2,402 research outputs found

    Gaussian decomposition of HI surveys. IV. Galactic intermediate- and high-velocity clouds

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    Traditionally IVC and HVC were defined to be concentrations of HI gas, with line-of-sight velocities that are inconsistent with data on the differential rotation of the Galaxy. We demonstrate that IVCs and HVCs can be identified from certain density enhancements in (V_C, FWHM) distribution of Gaussians, representing the Galactic HI 21 cm radio lines. We study the Gaussians, which parameters fall into the regions of the phase-space density enhancements about $(V_C, FWHM) = (-131, 27), (164, 26) and (-49 km/s, 23 km/s). The sky distribution of the Gaussians, corresponding to the first two concentrations, very well represents the sky distribution of HVCs, as obtained on the basis of the traditional definition of these objects. The Gaussians of the last concentration correspond to IVCs. Based on this identification, the division line between IVCs and HVCs can be drawn at about |V_C| = 74 km/s, and IVCs can be identified down to velocities of about |V_C| = 24 km/s. Traces of both IVCs and HVCs can also be seen in the sky distribution of Gaussians with FWHM = 7.3 km/s. In HVCs, these cold cores have small angular dimensions and low observed brightness temperatures T_b. In IVCs, the cores are both larger and brighter. This definition of IVCs and HVCs is less dependant than the traditional one, on the differential rotation model of the Galaxy. The consideration of line-width information may enable IVCs and HVCs to be better distinguished from each other, and from the ordinary Galactic HIComment: 9 pages, 7 figures. Accepted for publication in A&A. High-resolution version available at http://www.aai.ee/~urmas/ast/HVCc.pdf (12.4 MBaits

    Molecular gas in high-velocity clouds: revisited scenario

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    We report a new search for 12CO(1-0) emission in high-velocity clouds (HVCs) performed with the IRAM 30 m telescope. This search was motivated by the recent detection of cold dust emission in the HVCs of Complex C. Despite a spatial resolution which is three times better and sensitivity twice as good compared to previous studies, no CO emission is detected in the HVCs of Complex C down to a best 5 sigma limit of 0.16 K km/s at a 22'' resolution. The CO emission non-detection does not provide any evidence in favor of large amounts of molecular gas in these HVCs and hence in favor of the infrared findings. We discuss different configurations which, however, allow us to reconcile the negative CO result with the presence of molecular gas and cold dust emission. H2 column densities higher than our detection limit, N(H2) = 3x10^{19} cm^{-2}, are expected to be confined in very small and dense clumps with 20 times smaller sizes than the 0.5 pc clumps resolved in our observations according to the results obtained in cirrus clouds, and might thus still be highly diluted. As a consequence, the inter-clump gas at the 1 pc scale has a volume density lower than 20 cm^{-3} and already appears as too diffuse to excite the CO molecules. The observed physical conditions in the HVCs of Complex C also play an important role against CO emission detection. It has been shown that the CO-to-H2 conversion factor in low metallicity media is 60 times higher than at the solar metallicity, leading for a given H2 column density to a 60 times weaker integrated CO intensity. And the very low dust temperature estimated in these HVCs implies the possible presence of gas cold enough (< 20 K) to cause CO condensation onto dust grains under interstellar medium pressure conditions and thus CO depletion in gas-phase observations.Comment: 9 pages, 4 figures, Accepted for publication in A&

    High-resolution 21-cm observations of low-column density gas clumps in the Milky Way halo

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    We study the properties of low-column density gas clumps in the halo of the Milky Way based on high-resolution 21-cm observations. Using interferometric data from the WSRT and the VLA we study HI emission at low-, intermediate- and high radial velocities along four lines of sight towards quasars. Along these sightlines we previously detected weak CaII and NaI absorbers in their optical spectra. The analysis of the high-resolution HI data reveals the presence of several compact and cold clumps of neutral gas at velocities similar to the optical absorption. The clumps have narrow HI line widths in the range of 1.8 to 13 km/s, yielding upper limits for the kinetic temperature of the gas of 70 to 3700 K. The neutral gas has low HI column densities in the range of 5E18 to 3E19 1/cm^2. All clumps have angular sizes of only a few arcminutes. Our high-resolution 21-cm observations indicate that many of the CaII and NaI absorbers seen in our optical quasar spectra are associated with low-column density HI clumps at small angular scales. This suggests that next to the massive, high-column density neutral gas clouds in the halo (the common 21-cm LVCs, IVCs, and HVCs) there exists a population of low-mass, neutral gas structures in the halo that remain mostly unseen in the existing 21-cm all-sky surveys of IVCs and HVCs. The estimated thermal gas pressures of the detected HI clumps are consistent with what is expected from theoretical models of gas in the inner and outer Milky Way halo.Comment: 12 pages, 7 figure

    High-Velocity Clouds in the Nearby Spiral Galaxy M 83

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    We present deep HI 21-cm and optical observations of the face-on spiral galaxy M 83 obtained as part of a project to search for high-velocity clouds (HVCs) in nearby galaxies. Anomalous-velocity neutral gas is detected toward M 83, with 5.6x10^7 Msolar of HI contained in a disk rotating 40-50 km/s more slowly in projection than the bulk of the gas. We interpret this as a vertically extended thick disk of neutral material, containing 5.5% of the total HI within the central 8 kpc. Using an automated source detection algorithm to search for small-scale HI emission features, we find eight distinct, anomalous-velocity HI clouds with masses ranging from 7x10^5 to 1.5x10^7 Msolar and velocities differing by up to 200 km/s compared to the HI disk. Large on-disk structures are coincident with the optical spiral arms, while unresolved off-disk clouds contain no diffuse optical emission down to a limit of 27 r' mag per square arcsec. The diversity of the thick HI disk and larger clouds suggests the influence of multiple formation mechanisms, with a galactic fountain responsible for the slowly-rotating disk and on-disk discrete clouds, and tidal effects responsible for off-disk cloud production. The mass and kinetic energy of the HI clouds are consistent with the mass exchange rate predicted by the galactic fountain model. If the HVC population in M 83 is similar to that in our own Galaxy, then the Galactic HVCs must be distributed within a radius of less than 25 kpc.Comment: 30 pages, 23 figures; accepted for publication in ApJ. Some figures have been altered to reduce their siz

    A Single-Stage Approach to Anscombe and Aumann's Expected Utility

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    expected utility theory;decision analysis;revealed preference

    Comonotonic Book-Making with Nonadditive Probabilities

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    This paper shows how de Finetti's book-making principle, commonly used to justify additive subjective probabilities, can be modi-ed to agree with some nonexpected utility models.More precisely, a new foundation of the rank-dependent models is presented that is based on a comonotonic extension of the book-making principle.The extension excludes book-making only if all gambles considered induce a same rank-ordering of the states of nature through favorableness of their associated outcomes, and allows for nonadditive probabilities.Typical features of rank-dependence, hedging, ambiguity aversion, and pessimism and optimism, can be accommodated.Book-making;comonotonic;Choquet expected utility;ambiguity aversion;ordered vector space

    Origin(s) of the Highly Ionized High-Velocity Clouds Based on Their Distances

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    Previous HST and FUSE observations have revealed highly ionized high-velocity clouds (HVCs) or more generally low HI column HVCs along extragalactic sightlines over 70-90% of the sky. The distances of these HVCs have remained largely unknown hampering to distinguish a "Galactic" origin (e.g., outflow, inflow) from a "Local Group" origin (e.g., warm-hot intergalactic medium). We present the first detection of highly ionized HVCs in the Cosmic Origins Spectrograph (COS) spectrum of the early-type star HS1914+7134 (l = 103, b=+24) located in the outer region of the Galaxy at 14.9 kpc. Two HVCs are detected in absorption at v_LSR = -118 and -180 km/s in several species, including CIV, SiIV, SiIII, CII, SiII, OI, but HI 21-cm emission is only seen at -118 \km. Within 17 degrees of HS1914+7134, we found HVC absorption of low and high ions at similar velocities toward 5 extragalactic sight lines, suggesting that these HVCs are related. The component at -118 km/s is likely associated with the Outer Arm of the Milky Way. The highly ionized HVC at -180 km/s is an HVC plunging at high speed onto the thick disk of the Milky Way. This is the second detection of highly ionized HVCs toward Galactic stars, supporting a "Galactic" origin for at least some of these clouds.Comment: Submitted to the ApJ Letter
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