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TOPIC: Milky Way


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Posts: 131433
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RE: Milky Way
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Title: Masses for the Local Group and the Milky Way
Authors: Yang-Shyang Li (1), Simon D. M. White (2) ((1) Kapteyn Astronomical Institute, (2) Max-Plank-Institut für Astrophysik)

We use the very large Millennium Simulation of the concordance \LambdaCDM cosmogony to calibrate the bias and error distribution of Timing Argument estimators of the masses of the Local Group and of the Milky Way. From a large number of isolated spiral-spiral pairs similar to the Milky Way/Andromeda system, we find the interquartile range of the ratio of timing mass to true mass to be a factor of 1.8, while the 5% and 95% points of the distribution of this ratio are separated by a factor of 5.7. Here we define true mass as the sum of the "virial" masses M_{200} of the two dominant galaxies. For current best values of the distance and approach velocity of Andromeda this leads to a median likelihood estimate of the true mass of the Local Group of 5.27 x 10^12 \msun, or \log M_{LG}/M_\odot = 12.72, with an interquartile range of [12.58, 12.83] and a 5% to 95% range of [12.26, 13.01]. Thus a 95% lower confidence limit on the true mass of the Local Group is 1.81 x 10^12 \msun. A timing estimate of the Milky Way's mass based on the large recession velocity observed for the distant satellite Leo I works equally well, although with larger systematic uncertainties. It gives an estimated virial mass for the Milky Way of 2.43 x 10^12 \msun with a 95% lower confidence limit of 0.80 x 10^12 \msun.

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Posts: 131433
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Galactic high-velocity clouds
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Title: Distances to Galactic high-velocity clouds. I. Cohen Stream, complex GCP, cloud g1
Authors: B.P. Wakker (Wisconsin), D.G. York (Chicago), R. Wilhelm (Texas Tech), J.C. Barentine (Texas), P. Richter (Potsdam), T.C. Beers (Michigan State), Z. Ivezic (Washington), J.C. Howk (Notre Dame)

The high- and intermediate-velocity interstellar clouds (HVCs/IVCs) are tracers of energetic processes in and around the Milky Way. Clouds with near-solar metallicity about one kpc above the disk trace the circulation of material between disk and halo (the Galactic Fountain). The Magellanic Stream consists of gas tidally extracted from the SMC, tracing the dark matter potential of the Milky Way. Several other HVCs have low-metallicity and appear to trace the continuing accretion of infalling intergalactic gas. These assertions are supported by the metallicities (0.1 to 1 solar) measured for about ten clouds in the past decade. Direct measurements of distances to HVCs have remained elusive, however. In this paper we present four new distance brackets, using VLT observations of interstellar \CaII H and K absorption toward distant Galactic halo stars. We derive distance brackets of 5.0 to 11.7 kpc for the Cohen Stream (likely to be an infalling low-metallicity cloud), 9.8 to 15.1 kpc for complex GCP (also known as the Smith Cloud or HVC40-15+100 and with still unknown origin), 1.0 to 2.7 kpc for an IVC that appears associated with the return flow of the Fountain in the Perseus Arm, and 1.8 to 3.8 kpc for cloud g1, which appears to be in the outflow phase of the Fountain. Our measurements further demonstrate that the Milky Way is accreting substantial amounts of gaseous material, which influences the Galaxy's current and future dynamical and chemical evolution.

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Posts: 131433
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RSGC2
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The largest known swarm of red supergiant stars has been found near the central bulge of our galaxy. It offers a rare glimpse of massive stars on the verge of exploding.
Red supergiants are among the largest stars in the universe and in fact are second in size only to rare 'hypergiant' stars such Eta Carinae. Spanning several hundred times the diameter of the Sun, each could fit millions of Sun-like stars inside it.
These stellar titans are extremely rare. Only very massive stars, more than 10 times as heavy as the Sun, turn into red supergiants. And the red supergiant phase lasts just 100,000 years before ending in a supernova, a fleeting moment compared to the overall lifespan of the star.
Only about 200 red supergiants have been identified in our galaxy. In 2006, a team led by Don Figer of the Rochester Institute of Technology (RIT) in New York, NY, US, reported finding a massive cluster of thousands of stars that included 14 red supergiants the biggest collection of these rare stars then known.
Now, Ben Davies, also of RIT, and a team that includes Figer have identified an even larger group of 26 red supergiants.

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RSGC2
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About 40 stars near the centre of this image are red supergiants, appearing yellow in this infrared image from the Spitzer Space Telescope. Warm dust in the region glows red. The blue oval with a pink outline at top left may be the result of an ancient supernova and the larger blue patch below centre is a stellar nursery
Credit : B Davies/RIT/NASA



Title: A massive cluster of Red Supergiants at the base of the Scutum-Crux arm
Authors: Ben Davies (RIT), Don F. Figer (RIT), Rolf-Peter Kudritzki (IfA, Hawaii), John MacKenty (STScI), Francisco Najarro (CSIC, Madrid), Artemio Herrero (IAC, Spain)

We report on the unprecedented Red Supergiant (RSG) population of a massive young cluster, located at the base of the Scutum-Crux Galactic arm. We identify candidate cluster RSGs based on {\it 2MASS} photometry and medium resolution spectroscopy. With follow-up high-resolution spectroscopy, we use CO-bandhead equivalent width and high-precision radial velocity measurements to identify a core grouping of 26 physically-associated RSGs -- the largest such cluster known to-date. Using the stars' velocity dispersion, and their inferred luminosities in conjunction with evolutionary models, we argue that the cluster has an initial mass of ~40,000\msun, and is therefore among the most massive in the galaxy. Further, the cluster is only a few hundred parsecs away from the cluster of 14 RSGs recently reported by Figer et al (2006). These two RSG clusters represent 20% of all known RSGs in the Galaxy, and now offer the unique opportunity to study the pre-supernova evolution of massive stars, and the Blue- to Red-Supergiant ratio at uniform metallicity. We use GLIMPSE, MIPSGAL and MAGPIS survey data to identify several objects in the field of the larger cluster which seem to be indicative of recent region-wide starburst activity at the point where the Scutum-Crux arm intercepts the Galactic bulge. Future abundance studies of these clusters will therefore permit the study of the chemical evolution and metallicity gradient of the Galaxy in the region where the disk meets the bulge.

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-- Edited by Blobrana at 22:23, 2007-08-09

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Posts: 131433
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RE: Milky Way
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Is a second black hole lurking in the heart of the Milky Way? The evidence to date is inconclusive, but astronomers say a relatively simple test could settle the matter: look for a pair of stars fleeing the galaxy at break-neck speed.
Astronomers believe there is a colossal black hole weighing about 3.6 million times the Sun's mass at the centre of the Milky Way. But some contend there is also a second black hole there that weighs 1000 to 10,000 Suns.
The evidence comes from observations of a cluster of young stars located just a fraction of a light year from the monstrous black hole where gravitational forces should prevent any stars from forming. The cluster could have formed farther away and migrated there, however, if it contained a middleweight black hole that was gravitationally drawn towards the galactic centre.

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Posts: 131433
Date:
Old open clusters
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Title: Old open clusters in the inner Galaxy: FSR1744, FSR89 and FSR31
Authors: Charles Bonatto, Eduardo Bica

We establish the nature and derive fundamental and structural parameters of the recently catalogued objects FSR1744, FSR89 and FSR31. This work intends to provide clues to constrain the Galactic tidal disruption efficiency, improve statistics of the open cluster parameter space, and better define their age-distribution function inside the Solar circle. Properties of the objects are investigated by means of 2MASS colour-magnitude diagrams and stellar radial density profiles built with field star decontaminated photometry. Diagnostic-diagrams for structural parameters are used to help disentangle dynamical from high-background effects affecting such centrally projected open clusters. FSR1744, FSR89 and FSR31 are Gyr-class OCs located at Galactocentric distances 4.0 - 5.6kpc. Compared to nearby OCs, they have small core and limiting radii. With respect to the small number of OCs observed in the inner Galaxy, the emerging scenario in the near-infrared favours disruption driven by dynamical evolution rather than observational limitations associated with absorption and/or high background levels. Internally, the main processes associated with the dynamical evolution are, e.g. mass loss by stellar evolution, mass segregation and evaporation. Externally they are, e.g. tidal stress from the disk and bulge, and interactions with giant molecular clouds. FSR1744, FSR89 and FSR31 have structural parameters consistent with their Galactocentric distances, in the sense that tidally induced effects may have accelerated the dynamical evolution.

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Posts: 131433
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RE: Milky Way
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The most detailed survey of the entire sky at infrared wavelengths has been made by Japan's AKARI satellite. It will help astronomers produce a complete census of all nearby galaxies.
AKARI, which uses a 0.7-metre-wide mirror, launched into space in February 2006. Now, it has made the first all-sky infrared survey since the IRAS (Infrared Astronomical Satellite) mission, a joint project of the US, UK and the Netherlands, made such a map more than 20 years ago.

milkyway_1
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The disc of our Milky Way galaxy glows at infrared wavelengths in the all-sky survey by Japan's AKARI satellite
Credit: JAXA


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-- Edited by Blobrana at 18:13, 2007-07-11

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After pondering the weighty question of the mass of the Milky Way galaxy, astronomers have come up with an answer: 3 x 1042 kg.
That is, our galaxy weighs three times 10 to the power of 42 kilograms - a number written as 3 followed by 42 zeroes, which has echoes of author Douglas Adams's fictional answer to the question of life, the universe and everything in his series Hitchhiker's Guide to the Galaxy.

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Posts: 131433
Date:
Arches cluster
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Title: The proper motion of the Arches cluster with Keck Laser-Guide Star Adaptive Optics
Authors: Andrea Stolte, Andrea M. Ghez, Mark Morris, Jessica R. Lu, Wolfgang Brandner, Keith Matthews

We present the first measurement of the proper motion of the young, compact Arches cluster near the Galactic centre from near-infrared adaptive optics (AO) data taken with the recently commissioned laser-guide star (LGS) at the Keck 10-m telescope. The excellent astrometric accuracy achieved with LGS-AO provides the basis for a detailed comparison with VLT/NAOS-CONICA data taken 4.3 years earlier. Over the 4.3 year baseline, a spatial displacement of the Arches cluster with respect to the field population is measured to be 24.0 ± 2.2 mas, corresponding to a proper motion of 5.6 ± 0.5 mas/yr or 212 ± 20 km/s at a distance of 8 kpc. In combination with the known radial velocity of the cluster, we derive a 3D space motion of 232 ± 22 km/s of the Arches relative to the field. The large proper motion of the Arches cannot be explained with any of the closed orbital families observed in gas clouds in the bar potential of the inner Galaxy, but would be consistent with the Arches being on a transitional trajectory from x1 to x2 orbits. We investigate a cloud-cloud collision as the possible origin for the Arches cluster. The integration of the cluster orbit in the potential of the inner Galaxy suggests that the cluster formed in the central 200 pc, and still resides inside 200 pc today. A recent replenishment of the young stellar population in the inner few parsecs of the GC appears unlikely unless the present-day distance to the GC is smaller than ~30 pc, very close to the cluster's projected distance from the GC.

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Posts: 131433
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RE: Milky Way
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Radioactive iron, a window to the stars
 
Tracing massive stars in the Galaxy
Tracing massive stars in the Galaxy
 
ESAs orbiting gamma-ray observatory, Integral, has made a pioneering unequivocal discovery of radioactive iron-60 in our galaxy that provides powerful insight into the workings of massive stars that pervade and shape it.
Since late 2002, Integral has been collecting data from across the galaxy. It shows an enhancement in gamma rays at two characteristic energies, 1173 and 1333 kilo electron Volts. These are produced by radioactive decay of iron-60 into cobalt-60. Found drifting in space, the radioactive isotope has been sought for long. All past reported sightings of iron-60 have been subject to controversy. Now Integral has provided unequivocal evidence.

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Posts: 131433
Date:
IRAS 06361-0142
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Title: A new young stellar cluster embedded in a molecular cloud in the far outer Galaxy
Authors: Joao L. Yun, Ana Lopez-Sepulcre, Jose M. Torrelles

We report the discovery of a new young stellar cluster and molecular cloud located in the far outer Galaxy, seen towards IRAS 06361-0142, and we characterise their properties. Near-infrared images were obtained with VLT/ISAAC through JHKs filters, millimetre line observations of CO(1-0) were obtained with SEST, and VLA 6 cm continuum maps obtained from archive data. The cloud and cluster are located at a distance of 7 kpc and a Galactocentric distance of 15 kpc, well in the far outer Galaxy. Morphologically, IRAS 06361-0142 appears as a cluster of several tens of stars surrounded by a nearly spherical nebular cavity centred at the position of the IRAS source. The cluster appears composed of low and intermediate-mass, young reddened stars with a large fraction having cleared the inner regions of their circumstellar discs responsible for (H - Ks) colour excess. The observations are compatible with a 4 Myr cluster with variable spatial extinction between Av = 6 and Av = 13.

newcluster06361-0142
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J (blue), H (green), and Ks (red) colour composite image towards IRAS 06361-0142. The image covers about 70 × 70 arcsec². North is up and East to the left.

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