This chronographic map of the Milky Way’s halo system shows a color gradient that is associated with an age difference of roughly 2-2.5 G yrs
The 1st chronographic (age) map of the halo of the Milky Way galaxy has been created. 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 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 2 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. “This information can be used to tell us the assembly history of our galaxy,” Beers said. “We can now search for additional debris streams in the halo of the galaxy, based on their contrast in age, rather than simply their density contrast.”
According to Beers, “It is almost like having X-ray vision, as we can see through the numerous foreground stars and not be confused by the equally large numbers of background stars.” The researchers used stars with spectroscopy collected during the Sloan Digital Sky Survey, and from that, they determined the parameters that allow them to efficiently separate the BHB stars from other kinds of stars. From that, a relatively pure sample could be obtained, enabling a clear age map to be made based on their colors.
Daniela Carollo, left, Timothy Beers and Vinicius Placco
“We have confirmed one prediction, that the oldest stars, born shortly after the Big Bang, should be found near the center of the galaxy, and demonstrated in addition that searches for ancient stars in the region of the halo close to the solar neighborhood will be highly effective,” Beers said. “The assembly history of the galaxy, which is encoded in the ages of the members of the halo population, is now a story that can be explored and told more fully.”
Beers’ Galactic Archaeology group’s next detailed map will serve as a guide for astronomers to identify numerous new dwarf galaxy debris signatures and enable the development of a refined history of the assembly of the Milky Way + study more BHB stars via other sky surveys, as well as in the near future with the Large Synoptic Survey Telescope (LSST) under construction in Chile. http://news.nd.edu/news/62076-astrophysicists-produce-the-first-age-map-of-the-halo-of-the-milky-way/ http://iopscience.iop.org/article/10.1088/2041-8205/813/1/L16
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