ND: The First Chronographic Map of the Halo of the Milky Way

[bystander]

Astrophysicists produce the first age map of the halo of the Milky Way
University of Notre Dame | 2015 Oct 28

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University of Notre Dame astronomer Timothy Beers and his Galactic Archaeology group, which includes Notre Dame astronomers Daniela Carollo and Vinicius Placco, have led an international team of researchers that produced the first chronographic (age) map of the halo of the Milky Way galaxy. The halo, along with the disk and bulge, are the primary components of the galaxy. Using a sample of 4,700 blue horizontal-branch (BHB) stars from the Sloan Digital Sky Survey, the research team showed that the oldest stars are concentrated in the central region of the galaxy, confirming predictions from numerical simulations of galaxy assembly. The researchers have also shown that chronographic maps such as theirs can also be used to identify complex structures of stars still in the process of being added to the halo system of our galaxy.

The researchers used the colors of BHB stars, which burn helium in their cores, to produce the age map. The technique relies on the fact that the colors of BHB stars are related to their masses, which in turn are related to their ages. The research results allowed the team, for the first time, to demonstrate two primary results.

“The oldest stars in the galaxy are concentrated toward the center of the galaxy, as predicted by previous numerical simulations of the assembly of our Milky Way,” Beers said. “Surprisingly, the region of the oldest stars extends all the way to the halo region close to the sun. This Ancient Chronographic Sphere can now be explored in order to study the properties of these old stars, which will tell us about the chemistry of the early universe.”

The researchers have also resolved the ages of dwarf galaxies and their stellar debris, which was stripped from them due to their gravitational interaction with the Milky Way. ...

Chronography of the Milky Way's Halo System with Field Blue Horizontal-branch Stars - Rafael M. Santucci et al

• Astrophysical Journal Letters 813(1):L16 (2015 Nov 01) DOI: 10.1088/2041-8205/813/1/L16
  arXiv.org > astro-ph > arXiv:1510.08147 > 28 Oct 2015

[bystander]

Detailed Age Map Shows How Milky Way Came Together
University of Notre Dame | 2016 Sep 06

[img3="Age Structure of the Milky Way’s Halo"]http://news.nd.edu/assets/209907/test360f_lb2.gif[/img3][hr][/hr]

Using colors to identify the approximate ages of more than 130,000 stars in the Milky Way’s halo, University of Notre Dame astronomers have produced the clearest picture yet of how the galaxy formed more than 13.5 billion years ago.

Astrophysicist Daniela Carollo, research assistant professor in the Department of Physics at the University of Notre Dame, and Timothy Beers, Notre Dame Chair of Astrophysics, along with research assistant professor Vinicius Placco and their colleagues, published their findings in Nature Physics, including a chronographic (age) map that supports a hierarchical model of galaxy formation. That model, developed by theoreticians over the past few decades, suggests that the Milky Way formed by merging and accretion of small mini-halos containing stars and gas, and that the oldest of the Milky Way’s stars are at the center of the galaxy and younger stars and galaxies merged with the Milky Way, drawn in by gravity over billions of years. ...

The Age Structure of the Milky Way’s Halo - Daniela Carollo et al

• Nature Physics (online 05 Sep 2016) DOI: 10.1038/nphys3874
  arXiv.org > astro-ph > arXiv:1607.08628 > 28 Jul 2016