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Register a new userComparing Young Massive Clusters and their Progenitor Clouds in the Milky Way
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Young massive clusters (YMCs) have central stellar mass surface densities exceeding $10^{4} M_{odot} pc^{-2}$. It is currently unknown whether the stars formed at such high (proto)stellar densities. We compile a sample of gas clouds in the Galaxy which have sufficient gas mass within a radius of a few parsecs to form a YMC, and compare their radial gas mass distributions to the stellar mass distribution of Galactic YMCs. We find that the gas in the progenitor clouds is distributed differently than the stars in YMCs. The mass surface density profiles of the gas clouds are generally shallower than the stellar mass surface density profiles of the YMCs, which are characterised by prominent dense core regions with radii ~ 0.1 pc, followed by a power-law tail. On the scale of YMC core radii, we find that there are no known clouds with significantly more mass in their central regions when compared to Galactic YMCs. Additionally, we find that models in which stars form from very dense initial conditions require surface densities that are generally higher than those seen in the known candidate YMC progenitor clouds. Our results show that the quiescent, less evolved clouds contain less mass in their central regions than in the highly star-forming clouds. This suggests an evolutionary trend in which clouds continue to accumulate mass towards their centres after the onset of star formation. We conclude that a conveyor-belt scenario for YMC formation is consistent with the current sample of Galactic YMCs and their progenitor clouds.
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Aims. Young, massive stars have been found at projected distances R < 0.5 pc from supermassive black hole, Sgr A* at the center of our Galay. In recent years, increasing evidence has been found for the presence of young, massive stars also at R > 0.5 pc. Our goal in this work is a systematic search for young, massive star candidates throughout the entire region within R ~ 2.5 pc of the black hole. Methods. The main criterion for the photometric identification of young, massive early-type stars is the lack of CO-absorption in the spectra. We used narrow-band imaging with VLT/ISAAC to search for young, massive stars within ~2.5 pc of Sgr A*. Results. We have found 63 early-type star candidates at R < 2.5 pc, with an estimated erroneous identification rate of only about 20%. Considering their K-band magnitudes and interstellar extinction, they are candidates for Wolf-Rayet stars, supergiants, or early O-type stars. Of these, 31 stars are so far unknown young, massive star candidates, all of which lie at R>0.5pc. The surface number density profile of the young, massive star candidates can be well fit by a single power-law, with Gamma = 1.6 +- 0.17 at R < 2.5 pc, which is significantly steeper than that of the late-type giants that make up the bulk of the observable stars in the NSC. Intriguingly, this power-law is consistent with the power-law that describes the surface density of young, massive stars in the same brightness range at R < 0.5 pc. Conclusions. The finding of a significant number of newly identified early-type star candidates at the Galactic center suggests that young, massive stars can be found throughout the entire cluster which may require us to modify existing theories for star formation at the Galactic center. Follow-up studies are needed to improve the existing data and lay the foundations for a unified theory of star formation in the Milky Ways NSC.
The evolution of the Milky Way disk, which contains most of the stars in the Galaxy, is affected by several phenomena. For example, the bar and the spiral arms of the Milky Way induce radial migration of stars and can trap or scatter stars close to orbital resonances. External perturbations from satellite galaxies can also have a role, causing dynamical heating of the Galaxy, ring-like structures in the disk and correlations between different components of the stellar velocity. These perturbations can also cause phase wrapping signatures in the disk, such as arched velocity structures in the motions of stars in the Galactic plane. Some manifestations of these dynamical processes have already been detected, including kinematic substructure in samples of nearby stars, density asymmetries and velocities across the Galactic disk that differ from the axisymmetric and equilibrium expectations, especially in the vertical direction, and signatures of incomplete phase mixing in the disk. Here we report an analysis of the motions of six million stars in the Milky Way disk. We show that the phase-space distribution contains different substructures with various morphologies, such as snail shells and ridges, when spatial and velocity coordinates are combined. We infer that the disk must have been perturbed between 300 million and 900 million years ago, consistent with estimates of the previous pericentric passage of the Sagittarius dwarf galaxy. Our findings show that the Galactic disk is dynamically young and that modelling it as time-independent and axisymmetric is incorrect.
We present the first results from the science demonstration phase for the Hi-GAL survey, the Herschel key-project that will map the inner Galactic Plane of the Milky Way in 5 bands. We outline our data reduction strategy and present some science highlights on the two observed 2{deg} x 2{deg} tiles approximately centered at l=30{deg} and l=59{deg}. The two regions are extremely rich in intense and highly structured extended emission which shows a widespread organization in filaments. Source SEDs can be built for hundreds of objects in the two fields, and physical parameters can be extracted, for a good fraction of them where the distance could be estimated. The compact sources (which we will call cores in the following) are found for the most part to be associated with the filaments, and the relationship to the local beam-averaged column density of the filament itself shows that a core seems to appear when a threshold around A_V of about 1 is exceeded for the regions in the l=59{deg} field; a A_V value between 5 and 10 is found for the l=30{deg} field, likely due to the relatively larger distances of the sources. This outlines an exciting scenario where diffuse clouds first collapse into filaments, which later fragment to cores where the column density has reached a critical level. In spite of core L/M ratios being well in excess of a few for many sources, we find core surface densities between 0.03 and 0.5 g cm-2. Our results are in good agreement with recent MHD numerical simulations of filaments forming from large-scale converging flows.
We present a detailed analysis comparing the velocity fields in molecular clouds and the atomic gas that surrounds them in order to address the origin of the gradients. To that end, we present first-moment intensity-weighted velocity maps of the molecular clouds and surrounding atomic gas. The maps are made from high-resolution 13CO observations and 21-cm observations from the Leiden/Argentine/Bonn Galactic HI Survey. We find that (i) the atomic gas associated with each molecular cloud has a substantial velocity gradient---ranging within 0.02 to 0.07 km/s/pc---whether or not the molecular cloud itself has a substantial linear gradient (ii) If the gradients in the molecular and atomic gas were due to rotation, this would imply that the molecular clouds have less specific angular momentum than the surrounding HI by a factor of 1-6. (iii) Most importantly, the velocity gradient position angles in the molecular and atomic gas are generally widely separated---by as much as 130 degrees in the case of the Rosette Molecular Cloud. This result argues against the hypothesis that molecular clouds formed by simple top-down collapse from atomic gas.
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 Ways estimated H$_2$ mass. We track clouds in some spiral arms through multiple quadrants. The power index of Larsons first law, the size-linewidth relation, is consistent with 0.5 in all regions - possibly due to an observational bias - but clouds in the inner Galaxy systematically have significantly (~ 30%) higher linewidths at a given size, indicating that their linewidths are set in part by Galactic environment. The mass functions of clouds in the inner Galaxy versus the outer Galaxy are both qualitatively and quantitatively distinct. The inner Galaxy mass spectrum is best described by a truncated power-law with a power index of $gamma=-1.6pm0.1$ and an upper truncation mass $M_0 = (1.0 pm 0.2) times 10^7 M_odot$, while the outer Galaxy mass spectrum is better described by a non-truncating power law with $gamma=-2.2pm0.1$ and an upper mass $M_0 = (1.5 pm 0.5) times 10^6 M_odot$, indicating that the inner Galaxy is able to form and host substantially more massive GMCs than the outer Galaxy. Additionally, we have simulated how the Milky Way would appear in CO from extragalactic perspectives, for comparison with CO maps of other galaxies.
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