* Astronomy

Title: The spiral structure of our Milky Way Galaxy
Authors: L. G. Hou (NAOC), J. L. Han (NAOC), W. B. Shi (NAOC)
(Version v2)

The spiral structure of our Milky Way Galaxy is not yet known. HII regions and giant molecular clouds are the most prominent spiral tracers. We collected the spiral tracer data of our Milky Way from the literature, namely, HII regions and giant molecular clouds (GMCs). With weighting factors based on the excitation parameters of HII regions or the masses of GMCs, we fitted the distribution of these tracers with models of two, three, four spiral-arms or polynomial spiral arms. The distances of tracers, if not available from stellar or direct measurements, were estimated kinetically from the standard rotation curve of Brand & Blitz (1993) with R_0=8.5 kpc, and Theta_0=220 km s^{-1} or the newly fitted rotation curves with R_0=8.0 kpc and Theta_0=220 km s^{-1} or R_0=8.4 kpc and Theta_0=254 km s^{-1}. We found that the two-arm logarithmic model cannot fit the data in many regions. The three- and the four-arm logarithmic models are able to connect most tracers. However, at least two observed tangential directions cannot be matched by the three- or four-arm model. We composed a polynomial spiral arm model, which can not only fit the tracer distribution but also match observed tangential directions. Using new rotation curves with R_0=8.0 kpc and Theta_0=220 km s^{-1} and R_0=8.4 kpc and Theta_0=254 km s^{-1} for the estimation of kinematic distances, we found that the distribution of HII regions and GMCs can fit the models well, although the results do not change significantly compared to the parameters with the standard R_0 and Theta_0.

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Title: Interaction of HVCs with the Outskirts of Galactic Disks: Turbulence
Authors: A. Santillan, F.J. Sanchez-Salcedo, J. Kim, J. Franco, L. Hernandez-Cervantes

There exist many physical processes that may contribute to the driving of turbulence in galactic disks. Some of them could drive turbulence even in the absence of star formation. For example, hydrodynamic (HD) or magnetohydrodynamic (MHD) instabilities, frequent mergers of small satellite clumps, ram pressure, or infalling gas clouds. In this work we present numerical simulations to study the interaction of compact high velocity clouds (CHVC) with the outskirts of magnetised gaseous disks. With our numerical simulations we show that the rain of small HVCs onto the disk is a potential source of random motions in the outer parts of HI disks.

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Title: Discrete sources as the origin of the Galactic X-ray ridge emission
Authors: Revnivtsev M. (1,2), Sazonov S. (2,3), Churazov E. (3,2), Forman W. (4), Vikhlinin A. (4,2), Sunyaev R. (3,2) ((1) Excellence Cluster Universe, Garching, Germany; (2) IKI, Moscow, Russia, (3) MPA, Garching, Germany, (4) CfA, Cambridge, USA)

An unresolved X-ray glow (at energies above a few kiloelectronvolts) was discovered about 25 years ago and found to be coincident with the Galactic disk -the Galactic ridge X-ray emission. This emission has a spectrum characteristic of a 1e8 K optically thin thermal plasma, with a prominent iron emission line at 6.7 keV. The gravitational well of the Galactic disk, however, is far too shallow to confine such a hot interstellar medium; instead, it would flow away at a velocity of a few thousand kilometres per second, exceeding the speed of sound in gas. To replenish the energy losses requires a source of 10^{43} erg/s, exceeding by orders of magnitude all plausible energy sources in the Milky Way. An alternative is that the hot plasma is bound to a multitude of faint sources, which is supported by the recently observed similarities in the X-ray and near-infrared surface brightness distributions (the latter traces the Galactic stellar distribution). Here we report that at energies of 6-7 keV, more than 80 per cent of the seemingly diffuse X-ray emission is resolved into discrete sources, probably accreting white dwarfs and coronally active stars.

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