The Milky Way's central molecular zone (CMZ) has emerged in recent years as a unique laboratory for the study of star formation. Here we use the simulations presented in Tress et al. 2020 to investigate star formation in the CMZ. These simulations resolve the structure of the interstellar medium at sub-parsec resolution while also including the large-scale flow in which the CMZ is embedded. Our main findings are as follows. (1) While most of the star formation happens in the CMZ ring at $R\gtrsim100 {\, \rm pc}$, a significant amount also occurs closer to SgrA* at $R \lesssim 10{\, \rm pc}$. (2) Most of the star formation in the CMZ happens downstream of the apocentres, consistent with the "pearls-on-a-string" scenario, and in contrast to the notion that an absolute evolutionary timeline of star formation is triggered by pericentre passage. (3) Within the timescale of our simulations ($\sim100$ Myr), the depletion time of the CMZ is constant within a factor of $\sim2$. This suggests that variations in the star formation rate are primarily driven by variations in the mass of the CMZ, caused for example by AGN feedback or externally-induced changes in the bar-driven inflow rate, and not by variations in the depletion time. (4) We study the trajectories of newly born stars in our simulations. We find several examples that have age and 3D velocity compatible with those of the Arches and Quintuplet clusters. Our simulations suggest that these prominent clusters originated near the collision sites where the bar-driven inflow accretes onto the CMZ, at symmetrical locations with respect to the Galactic centre, and that they have already decoupled from the gas in which they were born.
The Central Molecular Zone (CMZ) is a ring-like accumulation of molecular gas in the innermost few hundred parsecs of the Milky Way, generated by the inward transport of matter driven by the Galactic bar. The CMZ is the most extreme star-forming environment in the Galaxy. The unique combination of large-scale dynamics and extreme interstellar medium conditions, characterised by high densities, temperatures, pressures, turbulent motions, and strong magnetic fields, make the CMZ an ideal region for testing current star and planet formation theories. We review the recent observational and theoret…
This review compiles the results of recent studies of molecular gas conditions in the central six hundred parsecs of our Galaxy. The review begins by placing our Galactic center into context with the rest of our galaxy. It next discusses the wealth of previous research on the Galactic center, before focusing on what is known about the molecular interstellar medium in this region. It focuses especially on a surge in interest in this region and new studies conducted in the last five years. It concludes by highlighting open questions that remain, and the potential for new facilities such as ALMA …
We apply an analytic theory for environmentally-dependent molecular cloud lifetimes to the Central Molecular Zone of the Milky Way. Within this theory, the cloud lifetime in the Galactic centre is obtained by combining the time-scales for gravitational instability, galactic shear, epicyclic perturbations and cloud-cloud collisions. We find that at galactocentric radii $\sim 45$-$120$ pc, corresponding to the location of the '100-pc stream', cloud evolution is primarily dominated by gravitational collapse, with median cloud lifetimes between 1.4 and 3.9 Myr. At all other galactocentric radii, g…
Molecular Gas Morphological Analogues for the Milky Way
The University of Texas at Austin（德克萨斯大学奥斯汀分校）；INAF-IAPS（意大利国家天体物理研究所国际天体物理中心）；INAF, Sezione di Lecce（意大利国家天体物理研究所莱切分部）；National Astronomical Observatory of Japan（日本国立天文台）；Max Planck Institute for Radio Astronomy（马克斯·普朗克射电天文研究所）；University of Kentucky（肯塔基大学）
In this paper we present the CMZoom Survey's catalog of compact sources (< 10'', ~0.4pc) within the Central Molecular Zone (CMZ). CMZoom is a Submillimeter Array (SMA) large program designed to provide a complete and unbiased map of all high column density gas (N(H$_2$) $\geq$ 10$^{23}$ cm$^{-2}$) of the innermost 500pc of the Galaxy in the 1.3mm dust continuum. We generate both a robust catalog designed to reduce spurious source detections, and a second catalog with higher completeness, both generated using a pruned dendrogram. In the robust catalog, we report 285 compact sources, or 816 in t…
To pinpoint the peak location of the synchrotron total intensity emission in a spiral arm, we use a map of the spiralarm locations (from the observed arm tangent). Thus In a typical spiral arm in Galactic Quadrant I, we find the peak of the synchrotron radiation to be located about 220 +/-40 pc away from the inner arm edge (hot dust lane) inside the spiral arm. While most of the galactic disk has a clockwise largescale magnetic field, we make a statistical analysis to delimitate more precisely the smaller reverse annulus wiith a counterclockwise galactic magnetic field. We find an annulus widt…
We study the relationship between the scale-height of the molecular gas disc and the turbulent velocity dispersion of the molecular interstellar medium within a simulation of a Milky Way-like galaxy in the moving-mesh code Arepo. We find that the vertical distribution of molecular gas can be described by a Gaussian function with a uniform scale-height of ~50 pc. We investigate whether this scale-height is consistent with a state of hydrostatic balance between gravity and turbulent pressure. We find that the hydrostatic prediction using the total turbulent velocity dispersion (as one would meas…
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 use hydrodynamical simulations to study the Milky Way's central molecular zone (CMZ). The simulations include a non-equilibrium chemical network, the gas self-gravity, star formation and supernova feedback. We resolve the structure of the interstellar medium at sub-parsec resolution while also capturing the interaction between the CMZ and the bar-driven large-scale flow out to $R\sim 5\kpc$. Our main findings are as follows: (1) The distinction between inner ($R\lesssim120$~pc) and outer ($120\lesssim R\lesssim450$~pc) CMZ that is sometimes proposed in the literature is unnecessary. Instead…
Three dimensional (3D) distribution of the volume-density molecular fraction, defined by fmol_rho=rho_H2/(rho_HI + rho_H2), is studied in the Milky Way Galaxy. The molecular front appears at galacto-centric distance of R \sim 8 kpc, where the phase transition from atomic to molecular hydrogen occurs suddenly with fmol_rho dropping from \sim 0.8 to 0.2 within a radial interval as narrow as \sim 0.5 kpc. The front in fmol_rho is much sharper than that for surface density molecular fraction. The front also appears in the vertical direction with a full width of the high-fmol_rho disk to be \sim 10…
The gas accretion and star-formation histories of galaxies like the Milky Way remain an outstanding problem in astrophysics. Observations show that 8 billion years ago, the progenitors to Milky Way-mass galaxies were forming stars 30 times faster than today and predicted to be rich in molecular gas, in contrast with low present-day gas fractions ($<$10%). Here we show detections of molecular gas from the CO(J=3-2) emission (rest-frame 345.8 GHz) in galaxies at redshifts z=1.2-1.3, selected to have the stellar mass and star-formation rate of the progenitors of today's Milky Way-mass galaxies. T…
Recent submillimeter and far-infrared wavelength observations of absorption in the rotational ground-state lines of various simple molecules against distant Galactic continuum sources have opened the possibility of studying the chemistry of diffuse molecular clouds throughout the Milky Way. In order to calculate abundances, the column densities of molecular and atomic hydrogen, HI, must be known. We aim at determining the atomic hydrogen column densities for diffuse clouds located on the sight lines toward a sample of prominent high-mass star-forming regions that were intensely studied with th…
The nuclear stellar disc (NSD) is, together with the nuclear star cluster (NSC) and the central massive black hole, one of the main components in the central parts of our Milky Way. However, until recently, only few studies of the stellar content of the NSD have been obtained due to extreme extinction and stellar crowding. With a dedicated KMOS (VLT, ESO) spectroscopic survey, we study the kinematics and global metallicities of the NSD based on the observations of K/M giant stars. We trace radial velocities and metallicities which were derived based on spectral indices (Na I and CO) along the …
We infer the absolute time dependence of kinematic gas temperature along a proposed orbit of molecular clouds in the Central Molecular Zone (CMZ) of the Galactic Center (GC). Ammonia gas temperature maps are one of the results of the "Survey of Water and Ammonia in the Galactic Center" (SWAG, PI: J. Ott); the dynamical model of molecular clouds in the CMZ was taken from Kruijssen et al. (2015). We find that gas temperatures increase as a function of time in both regimes before and after the cloud passes pericenter on its orbit in the GC potential. This is consistent with the recent proposal th…
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 …
We have studied the properties of molecular clouds in the second quadrant of the Milky Way Mid-plane from l$=$104$.\!\!^{\circ}$75 to l$=$119$.\!\!^{\circ}$75 and b$=-$5$.\!\!^{\circ}$25 to b$=$5$.\!\!^{\circ}$25 using the $^{12}$CO, $^{13}$CO, and C$^{18}$O $J=1-0$ emission line data from the Milky Way Imaging Scroll Painting project (MWISP). We have identified 857 and 300 clouds in the $^{12}$CO and $^{13}$CO spectral cubes, respectively, using the DENDROGRAM + SCIMES algorithms. The distances of the molecular clouds are estimated and the physical properties like masses, sizes, and surface d…
We study the time evolution of molecular clouds across three Milky Way-like isolated disc galaxy simulations at a temporal resolution of 1 Myr, and at a range of spatial resolutions spanning two orders of magnitude in spatial scale from ~10 pc up to ~1 kpc. The cloud evolution networks generated at the highest spatial resolution contain a cumulative total of ~80,000 separate molecular clouds in different galactic-dynamical environments. We find that clouds undergo mergers at a rate proportional to the crossing time between their centroids, but that their physical properties are largely insensi…
It is well known that the Galactic bar drives a gas inflow into the Central Molecular Zone, which fuels star formation, accretion onto the central super-massive black hole, and large-scale outflows. This inflow happens mostly through two symmetrical dust-lanes, similar to those often seen in external barred galaxies. Here we use the fact that the Milky Way dust-lanes have been previously identified in $^{12}$CO datacubes and a simple geometrical model to derive the first observational determination of the mass inflow rate into the Central Molecular Zone. We find that the time-averaged inflow r…
The Central Molecular Zone (CMZ) is a ring-like structure sitting at the center of the Milky Way. Using the 870 $μ$m continuum map from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL), we study the anisotropy of the density structure of the gas in the Central Molecular Zone (CMZ) using the 2D correlation function. To quantify the spatial anisotropy, we define the critical angle $θ_{\rm half}$, as well as the anisotropy parameter $A\equiv \fracπ{4θ_{\rm half}}-1$. We find that the density structure is strongly anisotropic at the large scale (~ 100 pc), and the degree of spatial an…
Being one of the most abundant atomic/molecular species observed in dense molecular gas, atomic carbon ($\mathrm{C}^0$) is a potential good tracer of molecular gas mass in many chemical/physical environments, though the $\mathrm{C^0}$ abundance variation outside the Galactic disk region is yet to be fully known. This paper presents a wide-field 500 GHz [CI] map of the Galactic central molecular zone (CMZ) obtained with the ASTE 10-m telescope. Principal component analysis and non-LTE multi-transition analysis have shown that the [CI] emission predominantly originates from the low-excitation ga…
We present the far-ultraviolet (FUV) fluorescent molecular hydrogen (H_2) emission map of the Milky Way Galaxy obtained with FIMS/SPEAR covering ~76% of the sky. The extinction-corrected intensity of the fluorescent H_2 emission has a strong linear correlation with the well-known tracers of the cold interstellar medium (ISM), including color excess E(B-V), neutral hydrogen column density N(H I), and H_alpha emission. The all-sky H_2 column density map was also obtained using a simple photodissociation region model and interstellar radiation fields derived from UV star catalogs. We estimated th…
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 investigate the temperature distribution of CO-dark molecular hydrogen (H2) in a series of disk galaxies simulated using the AREPO moving-mesh code. In conditions similar to those in the Milky Way, we find that H2 has a flat temperature distribution ranging from 10 - 100 K. At $T < 30$ K the gas is almost fully molecular and has a high CO content, whereas at $T > 30$ K, the H2 fraction spans a broader range and the CO content is small, allowing us to classify gas in these two regimes as CO-bright and CO-dark, respectively. The mean sound speed in the CO-dark H2 is 0.64 km/s, significantly l…
Star formation on galactic scales is known to be a slow process, but whether it is slow on smaller scales is uncertain. We cross-correlate 5469 giant molecular clouds (GMCs) from a new all-sky catalog with 256 star forming complexes (SFCs) to build a sample of 191 SFC-GMC complexes---collections of multiple clouds each matched to 191 SFCs. The total mass in stars harbored by these clouds is inferred from WMAP free-free fluxes. We measure the GMC mass, the virial parameter, the star formation efficiency $ε$ and the star formation rate per free-fall time $ε_{\rm ff}$. Both $ε$ and $ε_{\rm ff}$ r…
We analyze radial and azimuthal variations of the phase balance between the molecular and atomic ISM in the Milky Way. In particular, the azimuthal variations -- between spiral arm and interarm regions -- are analyzed without any explicit definition of spiral arm locations. We show that the molecular gas mass fraction, i.e., fmol=H2/ (HI+H2) in mass, varies predominantly in the radial direction: starting from ~100% at the center, remaining ~>50% (~>60%) to R~6kpc, and decreasing to ~10-20% (~50%) at R=8.5 kpc when averaged over the whole disk thickness (in the mid plane). Azimuthal, arm-intera…
Emission from carbon monoxide (CO) is ubiquitously used as a tracer of dense star forming molecular clouds. There is, however, growing evidence that a significant fraction of CO emission originates from diffuse molecular gas. Quantifying the contribution of diffuse CO-emitting gas is vital for understanding the relation between molecular gas and star formation. We examine the Galactic distribution of two CO-emitting gas components, a high column density component detected in 13CO and 12CO, and a low column density component detected in 12CO, but not in 13CO. The "diffuse" and "dense" component…
This study developed an automated identification procedure for compact clouds with broad velocity widths in the spectral line data cubes of highly crowded regions. The procedure was applied to the CO J=3-2 line data, obtained using the James Clerk Maxwell Telescope, to identify 184 high velocity dispersion compact clouds (HVCCs), which is a category of peculiar molecular clouds found in the central molecular zone of our galaxy. A list of HVCCs in the area -1.4°<l<+2.0°, -0.25°<b<+0.25° was presented with their physical parameters, CO J=3-2/J=1-0 intensity ratios, and morphological classificati…
The Central Molecular Zone (CMZ) spans the inner ~450 pc (3 degrees) of our Galaxy. This region is defined by its enhanced molecular emission and contains 5% of the entire Galaxy's molecular gas mass. However, the number of detected star forming sites towards the CMZ may be low for the amount of molecular gas that is present, and improved surveys of star formation indicators can help clarify this. With the Karl G Jansky Very Large Array (VLA), we conducted a blind survey of 6.7 GHz methanol masers spanning the inner 3deg x 40arcmin (450 pc x 100 pc) of the Galaxy. We detected 43 methanol maser…
We examine the role of the large-scale galactic-dynamical environment in setting the properties of giant molecular clouds in Milky Way-like galaxies. We perform three high-resolution simulations of Milky Way-like discs with the moving-mesh hydrodynamics code Arepo, yielding a statistical sample of $\sim 80,000$ giant molecular clouds and $\sim 55,000$ HI clouds. We account for the self-gravity of the gas, momentum and thermal energy injection from supernovae and HII regions, mass injection from stellar winds, and the non-equilibrium chemistry of hydrogen, carbon and oxygen. By varying the exte…
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…
By mapping the molecular fraction of the Galactic Center (GC), we quantitatively address the question of how much molecular and central the CMZ (Central Molecular Zone) is. For this purpose we analyze the CO and HI-line archival data, and determine the column- (surface-) and volume-molecular fractions, $f_{\rm mol}^Σ$ and $f_{\rm mol}^ρ$, which are the ratio of column-mass density of H$_2$ projected on the sky to that of total gas (H$_2$ + HI) from the line intensities, and the ratio of volume-mass densities of \Htwo\ to total gas from the brightness temperature, respectively. It is shown that…
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…
We investigate star formation at very early evolutionary phases in five massive clouds in the inner 500 pc of the Galaxy, the Central Molecular Zone. Using interferometer observations of H$_2$O masers and ultra-compact H II regions, we find evidence of ongoing star formation embedded in cores of 0.2 pc scales and $\gtrsim$10$^5$ cm$^{-3}$ densities. Among the five clouds, Sgr C possesses a high (9%) fraction of gas mass in gravitationally bound and/or protostellar cores, and follows the dense ($\gtrsim$10$^4$ cm$^{-3}$) gas star formation relation that is extrapolated from nearby clouds. The o…
A detailed comparison of HI and CO line cube data of the Galactic Center (GC) region from the archives is obtained. The central molecular zone (CMZ) is shown to be embedded in the HI disc (central HI zone, CHZ) of radius $\sim 320$ pc and vertical scale height $\sim 70$ pc. A radio continuum belt is shown to run parallel to molecular Arms I and II. The belt draws a double-infinity shape on the sky, connecting Sgr E ($l\sim -1^\circ.2)$, C, B1, B2 and Sgr D ($+1^\circ.2$), and is interpreted as a warping star-forming ring. The molecular Arms are closely associated with the HI arms on the longit…
The Milky Way Joins the Extragalactic World: I. PHANGS
The University of Texas at Austin（德克萨斯大学奥斯汀分校）；INAF-IAPS（意大利国家天体物理研究所空间天文与物理学中心）；INAF, Sezione di Lecce（意大利国家天体物理研究所莱切分部）；University of Kentucky（肯塔基大学）；National Astronomical Observatory of Japan（日本国立天文台）；Max Planck Institute for Radio Astronomy（马克斯·普朗克射电天文研究所）；The Ohio State University（俄亥俄州立大学）
Exploring the Sun's birth radius and the distribution of planet building blocks in the Milky Way galaxy: A multi-zone Galactic chemical evolution approach
We aim to compare variations in the full-UV dust extinction curve (912-3000 Angstrom), with the HI/H$_2$/total H content along diffuse Milky Way sightlines, to investigate possible connections between ISM conditions and dust properties. We combine an existing sample of 75 UV extinction curves based on IUE and FUSE data, with atomic and molecular column densities measured through UV absorption. The H$_2$ column density data are based on existing Lyman-Werner absorption band models from earlier work on the extinction curves. Literature values for the HI column density were compiled, and improved…
The central molecular zone (CMZ) plays an essential role in regulating the nuclear ecosystem of our Galaxy. To get an insight into the magnetic fields of the CMZ, we employ the Gradient Technique (GT), which is rooted in the anisotropy of magnetohydrodynamic turbulence. Our analysis is based on the data of multiple wavelengths, including molecular emission lines, radio 1.4 GHz continuum image, and Herschel 70 $μ$m image, as well as ionized [Ne II] and Paschen-alpha emissions. The results are compared with the observations of Planck 353 GHz and High-resolution Airborne Wideband Camera Plus (HWA…
The Central Molecular Zone (CMZ, the central 500 pc of the Milky Way) contains the largest reservoir of high-density molecular gas in the Galaxy, but forms stars at a rate 10-100 times below commonly-used star formation relations. We discuss recent efforts in understanding how the nearest galactic nucleus forms its stars. The latest models of the gas inflow, star formation, and feedback duty cycle reproduce the main observable features of the CMZ, showing that star formation is episodic and that the CMZ currently resides at a star formation minimum. Using orbital modelling, we derive the three…
The all-Galaxy CO survey of Dame, Hartmann, & Thaddeus (2001) is by far the most uniform, large-scale Galactic CO survey. Using a dendrogram-based decomposition of this survey, we present a catalog of 1064 massive molecular clouds throughout the Galactic plane. This catalog contains $2.5 \times 10^8$ solar masses, or $25^{+10.7}_{-5.8} \%$ of the Milky Way's estimated H$_2$ mass. We track clouds in some spiral arms through multiple quadrants. The power index of Larson's first law, the size-linewidth relation, is consistent with 0.5 in all regions - possibly due to an observational bias - but c…
YSO search toward the boundary of the Central Molecular Zone with near-infrared polarimetry
我们使用1.4米红外巡天望远镜上的近红外偏振照相机SIRPOL对中心分子区边界进行了近红外偏振测量，测量范围为（-1.4度$\lesssim l \lesssim$ -0.3度和1.0度$\lesssim l \lesssim$ 2.9度，$|b|\lesssim$ 0.1度）。根据Stokes $Q/I-U/I$平面上的星际偏振估计，我们选取了112个本征偏振源。选定的光源比$K_S=14.5$等亮，并且具有偏振不确定性$δP<1\,%$。当使用斯皮策空间望远镜中红外数据档案中的测光时，这些独特的偏振源中的十个与年轻恒星天体的光谱能量分布很好地拟合。然而，许多来源的光谱能量分布是遭受严重星际消光的正常恒星;这些可能是乌云后面的恒星。由于独特的偏振源和年轻天体候选者的数量很少，我们无法判断它们是否在中央分子带之外衰落。文献中许多大质量的年轻天体候选者只有很小的内禀极化。这可能表明它们的质量为4-15 M $_{\rm sun}$，其固有极化预计很小。
