* Astronomy

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RE: Sagittarius Dwarf Galaxy

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Farthest stars in Milky Way probably stolen from another galaxy

Half of the 11 farthest known stars in our galaxy that are located about 300,000 light-years from Earth might have been ripped from another galaxy, U.S. researchers said Wednesday.
Researchers at the Harvard-Smithsonian Center for Astrophysics used computer models to simulate how the Sagittarius dwarf, one of dozens of mini-galaxies that surround our galaxy, might move over the past eight billion years, by varying its initial velocity and angle of approach to the Milky Way.
The study showed the Sagittarius dwarf started with a weight of about 10 billion times the mass of our Sun, or about one percent of the Milky Way's mass.
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Title: The Sagittarius Impact on Light and Dark Structure in the Milky Way
Author: Chris W. Purcell

It is increasingly apparent that common merger events play a large role in the evolution of disk galaxies at all cosmic times, from the wet accretion of gas-filled dwarf galaxies during the era of peak star formation, to the collisions between large, dynamically-advanced spiral galaxies and their dry companion satellites, a type of interaction that continues to influence disk structure into the present day. We also live in a large spiral galaxy currently undergoing a series of impacts from an infalling, disrupting dwarf galaxy. As next-generation astrometry proposes to place our understanding of the Milky Way spiral structure on a much firmer footing, we analyze high-resolution numerical models of this disk-satellite interaction in order to assess the dynamical response of our home Galaxy to the Sagittarius dwarf impact, and possible implications for experiments hoping to directly detect dark matter passing through the Earth.

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Title: A Spatial Characterisation of the Sagittarius Dwarf Galaxy Tidal Tails
Authors: Matthew Newby, Nathan Cole, Heidi Jo Newberg, Travis Desell, Malik Magdon-Ismail, Boleslaw Szymanski, Carlos Varela, Benjamin Willett, Brian Yanny

We measure the spatial density of F turnoff stars in the Sagittarius dwarf tidal stream, from Sloan Digital Sky Survey (SDSS) data, using statistical photometric parallax. We find a set of continuous, consistent parameters that describe the leading Sgr stream's position, direction, and width for 15 stripes in the North Galactic Cap, and 3 stripes in the South Galactic Cap. We produce a catalogue of stars that has the density characteristics of the dominant leading Sgr tidal stream that can be compared with simulations. We find that the width of the leading (North) tidal tail is consistent with recent triaxial and axisymmetric halo model simulations. The density along the stream is roughly consistent common disruption models in the North, but possibly not in the South. We explore the possibility that one or more of the dominant Sgr streams has been mis-identified, and that one or more of the 'bifurcated' pieces is the real Sgr tidal tail, but we do not reach definite conclusions. If two dwarf progenitors are assumed, fits to the planes of the dominant and 'bifurcated' tidal tails favor an association of the Sgr dwarf spheroidal galaxy with the dominant Southern stream and the `bifurcated' stream in the North. In the North Galactic Cap, the best fit Hernquist density profile for the smooth component of the stellar halo is oblate, with a flattening parameter q = 0.53, and a scale length of r_0 = 6.73. The Southern data for both the tidal debris and the smooth component of the stellar halo do not match the model fits to the North, although the stellar halo is still overwhelmingly oblate. Finally, we verify that we can reproduce the parameter fits on the asynchronous Milkyway@home volunteer computing platform.

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Title: Carbon enrichment of the evolved stars in the Sagittarius dwarf spheroidal
Authors: Iain McDonald, Jennifer R. White, Albert A. Zijlstra, Lizette Guzman Ramirez, Cezary Szyszka, Jacobus Th. van Loon, Eric Lagadec, Olivia C. Jones

We present spectra of 1142 colour-selected stars in the direction of the Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy, of which 1058 were taken with VLT/FLAMES multi-object spectrograph and 84 were taken with the SAAO Radcliffe 1.9-m telescope grating spectrograph. Spectroscopic membership is confirmed (at >99% confidence) for 592 stars on the basis of their radial velocity, and spectral types are given. Very slow rotation is marginally detected around the galaxy's major axis. We identify five S stars and 23 carbon stars, of which all but four carbon stars are newly-determined and all but one (PQ Sgr) are likely Sgr dSph members. We examine the onset of carbon-richness in this metal-poor galaxy in the context of stellar models. We compare the stellar death rate (one star per 1000-1700 years) to known planetary nebula dynamical ages and find that the bulk population produce the observed (carbon-rich) planetary nebulae. We compute average lifetimes of S and carbon stars as 60-250 and 130-500 kyr, compared to a total thermal-pulsing asymptotic giant branch lifetime of 530-1330 kyr. We conclude by discussing the return of carbon-rich material to the ISM.

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Even though a dwarf galaxy clear across the Milky Way looks to be a mouse, it may have once been a bear that slashed through the Milky Way and created the galaxy's spiral arms, writes an Iowa State University astronomer in the journal Nature.
Curtis Struck, an Iowa State professor of physics and astronomy, uses a News & Views commentary in the Sept. 15 issue of Nature to add context and colour to a study published in the same issue by a research team led by Chris W. Purcell of the University of Pittsburgh.
The Purcell group reports that the Sagittarius Dwarf Elliptical Galaxy collided with the Milky Way, creating the galaxy's spiral arms, its central bar structure and the flaring at its outer disk. Along the way, the dwarf galaxy's stars were scattered and the galaxy shrunk to an object that's so small and unimpressive it's hard to see.
The Purcell group writes that dwarf galaxies such as the Sagittarius originally had massive amounts of dark matter. The original galaxy, in other words, may have been 100,000 times more massive than it is today. And that may have given it the power to shape our galaxy.