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Posts: 131433
Date:
Leo I
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Title: The Velocity Dispersion Profile of the Remote Dwarf Spheroidal Galaxy Leo I: A Tidal Hit and Run?
Authors: Mario Mateo, Edward W. Olszewski, Matthew G. Walker

We present kinematic results for a sample of 387 stars located near Leo I based on spectra obtained with the MMT's Hectochelle spectrograph near the MgI/Mgb lines. We estimate the mean velocity error of our sample to be 2.4 km/s, with a systematic error of < 1 km/s. We produce a final sample of 328 Leo I red giant members, from which we measure a mean heliocentric radial velocity of 282.9 ± 0.5 km/s, and a mean radial velocity dispersion of 9.2 ± 0.4 km/s for Leo I. The dispersion profile of Leo I is flat out to beyond its classical `tidal' radius. We fit the profile to a variety of equilibrium dynamical models and can strongly rule out models where mass follows light. Two-component Sersic+NFW models with tangentially anisotropic velocity distributions fit the dispersion profile well, with isotropic models ruled out at a 95% confidence level. The mass and V-band mass-to-light ratio of Leo I estimated from equilibrium models are in the ranges 5-7 x 10^7 M_sun and 9-14 (solar units), respectively, out to 1 kpc from the galaxy centre. Leo I members located outside a `break radius' (about 400 arcsec = 500 pc) exhibit significant velocity anisotropy, whereas stars interior appear to have isotropic kinematics. We propose the break radius represents the location of the tidal radius of Leo I at perigalacticon of a highly elliptical orbit. Our scenario can account for the complex star formation history of Leo I, the presence of population segregation within the galaxy, and Leo I's large outward velocity from the Milky Way. The lack of extended tidal arms in Leo I suggests the galaxy has experienced only one perigalactic passage with the Milky Way, implying that Leo I may have been injected into its present orbit by a third body a few Gyr before perigalacticon.

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Posts: 131433
Date:
Hercules dwarf spheroidal galaxy
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Title: The Elongated Structure of the Hercules dSph from Deep LBT Imaging
Authors: Matthew G. Coleman, Jelte T. A. De Jong, Nicolas F. Martin, Hans-Walter Rix, David J. Sand, Eric F. Bell, Richard W. Pogge, David J. Thompson, H. Hippelein, E. Giallongo, R. Ragazzoni, Andrea DiPaola, Jacopo Farinato, Riccardo Smareglia, Vincenzo Testa, Jill Bechtold, John M. Hill, Peter M. Garnavich, Richard F. Green

We present a deep, wide-field photometric survey of the newly-discovered Hercules dwarf spheroidal galaxy, based on data from the Large Binocular Telescope. Images in B, V and r were obtained with the Large Binocular Camera covering a 23' times 23' field of view to a magnitude of ~25.5 (5 sigma). This permitted the construction of colour-magnitude diagrams that reach approximately 1.5 magnitudes below the Hercules main sequence turnoff. Three-filter photometry allowed us to preferentially select probable Hercules member stars, and examine the structure of this system at a previously unattained level. We find that the Hercules dwarf is highly elongated (3:1), considerably more so than any other dSph satellite of the Milky Way except the disrupting Sagittarius dwarf. While we cannot rule out that the unusual structure is intrinsic to Hercules as an equilibrium system, our results suggest tidal disruption as a likely cause of this highly elliptical structure. Given the relatively large Galactocentric distance of this system (132 ± 12 kpc), signs of tidal disruption would require the Hercules dwarf to be on a highly eccentric orbit around the Milky Way.

Hercules dwarf
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Posts: 131433
Date:
The Magellanic Stream
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Title: The Origin of the Magellanic Stream and Its Leading Arm
Authors: David L. Nidever, Steven R. Majewski, W. Butler Burton

We explore the Magellanic Stream (MS) using a Gaussian decomposition of the HI velocity profiles in the Leiden-Argentine-Bonn (LAB) all-sky HI survey. This exposes the MS to be composed of two filaments distinct spatially (as first pointed out by Putman et al.) and in velocity. Using the velocity coherence of the filaments, one can be traced back to its origin in what we identify as the SE HI Overdensity (SEHO) of the Large Magellanic Cloud (LMC), which includes 30 Doradus. Parts of the Leading Arm (LA) can also be traced back to the SEHO in velocity and position. Therefore, at least one-half of the trailing Stream and most of the LA originates in the LMC, contrary to previous assertions that both the MS and the LA originate in the Small Magellanic Cloud (SMC) and/or in the Magellanic Bridge. The two MS filaments show strong periodic, undulating spatial and velocity patterns that we speculate are an imprint of the LMC rotation curve. If true, then the drift rate of the Stream gas away from the Magellanic Clouds is ~71 km/s and the age of the MS is ~1.3 Gyr. The Staveley-Smith et al. high-resolution HI data of the LMC show gas outflows from supergiant shells in the SEHO which seem to be creating the LA and LMC filament of the MS. This method for blowing out LMC gas has not previously been accounted for but probably plays an important role in creating the MS and LA.

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Posts: 131433
Date:
Willman 1
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New measurements of a faint companion to the Milky Way suggest that it is by far the smallest galaxy ever found. The discovery hints that many more very small and dim galaxies may be lurking out there.
Computer simulations show that the process of building our Milky Way should have left hundreds of small galaxies hovering nearby. But astronomers have only been able to find about 20, creating a puzzling mismatch between theory and observations.
Some scientists have proposed various reasons why the smallest satellite galaxies might be even dimmer than originally expected, allowing them to elude detection.
One idea is that galaxies below a limiting weight of about 10 million Suns are unable to form many stars, making them effectively invisible. The hypothesis was bolstered by the fact that no satellite galaxy below that mass had ever been seen.
Now astronomers appear to have found a galaxy that smashes that lower-mass limit. Willman 1, or Wil 1 for short, was discovered in 2004, orbiting 120,000 light years from the Milky Way, but, at the time, its mass was unknown. New observations of the object have been made by a team of astronomers led by Nicolas Martin of the Max Planck Institute for Astronomy in Heidelberg, Germany.

Willman 1
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Credit: Beth Willman/New York University/Sloan Digital Sky Survey

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Posts: 131433
Date:
Hercules Stream
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New research on the composition and age of a group of stars known as the Hercules Stream raises questions about one of the prevailing theories as to how our galaxy formed, and gives new clues about how these mysterious streams really were created.
One theory for the formation of our Milky Way galaxy is that it results from the mergers of numerous, smaller dwarf galaxies. Many astronomers believe that star streams such as Herculesstars that move at the same velocity in a band or streamare thought to be remnants of those mergers. New research from the University of Michigan doesn't refute those ideas, but does suggest astronomers need to take a much closer look.
Thomas Bensby, University of Michigan research fellow in astronomy, said it follows that if the Hercules Stream is actually a remnant of a dwarf galaxy that merged with others to make the Milky Way, one would expect its stars to result from different star formation histories than the stars in the Milky Way.

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Posts: 131433
Date:
RE: Milky Way Companions
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 Newly discovered stellar streams that arc around our galaxy might be the remnants of cannibalised star clusters and galaxies, scientists announced today.
The stellar streams findings, described by Caltech's Carl Grillmair here at a meeting of the American Astronomical Society, reveal our galaxy can be a dangerous place for passersby.
Stellar streams are thought to form over billions of years as our galaxy's gravity slowly tears apart globular clusters and even dwarf galaxies. The stars, which were once packed tightly together, are now separated by light-years, trailing one another as they jet at high speeds through the galactic halo.

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Posts: 131433
Date:
Giant stellar streams
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An astronomer at the Spitzer Science Centre has discovered three giant stellar streams arcing high over the Milky Way. Remnants of cannibalised galaxies and star clusters, the streams are between 13,000 and 130,000 light-years distant from Earth and extend over much of the northern sky. The new results are being presented by Carl Grillmair at this week's meeting of the American Astronomical Society in Honolulu, Hawaii.
Two of the newly discovered streams are almost certainly the remains of ancient star clusters. Known to astronomers as globular clusters, these giant stellar cities contain between tens of thousands and millions of stars. Though only about 150 globular clusters orbit the Milky Way today, they may once have numbered in the thousands.
Over billions of years, the relentless gravitational stresses inflicted on them by our galaxy have slowly torn them apart, leaving behind long, thin streams of stars. Once crowded so closely together that they could sometimes actually collide, these stars are now separated by many light-years, trailing one another at half a million miles an hour through the dark and lonely reaches of the galactic halo.

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Posts: 131433
Date:
RE: Milky Way Companions
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Title: Cosmic Ménage à Trois: The Origin of Satellite Galaxies On Extreme Orbits
Authors: L. V. Sales, J. F. Navarro, M. G. Abadi, M. Steinmetz
(Version v2)

We examine the orbits of satellite galaxies identified in a suite of N-body/gasdynamical simulations of the formation of L_* galaxies in a LCDM universe. Most satellites follow conventional orbits; after turning around, they accrete into their host halo and settle on orbits whose apocentric radii are steadily eroded by dynamical friction. However, a number of outliers are also present, we find that ~1/3 of satellites identified at z=0 are on unorthodox orbits, with apocenters that exceed their turnaround radii. This population of satellites on extreme orbits consists typically of the faint member of a satellite pair that has been ejected onto a highly-energetic orbit during its first approach to the primary. Since the concurrent accretion of multiple satellite systems is a defining feature of hierarchical models of galaxy formation, we speculate that this three-body ejection mechanism may be the origin of (i) some of the newly discovered high-speed satellites around M31 (such as Andromeda XIV); (ii) some of the distant fast-receding Local Group members, such as Leo I; and (iii) the oddly isolated dwarf spheroidals Cetus and Tucana in the outskirts of the Local Group. Our results suggest that care must be exercised when using the orbits of the most weakly bound satellites to place constraints on the total mass of the Local Group.

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Posts: 131433
Date:
Bootes dwarf
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Title: A Pair of Bootes: A New Milky Way Satellite
Authors: S.M. Walsh, H. Jerjen, B. Willman

As part of preparations for a southern sky search for faint Milky Way dwarf galaxy satellites, we report the discovery of a stellar overdensity in the Sloan Digital Sky Survey Data Release 5, lying at an angular distance of only 1.5 degrees from the recently discovered Bootes dwarf. The overdensity was detected well above statistical noise by employing a sophisticated data mining algorithm and does not correspond to any catalogued object. Overlaid isochrones using stellar population synthesis models show that the colour-magnitude diagram of that region has the signature of an old (12 Gyr), metal-poor (Fe/H} -2.0) stellar population at a tentative distance of 60 kpc, evidently the same heliocentric distance as the Bootes dwarf. We estimate the new object to have a total magnitude of M_{V}~ -3.1 ±1.1 mag and a half-light radius of r_{h}=4'.1±1'.6 (72±28 pc) placing it in an apparent 40 <100 pc void between globular clusters and dwarf galaxies, occupied only by another recently discovered Milky Way Satellite, Coma Berenices.

Image1

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Posts: 131433
Date:
The Monoceros Ring
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Title: Does the Second Caustic Ring of Dark Matter Cause the Monoceros Ring of Stars ?
Authors: Aravind Natarajan, Pierre Sikivie

Caustic rings of dark matter were predicted to exist in the plane of the Galaxy at radii a_n \simeq 40 {
m kpc}/n for n = 1,2,3 ... The recently discovered Monoceros Ring of stars is located near the n=2 caustic, prompting us to consider a possible connection between these two objects. We identify two processes through which the Monoceros Ring of stars may have formed. One process is the migration of gas to an angular velocity minimum at the caustic leading to enhanced star formation there. The other is the adiabatic deformation of star orbits as the caustic slowly grows in mass and radius. The second process predicts an order 100% enhancement of the density of disk stars at the location of the caustic ring.

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