the diffuse CO 1-0 gas. Our results support the idea that some bone-like filaments, greater than 10 pc (labeled elsewhere as bones, mst, herschel) and smaller than 100 pc, are observed near the potential minimum of the Scutum spiral arm (bone-like filaments are offset outward from masers by about 200 pc).
CO-dark gas and molecular filaments in Milky Way type galaxies
我们使用移动网格代码AREPO耦合到一个含时的化学网络来研究分子气体在模拟的螺旋星系中的形成和破坏。这使我们能够确定CO排放不跟踪的气体的特性。我们的极高分辨率AREPO模拟使我们能够捕捉光盘的化学演变，而无需求助于参数化的“聚集因子”。我们通过我们模拟的盘星系计算了H2和CO柱密度，并估算了CO发射和CO-H2转换因子。我们发现，在类似的条件下，在当地的星际介质，大约42%的总分子质量应该在CO暗区，在合理的协议与观测估计。这部分是几乎不敏感的CO积分强度阈值用于区分CO亮和CO暗气体，只要这个阈值是小于10 K km/s。CO暗分子气体主要存在于极长（>100 pc）的细丝中，这些细丝被星系切变拉伸在旋臂之间。只有这些细丝的中心在CO中是明亮的，这表明在银河系中观察到的非连续分子云可能只是更大结构的一小部分。CO暗分子气体主要存在于部分分子相中，占总圆盘质量预算的很大一部分。在环境紫外线场较高或表面密度较低的模拟中，暗气体比例较高，这意味着具有这些条件的外部星系可能具有更大比例的暗气体。
We present a statistical study of the relative orientation in the plane of the sky between interstellar magnetic fields and filaments hosting cold clumps. For the first time, we consider both the density of the environment and the density contrast between the filaments and their environment. Moreover, we geometrically distinguish between the clumps and the remaining portions of the filaments. We infer the magnetic field orientations in the filaments and in their environment from the Stokes parameters, assuming optically thin conditions. Thus, we analyze the relative orientations between filame…
The Milky and its Gas: Cold Fountains and Accretion
银河系正在从下落的高速云中获取气体。这些物质进入了一个盘-晕界面，在许多地方都充满了HI云，这些云是通过与恒星形成有关的过程从盘中喷射出来的。史密斯云是一个非凡的例子，高速云将相对低金属含量的气体$>10^6$ M $_{\odot}$带入银河系。它可能是一个更大的流的一部分，其中的组件现在正在通过磁盘。
The ubiquity of very thin and lengthy cold neutral media (CNM) has been reported by multiple authors in the HI community. Yet, the reason of how the CNM can be so long and lengthy is still in debate. In this paper, we recognize a new type of instability due to the attractive nature of the pressure force in the unstable phase. We provide a new estimation of the average CNM filament aspect ratio with the consideration of force balances at the phase boundary, which is roughly 5-20 in common CNM environment. We show that most of the cold filaments are less filamentary than what usually predicted v…
A recent publication (Li et al. 2021) discovered one of the largest filamentary neutral hydrogen features dubbed Cattail from high resolution FAST observations that might be a new galactic arm of our own Milky Way. However in the analysis, it was suggested that this neutral hydrogen feature is cold despite having 12km/s total linewidth. We evaluate the probability whether the Cattail is actually cold neutral media via the newly developed Velocity Decomposition Algorithm (Yuen et al. 2021a) and Force Balancing Model (Ho et al. 2021a). We discovered that even with the inclusion of the galactic s…
Formation process(es) of galactic bulges are not yet clarified although several mechanisms have been proposed. In a previous study, we suggested one possibility that galactic bulges have been formed from the cold gas inflowing through surrounding hot halo gas in massive dark matter halos at high redshifts. It was shown that this scenario leads to the bulge-to-total stellar mass ratio increasing with the galaxy mass, in agreement with the well-known observed trend. We here indicate that it also reproduces recent observational results that the mean stellar age of the bulge increases with the gal…
Data of $^{12}$CO/$^{13}$CO/C$^{18}$O $J=1\to0$ emission toward the Galactic plane region of $l=35^\circ$ to $45^\circ$ and $b= -5^\circ$ to $+5^\circ$ are available with the Milky Way Imaging Scroll Painting (MWISP) project. Using the data, we found a giant molecular filament (GMF) around $l\approx38\sim42^\circ$, $b\approx-3.5\sim0^\circ$, $V_{LSR} \approx 27 \sim 40$ 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 $10^{21}$ cm$^{-2}$, this GMF is very cold and…
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 $\sim$ 16,000 M$_{\odot}$, yet hosts only 15 potentially protostellar 70 $μ$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 sta…
Multi-phase filamentary structures around Brightest Cluster Galaxies are likely a key step of AGN-feedback. We observed molecular gas in 3 cool cluster cores: Centaurus, Abell S1101, and RXJ1539.5 and gathered ALMA and MUSE data for 12 other clusters. Those observations show clumpy, massive and long, 3--25 kpc, molecular filaments, preferentially located around the radio bubbles inflated by the AGN (Active Galactic Nucleus). Two objects show nuclear molecular disks. The optical nebula is certainly tracing the warm envelopes of cold molecular filaments. Surprisingly, the radial profile of the H…
We present a study of the filamentary structure in the emission from the neutral atomic hydrogen (HI) at 21 cm across velocity channels in the 40'' and 1.5-km/s resolution position-position-velocity cube resulting from the combination of the single-dish and interferometric observations in The HI/OH/Recombination (THOR) line survey. Using the Hessian matrix method in combination with tools from circular statistics, we find that the majority of the filamentary structures in the HI emission are aligned with the Galactic plane. Part of this trend can be assigned to long filamentary structures that…
The hot intracluster medium (ICM) is thought to be quiescent with low observed velocity dispersions. Surface brightness fluctuations of the ICM also suggest that its turbulence is subsonic with a Kolmogorov scaling relation, indicating that the viscosity is suppressed and the kinetic energy cascades to small scales unscathed. However, recent observations of the cold gas filaments in galaxy clusters find that the scaling relations are steeper than that of the hot plasma, signaling kinetic energy losses and the presence of supersonic flows. In this work we use high-resolution simulations to expl…
We report the discovery of a velocity coherent, kpc-scale molecular structure towards the Galactic center region with an angular extent of 30deg and an aspect ratio of 60:1. The kinematic distance of the CO structure ranges between 4.4 to 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 sub-branch of the Norma arm or an inter-arm giant molecular filament, likely to be a kpc-scale feather (or spur) of the Milky Way, similar to those observed in nearby spiral galaxies. The filamentary cloud is at …
Star formation primarily occurs in filaments where magnetic fields are expected to be dynamically important. The largest and densest filaments trace spiral structure within galaxies. Over a dozen of these dense ($\sim$10$^4$\,cm$^{-3}$) and long ($>$10\,pc) filaments have been found within the Milky Way, and they are often referred to as "bones." Until now, none of these bones have had their magnetic field resolved and mapped in their entirety. We introduce the SOFIA legacy project FIELDMAPS which has begun mapping $\sim$10 of these Milky Way bones using the HAWC+ instrument at 214\,$μ$m and 1…
Observations have discovered numerous giant molecular filaments (GMFs) in the Milky Way. However, their role in the Galactic star formation and Galaxy-scale evolution of dense gas is still unknown. 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. We identify and classify the young stellar object (YSO) population of each GMF using multi-wavelength photometry from near- to far-infrared. We estimate the total SFRs assumin…
Theory suggests that there are two primary modes of accretion through which dark matter halos acquire the gas to form and fuel galaxies, hot and cold mode accretion. In cold mode accretion, gas streams along cosmic web filaments to the center of the halo, allowing for the efficient delivery of star-forming fuel. Recently, two QSO-illuminated HI Lyman alpha (Lyα) emitting objects were reported to have properties of cold, rotating structures (Martin et al. 2015, Martin et al. 2016). However, the spatial and spectral resolution available was insufficient to constrain radial flows associated with …
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 $\gtrsim 100$ 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 $\rm kpc^{2}$ should be id…
Dwarf spheroidal galaxies (dSph) orbiting the Milky Way are complex objects often with complicated star formation histories and internal dynamics. In this work, we search for stellar substructures in four of the classical dSph satellites of the Milky Way: Sextans, Carina, Leo I, and Leo II. We apply two methods to search for stellar substructure: the minimum spanning tree method, which helps us to find and quantify spatially connected structures, and the "brute-force" method, which is able to find elongated stellar substructures. We detected the previously known substructure in Sextans, and al…
The centre of the Milky Way is the site of several high-energy processes that have strongly impacted the inner regions of our Galaxy. Activity from the super-massive black hole, Sgr A*, and/or stellar feedback from the inner molecular ring expel matter and energy from the disc in the form of a galactic wind. Multiphase gas has been observed within this outflow, from hot highly-ionized, to warm ionized and cool atomic gas. To date, however, there has been no evidence of the cold and dense molecular phase. Here we report the first detection of molecular gas outflowing from the centre of our Gala…
Context. There are significant amounts of H2 in the Milky Way. Due to its symmetry H2 does not radiate at radio frequencies. CO is thought to be a tracer for H2, however CO is formed at significantly higher opacities than H2. Thus, toward high Galactic latitudes significant amounts of H2 are hidden and called CO-dark. Aims. We demonstrate that the dust-to-gas ratio is a tool to identify locations and column densities of CO-dark H2. Methods. We adopt the hypothesis of a constant E(B-V)/NH ratio, independent of phase transitions from HI to H2. We investigate the Doppler temperatures TD, from a G…
In the local universe, a large fraction of the baryon content is believed to exist as diffuse gas in filaments. While this gas is directly observable in X-ray emission around clusters of galaxies, it is primarily studied through its UV absorption. Recently, X-ray observations of large-scale filaments connecting to the cosmic web around the nearby ($z=0.05584$) cluster Abell 133 were reported. One of these filaments is intersected by the sightline to quasar [VV98] J010250.2$-$220929, allowing for a first-ever census of cold, cool, and warm gas in a filament of the cosmic web where hot gas has b…
We study the stacked filaments connecting group-mass halo pairs, using dark-matter-only N-body simulations. We calculate the dark matter over-density profile of these stacked filaments at different redshifts as a function of the distance perpendicular to the filament axis. A four-parameter universal functional form, including three comoving scale radii and one amplitude parameter (core density), provides a good fit out to a radius of 20 cMpc/h for stacked filaments over a range of redshifts, lengths and masses. The scale radii are approximately independent of redshift but increase as power-law…
Chemical bimodality of the Milky Way (MW) disc stars constitutes one of the most remarkable properties of MW. The cold accretion theory for the cosmological gas accretion provides one viable explanation to this phenomenon. In this scenario, the rapid cold-mode accretion in the early epoch creates the first generation stars relatively rich in $α$-elements(O,Mg,Si,S,Ca,etc) and later cooling flow produces iron-rich second generation stars, creating the bimodality in the [$α$/Fe] ratio. We employ a cosmologically motivated chemical evolution model for disc galaxies to elucidate the role played by…
