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TOPIC: Milky Way
Blobrana


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RE: Milky Way
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Title: ON the Nature of the Local Spiral Arm of the Milky Way
Authors: Y. Xu, J. J. Li, M. J. Reid, K. M. Menten, X. W. Zheng, A. Brunthaler, L. Moscadelli, T. M. Dame, B. Zhang

Trigonometric parallax measurements of nine water masers associated with the Local arm of the Milky Way were carried out as part of the BeSSeL Survey using the VLBA. When combined with 21 other parallax measurements from the literature, the data allow us to study the distribution and 3-dimensional motions of star forming regions in the spiral arm over the entire northern sky. Our results suggest that the Local arm does not have the large pitch angle characteristic of a short spur. Instead its active star formation, overall length (>5 kpc), and shallow pitch angle (~10 degrees) suggest that it is more like the adjacent Perseus and Sagittarius arms; perhaps it is a branch of one of these arms. Contrary to previous results, we find the Local arm to be closer to the Perseus than to the Sagittarius arm, suggesting that a branching from the former may be more likely. An average peculiar motion of near-zero toward both the Galactic center and north Galactic pole, and counter rotation of ~ 5 km/s were observed, indicating that the Local arm has similar kinematic properties as found for other major spiral arms.

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Blobrana


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Title: ARGOS IV: The Kinematics of the Milky Way Bulge
Authors: M. Ness, K. Freeman, E. Athanassoula, E. Wylie de Boer, J. Bland Hawthorn, M. Asplund, G.F Lewis, D. Yong, R.R. Lane, L.L Kiss, R. IbataVandenberg

We present the kinematic results from our ARGOS spectroscopic survey of the Galactic bulge of the Milky Way. Our aim is to understand the formation of the Galactic bulge. We examine the kinematics of about 17,400 stars in the bulge located within 3.5 kpc of the Galactic centre, identified from the 28,000 star ARGOS survey. We aim to determine if the formation of the bulge has been internally driven from disk instabilities as suggested by its boxy shape, or if mergers have played a significant role as expected from Lambda CDM simulations. From our velocity measurements across latitudes b = -5 deg, -7.5 deg and -10 deg we find the bulge to be a cylindrically rotating system that transitions smoothly out into the disk. Within the bulge, we find a kinematically distinct metal-poor population ([Fe/H] < -1.0) that is not rotating cylindrically. The 5% of our stars with [Fe/H] < -1.0 are a slowly rotating spheroidal population, which we believe are stars of the metal weak thick disk and halo which presently lie in the inner Galaxy. The kinematics of the two bulge components that we identified in ARGOS paper III (mean [Fe/H] = -0.25 and [Fe/H] = +0.15, respectively) demonstrate that they are likely to share a common formation origin and are distinct from the more metal poor populations of the thick disk and halo which are colocated inside the bulge. We do not exclude an underlying merger generated bulge component but our results favour bulge formation from instabilities in the early thin disk.

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Blobrana


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Milky Way bubbles
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Giant Milky Way bubbles blown by black hole merger

A tiny galaxy that collided with the Milky Way spawned two huge bubbles of high-energy particles that now tower over the centre of our galaxy. This new model for the birth of the mysterious bubbles also explains discrepancies in the ages of stars at the galactic middle.
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Blobrana


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RE: Milky Way
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Runaway stars to fill in the blanks in Milky Way map

Guides to the galaxy might call it Zona Galactica Incognita - the half of our home galaxy we know little about. Indeed, the Milky Way is one of the least charted spiral galaxies in the nearby universe. Now it seems that stars kicked out of their birth clusters can help fill in the void and create the first proper map of the entire galaxy.
Young star clusters and clouds of hydrogen that formed in our galaxy help trace the shapes of the Milky Way's arms, so astronomers are reasonably certain that it has a spiral structure. Observations of stellar motion show that there is a supermassive black hole at its core. 
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Blobrana


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First "Bone" of the Milky Way Identified

Our Milky Way is a spiral galaxy - a pinwheel-shaped collection of stars, gas and dust. It has a central bar and two major spiral arms that wrap around its disk. Since we view the Milky Way from the inside, its exact structure is difficult to determine. Astronomers have identified a new structure in the Milky Way: a long tendril of dust and gas that they are calling a "bone."
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Blobrana


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Astronomers identify the stellar patrons of the Milky Way bar

Forget the restaurant at the end of the Universe - astronomers now have the clearest understanding yet of the bar at the center of the Milky Way.
Scientists with the Sloan Digital Sky Survey III (SDSS-III) have announced the discovery of hundreds of stars rapidly moving together in long, looping orbits around the center of our Galaxy.
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Blobrana


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Title: ARGOS III: Stellar Populations in the Galactic Bulge of the Milky Way
Authors: M. Ness, K. Freeman, E. Athanassoula, E. Wylie de Boer, J. Bland Hawthorn, M. Asplund, G. F. Lewis, D. Yong, R. R. Lane, L. L. Kiss

We present the metallicity results from the ARGOS spectroscopic survey of the Galactic bulge. Our aim is to understand the formation of the Galactic bulge: did it form via mergers, as expected from Lambda CDM theory, or from disk instabilities, as suggested by its boxy/peanut shape, or both? We have obtained spectra for 28,000 stars at a spectral resolution of R = 11,000. From these spectra, we have determined stellar parameters and distances to an accuracy of < 1.5 kpc. The stars in the inner Galaxy span a large range in [Fe/H], -2.8 < [Fe/H] < +0.6. From the spatial distribution of the red clump stars as a function of [Fe/H] (Ness et al. 2012a), we propose that the stars with [Fe/H] > -0.5 are part of the boxy/peanut bar/bulge. We associate the lower metallicity stars ([Fe/H] < -0.5) with the thick disk, which may be puffed up in the inner region, and with the inner regions of the metal-weak thick disk and inner halo. For the bulge stars with [Fe/H] > -0.5, we find two discrete populations; (i) stars with [Fe/H] ~ -0.25 which provide a roughly constant fraction of the stars in the latitude interval b = -5 deg to -10 deg, and (ii) a kinematically colder, more metal-rich population with mean [Fe/H] ~ +0.15 which is more prominent closer to the plane. The changing ratio of these components with latitude appears as a vertical abundance gradient of the bulge. We attribute both of these bulge components to instability-driven bar/bulge formation from the thin disk. We do not exclude a weak underlying classical merger-generated bulge component, but see no obvious kinematic association of any of our bulge stars with such a classical bulge component. 

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Blobrana


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Mass of Dark Matter Revealed by Precise Measurements of the Galaxy

A research team, led by Associate Professor Mareki Honma from the National Astronomical Observatory of Japan (NAOJ), has succeeded in precisely determining the astronomical yardstick for the Galaxy based upon the precise distance measurements with VERA from NAOJ and other advanced radio telescopes. The new findings are that the distance from the sun to the Galactic center is 26,100 light-years, and that the Galactic rotation velocity in the solar system is 240km/s.
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Blobrana


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Galactic Spiral Arms
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Title: Structured Molecular Gas Reveals Galactic Spiral Arms
Authors: Tsuyoshi Sawada, Tetsuo Hasegawa, Jin Koda

We explore the development of structures in molecular gas in the Milky Way by applying the analysis of the brightness distribution function (BDF) and the brightness distribution index (BDI) in the archival data from the Boston University-Five College Radio Astronomy Observatory 13CO J=1-0 Galactic Ring Survey. The BDI measures the fractional contribution of spatially confined bright molecular emission over faint emission extended over large areas. This relative quantity is largely independent of the amount of molecular gas and of any conventional, pre-conceived structures, such as cores, clumps, or giant molecular clouds. The structured molecular gas traced by higher BDI is located continuously along the spiral arms in the Milky Way in the longitude-velocity diagram. This clearly indicates that molecular gas changes its structure as it flows through the spiral arms. Although the high-BDI gas generally coincides with H II regions, there is also some high-BDI gas with no/little signature of ongoing star formation. These results support a possible evolutionary sequence in which unstructured, diffuse gas transforms itself into a structured state on encountering the spiral arms, followed by star formation and an eventual return to the unstructured state after the spiral arm passage.

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Blobrana


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Posts: 131433
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RE: Milky Way
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A fresh look at our Galaxy points to a chaotic past and a violent end.

Astronomers are still arguing about the precise sequence of events during the Milky Ways birth, but every-one agrees that the story began with dark matter. The stuff is everywhere, even though it is invisible and no one yet knows what it is. It outweighs ordinary matter - stars, gas and everything else made of atoms - by a factor of about five, and yet can be detected only through its gravitational pull on visible stars and galaxies. Astronomers have known since the 1970s that the Milky Way, like every other galaxy, is wrapped in a vast cocoon of dark matter; without it, the gravity generated by ordinary matter would be nowhere near enough to hold the Galaxy together.
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