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Title: Tracing chemical evolution over the extent of the Milky Way's Disk with APOGEE Red Clump Stars
Author: David L. Nidever, Jo Bovy, Jonathan C. Bird, Brett H. Andrews, Michael Hayden, Jon Holtzman, Steven R. Majewski, Verne Smith, Annie C. Robin, Ana E. Garcia Perez, Katia Cunha, Carlos Allende Prieto, Gail Zasowski, Ricardo P. Schiavon, Jennifer A. Johnson, David H. Weinberg, Diane Feuillet, Donald P. Schneider, Matthew Shetrone, Jennifer Sobeck, D. A. Garcia-Hernandez, O. Zamora, Hans-Walter Rix, Timothy C. Beers, John C. Wilson, Robert W. O'Connell, Ivan Minchev, Cristina Chiappini, Friedrich Anders, Dmitry Bizyaev, Howard Brewington, Garrett Ebelke, Peter M. Frinchaboy, Jian Ge, Karen Kinemuchi, Elena Malanushenko, Viktor Malanushenko, Moses Marchante, Szabolcs Meszaros, Daniel Oravetz, Kaike Pan, Audrey Simmons, Michael F. Skrutskie

We employ the first two years of data from the near-infrared, high-resolution SDSS-III/APOGEE spectroscopic survey to investigate the distribution of metallicity and alpha-element abundances of stars over a large part of the Milky Way disk. Using a sample of ~10,000 kinematically-unbiased red-clump stars with ~5% distance accuracy as tracers, the [alpha/Fe] vs. [Fe/H] distribution of this sample exhibits a bimodality in [alpha/Fe] at intermediate metallicities, -0.9<[Fe/H]<-0.2, but at higher metallicities ([Fe/H]=+0.2) the two sequences smoothly merge. We investigate the effects of the APOGEE selection function and volume filling fraction and find that these have little qualitative impact on the alpha-element abundance patterns. The described abundance pattern is found throughout the range 5<R<11 kpc and 0<|Z|<2 kpc across the Galaxy. The [alpha/Fe] trend of the high-alpha sequence is surprisingly constant throughout the Galaxy, with little variation from region to region (~10%). Using simple galactic chemical evolution models we derive an average star formation efficiency (SFE) in the high-alpha sequence of ~4.5E-10 1/yr, which is quite close to the nearly-constant value found in molecular-gas-dominated regions of nearby spirals. This result suggests that the early evolution of the Milky Way disk was characterized by stars that shared a similar star formation history and were formed in a well-mixed, turbulent, and molecular-dominated ISM with a gas consumption timescale (1/SFE) of ~2 Gyr. Finally, while the two alpha-element sequences in the inner Galaxy can be explained by a single chemical evolutionary track this cannot hold in the outer Galaxy, requiring instead a mix of two or more populations with distinct enrichment histories.

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Title: Vertical density waves in the Milky Way disc induced by the Sagittarius Dwarf Galaxy
Authors: Facundo A. Gómez, Ivan Minchev, Brian W. O'Shea, Timothy C. Beers, James S. Bullock, Chris W. Purcell

Recently, Widrow and collaborators announced the discovery of vertical density waves in the Milky Way disk. Here we investigate a scenario where these waves were induced by the Sagittarius dwarf galaxy as it plunged through the Galaxy. Using numerical simulations, we find that the Sagittarius impact produces North-South asymmetries and vertical wave-like behaviour that qualitatively agrees with what is observed. The extent to which vertical modes can radially penetrate into the disc, as well as their amplitudes, depend on the mass of the perturbing satellite. We show that the mean height of the disc is expected to vary more rapidly in the radial than in the azimuthal direction. If the observed vertical density asymmetry is indeed caused by vertical oscillations, we predict radial and azimuthal variations of the mean vertical velocity, correlating with the spatial structure. These variations can have amplitudes as large as 8 km/s.

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Title: Evidence of a bisymmetric spiral in the Milky Way
Authors: Charles Francis, Erik Anderson

Context: Because of our viewing point within the Galactic disc, it is extremely difficult to observe the spiral structure of the Milky Way.
Aims: To clarify the structure of the Galaxy by re-examination of gas distributions and data from 2MASS; to determine stream memberships among local stars and to show the relationship between streaming motions and spiral structure.
Methods: We extend the spiral pattern found from neutral gas towards the Galactic centre using data from 2MASS. We select a population of 23 075 local disc stars for which complete kinematic data is available. We plot eccentricity against the true anomaly for stellar orbits and identify streams as dense regions of the plot. We reconstruct the spiral pattern by replacing each star at a random position of the inward part of its orbit.
Results: We find evidence in 2MASS of a bar of length 4.2 ± 0.1 kpc at angle 30 ± 10°. We extend spiral structure by more than a full turn toward the Galactic centre, and confirm that the Milky Way is a two-armed grand-design bisymmetric spiral with pitch angle 5.56 ± 0.06°. Memberships of kinematic groups are assigned to 98% of local disc stars and it is seen that the large majority of local stars have orbits aligned with this spiral structure.

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Title: The Milky Way has no thick disk
Authors: Jo Bovy (IAS), Hans-Walter Rix (MPIA), David W. Hogg (NYU, MPIA)

Different stellar sub-populations of the Milky Way's stellar disk are known to have different vertical scale heights, their thickness increasing with age. Using SEGUE spectroscopic survey data, we have recently shown that mono-abundance sub-populations, defined in the [\alpha/Fe]-[Fe/H] space, are well described by single exponential spatial-density profiles in both the radial and the vertical direction; therefore any star of a given abundance is clearly associated with a sub-population of scale height h_z. Here, we work out how to determine the stellar surface-mass density contributions at the solar radius R_0 of each such sub-population, accounting for the survey selection function, and for the fraction of the stellar population mass that is reflected in the spectroscopic target stars given populations of different abundances and their presumed age distributions. Taken together, this enables us to derive \Sigma_{R_0}(h_z), the surface-mass contributions of stellar populations with scale height h_z. Surprisingly, we find no hint of a thin-thick disk bi-modality in this mass-weighted scale-height distribution, but a smoothly decreasing function, approximately \Sigma_{R_0}(h_z)\propto \exp(-h_z), from h_z ~ 200 pc to h_z ~ 1 kpc. As h_z is ultimately the structurally defining property of a thin or thick disk, this shows clearly that the Milky Way has a continuous and monotonic distribution of disk thicknesses: there is no 'thick disk' sensibly characterized as a distinct component. We discuss how our result is consistent with evidence for seeming bi-modality in purely geometric disk decompositions, or chemical abundances analyses.

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Title: The Sagittarius impact as an architect of spirality and outer rings in the Milky Way
Authors: Chris W. Purcell, James S. Bullock, Erik Tollerud, Miguel Rocha, Sukanya Chakrabarti

Like many galaxies of its size, the Milky Way is a disk with prominent spiral arms rooted in a central bar, although our knowledge of its structure and origin is incomplete. Traditional attempts to understand the Galaxy's morphology assume that it has been unperturbed by major external forces. Here we report simulations of the response of the Milky Way to the infall of the Sagittarius dwarf galaxy (Sgr), which results in the formation of spiral arms, influences the central bar and produces a flared outer disk. Two ring-like wrappings emerge towards the Galactic anti-Center in our model that are reminiscent of the low- latitude arcs observed in the same area of the Milky Way. Previous models have focused on Sgr itself to reproduce the dwarf's orbital history and place associated constraints on the shape of the Milky Way gravitational potential, treating the Sgr impact event as a trivial influence on the Galactic disk. Our results show that the Milky Way's morphology is not purely secular in origin and that low-mass minor mergers predicted to be common throughout the Universe probably have a similarly important role in shaping galactic structure.

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Most bright stars in our Milky Way Galaxy reside in a disk. 

milkywayband_gleason.jpg

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Since our Sun also resides in this disk, these stars appear to us as a diffuse band that circles the sky. The panorama of a northern band of the Milky Way's disk covers 90 degrees and is a digitally created mosaic of several independent exposures. Scrolling right will display the rest of this spectacular picture. Visible are many bright stars, dark dust lanes, red emission nebulae, blue reflection nebulae, and clusters of stars. In addition to all this matter that we can see, astronomers suspect that there exists even more dark matter that we cannot see.



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