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Register a new userA Uniform Catalog of Molecular Clouds in the Milky Way
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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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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.
Throughout the Milky Way, molecular clouds typically appear filamentary, and mounting evidence indicates that this morphology plays an important role in star formation. What is not known is to what extent the dense filaments most closely associated with star formation are connected to the surrounding diffuse clouds up to arbitrarily large scales. How are these cradles of star formation linked to the Milky Ways spiral structure? Using archival Galactic plane survey data, we have used multiple datasets in search of large-scale, velocity-coherent filaments in the Galactic plane. In this paper, we present our methods employed to identify coherent filamentary structures first in extinction and confirmed using Galactic Ring Survey data. We present a sample of seven Giant Molecular Filaments (GMFs) that have lengths of order $sim$100 pc, total masses of 10$^4$ - 10$^5$ M$_{odot}$, and exhibit velocity coherence over their full length. The GMFs we study appear to be inter-arm clouds and may be the Milky Way analogues to spurs observed in nearby spiral galaxies. We find that between 2 and 12% of the total mass (above $sim$10$^{20}$ cm$^{-2}$) is dense (above 10$^{22}$ cm$^{-2}$), where filaments near spiral arms in the Galactic midplane tend to have higher dense gas mass fractions than those further from the arms.
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 the HIFI instrument onboard Herschel. Based on Jansky Very Large Array data, we employ the 21 cm HI absorption-line technique to construct profiles of the HI opacity versus radial velocity toward our target sources. These profiles are combined with lower resolution archival data of extended HI emission to calculate the HI column densities of the individual clouds along the sight lines. We employ Bayesian inference to estimate the uncertainties of the derived quantities. Our study delivers reliable estimates of the atomic hydrogen column density for a large number of diffuse molecular clouds at various Galactocentric distances. Together with column densities of molecular hydrogen derived from its surrogates observed with HIFI, the measurements can be used to characterize the clouds and investigate the dependence of their chemistry on the molecular fraction, for example.
We report the relative abundances of the three stable isotopes of silicon, $^{28}$Si, $^{29}$Si and $^{30}$Si, across the Galaxy using the $v = 0, J = 1 to 0$ transition of silicon monoxide. The chosen sources represent a range in Galactocentric radii ($R_{rm GC}$) from 0 to 9.8 kpc. The high spectral resolution and sensitivity afforded by the GBT permit isotope ratios to be corrected for optical depths. The optical-depth-corrected data indicate that the secondary-to-primary silicon isotope ratios $^{29}{rm Si}/^{28}{rm Si}$ and $^{30}{rm Si}/^{28}{rm Si}$ vary much less than predicted on the basis of other stable isotope ratio gradients across the Galaxy. Indeed, there is no detectable variation in Si isotope ratios with $R_{rm GC}$. This lack of an isotope ratio gradient stands in stark contrast to the monotonically decreasing trend with $R_{rm GC}$ exhibited by published secondary-to-primary oxygen isotope ratios. These results, when considered in the context of the expectations for chemical evolution, suggest that the reported oxygen isotope ratio trends, and perhaps that for carbon as well, require further investigation. The methods developed in this study for SiO isotopologue ratio measurements are equally applicable to Galactic oxygen, carbon and nitrogen isotope ratio measurements, and should prove useful for future observations of these isotope systems.
Using data from the Galactic All-Sky Survey, we have compared the properties and distribution of HI clouds in the disk-halo transition at the tangent points in mirror-symmetric regions of the first quadrant (QI) and fourth quadrant (QIV) of the Milky Way. Individual clouds are found to have identical properties in the two quadrants. However, there are 3 times as many clouds in QI as in QIV, their scale height is twice as large, and their radial distribution is more uniform. We attribute these major asymmetries to the formation of the clouds in the spiral arms of the Galaxy, and suggest that the clouds are related to star formation in the form of gas that has been lifted from the disk by superbubbles and stellar feedback, and fragments of shells that are falling back to the plane.
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