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Register a new userThe Milky Ways nuclear stellar disc: A dynamically cool and metal-rich component formed from the Central Molecular Zone?
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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 NSD and compare those with a Galactic Bulge sample of APOGEE (DR16) and data from the NSC. We find that the metallicity distribution function and the fraction of metal-rich and metal-poor stars in the NSD are different from the corresponding distributions and ratios of the NSC and the Galactic Bulge. By tracing the velocity dispersion as a function of metallicity, we clearly see that the NSD is kinematically cool and that the velocity dispersion decreases with increasing metallicity contrary to the inner Bulge sample of APOGEE ($rm |b| < 4^{o}$). Using molecular gas tracers ($rm H_{2}CO$, CO(4-3)) of the Central Molecular Zone (CMZ) we find an astonishing agreement between the gas rotation and the rotation of the metal-rich population indicating that the metal-rich stars could have formed from gas in the CMZ. On the other hand, the metal-poor stars show a much slower rotation profile with signs of counter-rotation indicating a different origin of these stars. Coupling kinematics with global metallicities, our results demonstrate that the NSD is chemically and kinematically distinct with respect to the inner Bulge indicating a different formation scenario.
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The nuclear stellar disc (NSD) is a flattened stellar structure that dominates the gravitational potential of the Milky Way at Galactocentric radii $30 lesssim R lesssim 300{, rm pc}$. In this paper, we construct axisymmetric Jeans dynamical models of the NSD based on previous photometric studies and we fit them to line-of-sight kinematic data of APOGEE and SiO maser stars. We find that (i) the NSD mass is lower but consistent with the mass independently determined from photometry by Launhardt et al. (2002). Our fiducial model has a mass contained within spherical radius $r=100{, rm pc}$ of $M(r<100{, rm pc}) = 3.9 pm 1 times 10^8 {, rm M_odot}$ and a total mass of $M_{rm NSD} = 6.9 pm 2 times 10^8 {, rm M_odot}$. (ii) The NSD might be the first example of a vertically biased disc, i.e. with ratio between the vertical and radial velocity dispersion $sigma_z/sigma_R>1$. Observations and theoretical models of the star-forming molecular gas in the central molecular zone suggest that large vertical oscillations may be already imprinted at stellar birth. However, the finding $sigma_z/sigma_R > 1$ depends on a drop in the velocity dispersion in the innermost few tens of parsecs, on our assumption that the NSD is axisymmetric, and that the available (extinction corrected) stellar samples broadly trace the underlying light and mass distributions, all of which need to be established by future observations and/or modelling. (iii) We provide the most accurate rotation curve to date for the innermost $500 {, rm pc}$ of our Galaxy.
The Milky Ways 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 $Rgtrsim100 {, rm pc}$, 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.
We use hydrodynamical simulations to study the Milky Ways 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 $Rsim 5kpc$. Our main findings are as follows: (1) The distinction between inner ($Rlesssim120$~pc) and outer ($120lesssim Rlesssim450$~pc) CMZ that is sometimes proposed in the literature is unnecessary. Instead, the CMZ is best described as single structure, namely a star-forming ring with outer radius $Rsimeq 200$~pc which includes the 1.3$^circ$ complex and which is directly interacting with the dust lanes that mediate the bar-driven inflow. (2) This accretion can induce a significant tilt of the CMZ out of the plane. A tilted CMZ might provide an alternative explanation to the $infty$-shaped structure identified in Herschel data by Molinari et al. 2011. (3) The bar in our simulation efficiently drives an inflow from the Galactic disc ($Rsimeq 3$~kpc) down to the CMZ ($Rsimeq200$~pc) of the order of $1rm,M_odot,yr^{-1}$, consistent with observational determinations. (4) Supernova feedback can drive an inflow from the CMZ inwards towards the circumnuclear disc of the order of $sim0.03,rm M_odot,yr^{-1}$. (5) We give a new interpretation for the 3D placement of the 20 and 50 km s$^{-1}$ clouds, according to which they are close ($Rlesssim30$~pc) to the Galactic centre, but are also connected to the larger-scale streams at $Rgtrsim100$~pc.
We present the results of a large-scale proper motion study of the central ~36x16 of the Milky Way, based on our high angular resolution GALACTICNUCLEUS survey (epoch 2015) combined with the HST Paschen-alpha survey (epoch 2008). Our catalogue contains roughly 80,000 stars, an unprecedented kinematic data set for this region. We describe the data analysis and the preparation of the proper motion catalogue. We verify the catalogue by comparing our results with measurements from previous work and data. We provide a preliminary analysis of the kinematics of the studied region. Foreground stars in the Galactic Disc can be easily identified via their small reddening. Consistent with previous work and with our expectations, we find that stars in the nuclear stellar disc have a smaller velocity dispersion than Bulge stars, in particular in the direction perpendicular to the Galactic Plane. The rotation of the nuclear stellar disc can be clearly seen in the proper motions parallel to the Galactic Plane. Stars on the near side of the nuclear stellar disc are less reddened than stars on its far side. Proper motions enable us to detect co-moving groups of stars that may be associated with young clusters dissolving in the Galactic Centre that are difficult to detect by other means. We demonstrate a technique based on a density clustering algorithm that can be used to find such groups of stars.
In this paper we present the CMZoom Surveys 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 the high completeness catalog. These sources have effective radii between 0.04-0.4 pc, and are the potential progenitors of star clusters. The masses for both catalogs are dominated by the Sagittarius B2 cloud complex, where masses are likely unreliable due to free-free contamination, uncertain dust temperatures, and line-of-sight confusion. Given the survey selection and completeness, we predict that our robust catalog accounts for more than ~99% of compact substructure capable of forming high mass stars in the CMZ. This catalog provides a crucial foundation for future studies of high-mass star formation in the Milky Ways Galactic Center.
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