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Stellar Archaeology Traces Milky Way's History

Jason Kalirai of the Space Telescope Science Institute and The Johns Hopkins University's Center for Astrophysical Sciences, both in Baltimore, Md., has found the next best thing to a star's birth certificate.
Using a new technique, Kalirai probed the burned-out relics of Sun-like stars, called white dwarfs, in the inner region of our Milky Way galaxy's halo. The halo is a spherical cloud of stars surrounding our galaxy's disk. Those stars, his study reveals, are 11.5 billion years old, younger than the first generation of Milky Way stars. They formed more than 2 billion years after the birth of the universe 13.7 billion years ago. Previous age estimates, based on analyzing normal stars in the inner halo, ranged from 10 billion to 14 billion years. Kalirai's study reinforces the emerging view that our galaxy's halo is composed of a layer-cake structure that formed in stages over billions of years.

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Title: Two distinct halo populations in the solar neighbourhood. III. Evidence from stellar ages and orbital parameters
Authors: William J. Schuster (1), Edmundo Moreno (2), Poul E. Nissen (3), Barbara Pichardo (2) ((1) Observatorio Astronomico Nacional, Universidad Nacional Autonoma de Mexico, Ensenada, Mexico, (2) Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, Mexico, D.F., Mexico, (3) Department of Physics and Astronomy, University of Aarhus, Aarhus, Denmark)

In Papers I and II of this series, the existence of two distinct halo populations of stars have been found in the solar neighbourhood. Precise relative ages and orbital parameters are determined for 67 halo and 16 thick-disk stars having metallicities in the range -1.4 < [Fe/H] < -0.4 to better understand the context of the two halo populations in the formation and evolution of the Galaxy. Ages are derived by comparing the positions of stars in the logT_{eff}-log(g) diagram with isochrones from the Y models interpolated to the exact [Fe/H] and [alpha/Fe] values of each star. Possible systematic errors in T_{eff} and log(g) are considered and corrected. With space velocities from Paper I as initial conditions, orbital integrations have been carried out using a detailed, observationally constrained Milky Way model including a bar and spiral arms. The 'high-alpha' halo stars have ages 2-3 Gyr larger than the `low-alpha' ones. The orbital parameters show very distinct differences between the 'high-alpha' and 'low-alpha' halo stars. The 'low-alpha' ones have r_{max}'s to 30-40 kpc, z_{max}'s to approx. 18 kpc, and e_{max}'s clumped at values greater than 0.85, while the 'high-alpha' ones, r_{max}'s to about 16 kpc, z_{max}'s to 6-8 kpc, and e_{max} more or less uniformly distributed over 0.4-1.0. A dual in situ-plus-accretion formation scenario best explains the existence and characteristics of these two halo populations, but one remaining defect is that this model is not consistent regarding the r_{max}'s obtained for the in situ 'high-alpha' component; the predicted values are too small. It appears that omega Cen may have contributed in a significant way to the existence of the 'low-alpha' component; recent models, including dynamical friction and tidal stripping, have produced orbital parameters as great as those of the 'low-alpha' component.

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Title: Evolution of the Milky Way halo by accretion of dwarf satellite galaxies
Authors: Mykola Petrov, Gerhard Hensler

Within the Cold Dark Matter scenario the hierarchical merging paradigm is the natural result to form massive galactic halos by the minor mergers of sub-halos and, by this, inherently their stellar halo. Although this must be also invoked for the Milky Way, the context of chemical and kinematic coherence of halo stars and dwarf spheroidal galaxies is yet unsolved a focus of present-day research. To examine this issue we model the chemo-dynamical evolution of the system of satellites selected from the cosmological Via Lactea II simulations to be similar for the Milky Way environment but at an early epoch.

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Many of the Milky Way's oldest stars are refugees from other older galaxies torn apart by collisions.

Some of the oldest stars in the Milky Way are immigrants from other galaxies, survivors of cataclysmic collisions that tore apart smaller systems about 5 billion years ago.
Scientists developed a sophisticated new computer simulation that recreated the Milky Way's origin and evolution stemming back as far as time began, at least in our universe, with the Big Bang explosion roughly 13 billion years ago.
The model reveals new details about the halo of debris around the Milky Way and the clumps of stars and other matter it contains.

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Astronomers Discover Clue to Origin of Milky Way Gas Clouds

A surprising discovery that hydrogen gas clouds found in abundance in and above our Milky Way Galaxy have preferred locations has given astronomers a key clue about the origin of such clouds, which play an important part in galaxy evolution.
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Milky Way Outer Disk
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Title: Where Does the Disk Turn Into the Halo? Cool H I in the Outer Milky Way Disk
Authors: John M. Dickey

Using H I absorption spectra taken from the recent surveys of 21-cm line and continuum emission in the Galactic plane, the distribution of cool atomic clouds in the outer disk of the Milky Way is revealed. The warp of the midplane is clearly seen in absorption, as it is in emission, and the cool, neutral medium also shows flaring or increase in scale height with radius similar to that of the warm atomic hydrogen. The mixture of phases, as measured by the fraction of H I in the cool clouds relative to the total atomic hydrogen, stays nearly constant from the solar circle out to about 25 kpc radius. Assuming cool phase temperature ~50 K this indicates a mixing ratio of 15% to 20% cool H I, with the rest warm.

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We call it home, but the Milky Way can still surprise us. It does not have just one halo of stars, as we thought, but two. The finding calls into question our theories for how our galaxy formed.
Daniela Carollo at the Torino Observatory in Italy and her colleagues were measuring the metal content and motion of 20,000 stars in the Milky Way, observed by the Sloan Digital Sky Survey, when they made their discovery.
They found that the halo can be divided into two distinct regions, rotating in opposite directions, and containing stars of different chemical composition.

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Milky Way Galaxy Wears Two Halos of Stars
Though shaped like a spinning disk, our galaxy wears two crowns of stars earned by devouring other galaxies.
Though its devoured and destroyed countless smaller galaxies in its nearly 14-billion-year history, the Milky Way has earned itself two halos of stars, according to a Texas Tech researcher.
Ronald Wilhelm, an assistant professor of physics and co-author of the report, Two Stellar Components in the Halo of the Milky Way, said that though our galaxy is shaped like a flat disk of up to 400 billion stars rotating clockwise, it also wears two crowns of stars that make up a spherical haze and envelops the galaxys disk. Though others have suggested that the galaxy wears two halos of stars before, this is the first definitive proof of two different halos, he said.
The report will be published in the Dec. 13 issue of the journal, Nature.

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galx_e6
Credit DSS-II, Masashi Chiba, Tohoku University, Japan

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Using 20,000 stars observed by the Sloan Digital Sky Survey (SDSS-II), an international team of astronomers has discovered that the outer Milky Way is a mix of two distinct components rotating in opposite directions.

"By examining the motions and chemical makeup of the stars, we can see that the inner and outer halos are quite different beasts and they probably formed in different ways at different times" - Daniela Carollo, a researcher at the Italys Torino Observatory and the Australian National University.

Two stellar components in the halo of the Milky Way, the paper describing the discovery, appears in the 13 December 2007 edition of the journal Nature. In addition to Carollo, the lead author, the teams principal investigators include Timothy C. Beers of Michigan State University, John E. Norris of The Australian National University and Masashi Chiba of Japans Tohoku University.

"Although it was once considered a single structure, an analysis of the 20,000 stars from SDSS-II shows that the halo is clearly divisible into two, broadly overlapping components. The discovery gives us a much clearer picture of the formation of the first objects in our Galaxy and in the entire Universe" - Timothy C. Beers.

The discovery was a year in the making. Beers explained that Carollo came to Michigan State University in 2006 to carry out a straightforward comparison between SDSS-II measurements and data about the standard model of galaxy formation.

"But as we looked at the velocity and chemical composition of the halo data more closely, the evidence for distinct inner and outer halos became unmistakable."

To get a sense of one dimension rotational speed examine our Sun. It is a part of a disk of stars orbiting around the centre of the Galaxy at an astounding 500,000 miles per hour. The inner halo, located well outside the disk, rotates in the same direction, but more slowly, at 50,000 miles per hour. The outer halo, the most remote of these components, spins in the opposite direction, at about 100,000 miles per hour.
The difference in chemical composition opens another door to discovery of the Galaxys birth. According to Carollo, inner-halo stars contain three times more heavy atoms such as iron and calcium than outer-halo stars. These heavy atoms were forged in early forming massive stars and ejected during the stars death a supernova explosion. Later forming stars, such as those observed in this study, form from gas that has recorded the composition of all of the stellar generations that preceded it.

"Inner- and outer-halo stars both pass near the sun as they move through the Galaxy. Measuring both their motions and their chemical compositions allows us to separate them into these two halo families" - Daniela Carollo.

"Daniela, Tim, and the rest of their team have proven without a doubt that the Milky Way halo was not assembled all at once at early times. Astronomers have suspected for about 30 years that the outer parts of the Milky Way galaxy were assembled over time from many smaller protogalactic pieces. Now the SDSS-II team has shown that this is true even in the inner parts of the galaxy, and theyve done it with a sample of stars large enough to banish all doubt" - Jason Tumlinson, Yale University.

Tumlinson, a galaxy formation theorist who was not part of the discovery team, said it was possible that stars from the outer halo are among the oldest parts of the Milky Way.

"Speaking as a theorist who tries to build computer models of the construction of the Milky Way, I find this result inspiring in its statistical power, and yet frustrating at the same time as we are still struggling to build models with enough detail to adequately explain the observations."

What does it all mean? Chiba and the team believe that the inner halo formed first from the collision of smaller but massive galaxies that rotated with the Galaxy. The outer halo formed later from small galaxies orbiting the Milky Way in the reverse direction. These were torn apart by the Milky Ways gravitational forces, dispersing their stars into the halo. We still have a lot to understand - Masashi Chiba.

Beers and Norris have spent years searching for the most chemically primitive stars in the galaxy. Their heavy atoms may come from just a handful of early supernova, so their patterns of chemical elements provide archaeological clues to the properties of stars that formed in the first billion years after the Big Bang.
These fossils of the early universe are extremely rare, Norris explained, so finding them remains a classic needle-in-a-haystack problem. The identification of a chemically distinct outer halo gives us a much better way to search the haystack.

More data are on the way. The Sloan Extension for Galactic Understanding and Exploration (SEGUE) is more than halfway to its goal of measuring motions and compositions for 250,000 stars.

"With 10 times as many stars, there are bound to be more surprises in store" - Timothy C. Beers.

The Sloan Digital Sky Survey is the most ambitious survey of the sky ever undertaken, involving more than 300 astronomers and engineers at 25 institutions around the world. SDSS-II, which runs from 20052008, is comprised of three complementary projects. The Legacy Survey is completing the original SDSS map of half the northern sky, determining the positions, brightness, and colours of hundreds of millions of celestial objects and measuring distances to more than a million galaxies and quasars. SEGUE (Sloan Extension for Galactic Understanding and Exploration) is mapping the structure and stellar makeup of the Milky Way Galaxy. The Supernova Survey repeatedly scans a stripe along the celestial equator to discover and measure supernovae and other variable objects, probing the accelerating expansion of the cosmos. All three surveys are carried out with special purpose instruments on the 2.5-metre telescope at Apache Point Observatory in New Mexico.

Source: Sloan Digital Sky Survey

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