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Making (galactic) history with big data: First global age map of the Milky Way

January 08, 2016

Using completely new ways of deducing the ages of so-called red giant stars from observational data, astronomers have created the first large-scale map that shows stellar ages in the Milky Way. Determining the ages of nearly 100 000 red giant stars, at distances of up to 50 000 light-years from the galactic center, the astronomers, led by Melissa Ness and Marie Martig of the Max Planck Institute for Astronomy, were able to test key ideas about the growth of the Milky Way. Notably, the map confirms that our home galaxy has grown inside out: in the present epoch, most old stars can be found in the middle, more recently formed ones in the outskirts.
Background information Additional images and downloads In-depth description of the results
Figure 1: Age distribution for a sample of red giant stars ranging from the galactic center to the outskirts of the Milky Way, analyzed by Melissa Ness and colleagues. The sample is embedded in a simulation of a Milky Way-like galaxy. Age is color coded, with the youngest stars shown in blue, the oldest stars in red, and middle-aged stars in green. The age distribution, including the obvious fact that the oldest stars are concentrated closer to the galactic center, confirms current models of galactic growth that have the Milky Way growing from the inside out. Zoom Image
Figure 1: Age distribution for a sample of red giant stars ranging from the galactic center to the outskirts of the Milky Way, analyzed by Melissa Ness and colleagues. The sample is embedded in a simulation of a Milky Way-like galaxy. Age is color coded, with the youngest stars shown in blue, the oldest stars in red, and middle-aged stars in green. The age distribution, including the obvious fact that the oldest stars are concentrated closer to the galactic center, confirms current models of galactic growth that have the Milky Way growing from the inside out.

In the past decades, powerful astronomical surveys have provided astronomers with data about millions of astronomical objects, allowing for large-scale statistical analysis. But big data of this kind is only as good as the tools available for analysis. Now, Melissa Ness and Marie Martig of the Max Planck Institute for Astronomy have added two powerful new tools to astronomy's arsenal: Using sample data from the APOGEE survey (part of the Sloan Digital Sky Survey) and NASA's Kepler Space Telescope, Ness and Martig devised two independent methods for determing the age of a red giant star from its spectrum (that is, from the properties of its light).

Using these methods, the astronomers were able to estimate the ages of nearly 100 000 stars that had been observed with the APOGEE survey. The result is an age map of the Milky Way, showing exactly which regions of our galaxy harbour young, old or middle-age stars (cf. figure 1). The map provides a representative cross section from the center of the Milky Way to the outskirts at a distance of 65 000 light-years from the galactic center.

With an age map of this kind, current models of how our home galaxy came into being and evolved can be put to the test. For instance, such models predict that stellar disks, the dominant stellar component of galaxies like the Milky Way, should have formed from the inside out: so, one would expect to find the older stars closer to the galactic center, and the younger stars at the outside. The map confirms this distribution.

Also, at any given radius, the younger stars are typically found closer to the galactic plane than their older cousins. This is also confirmed by the age map of Ness and colleagues.

Techniques such as these promise much greater advantages once the data from future surveys such as APOGEE-2 or ESA's Gaia mission are in. Eventually, it could help astronomers to reconstruct the entire star formation history of our galaxy: how many stars within our galaxy were formed at different times of galactic history, and in which regions, and how these stars have enriched our galaxy's raw material with the various elements they produce via nuclear fusion (thus enabling the production of heavier elements, of planets and, eventually, of living beings).

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Background information

The results described here are published in Ness et al. The Cannon: A Data-driven Approach to Stellar Label Determination (published, Astrophysical Journal), Spectroscopic determination of masses (and implied ages) for red giants (Preprint, submitted to Astrophysical Journa), and Martig et al. Red giant masses and ages derived from carbon and nitrogen abundances (accepted for publication in MNRAS).

Melissa Ness will present these results at a press conference at the annual meeting of the American Astronomical Society on Friday, January 8, after 10:15 am local time (16:15 CET). The press conference can be seen live as webcast. Please for access to the webcast.

An in-depth description of the research and scientific background is available here.

The SDSS press release describing these results can be found here.

The researchers from the Max Planck Institute for Astronomy involved in the research were

Melissa Ness, Marie Martig, Hans-Walter Rix, David Hogg (also New York University),  Morgan Fouesneau and Anna Y.Q. Ho (also Caltech)

in collaboration with

Marc H. Pinsonneault (Ohio State University), Szabolcs Mészáros (ELTE Gothard Astrophysical Observatory, Hungary), D. A. García-Hernández and Olga Zamora (both Instituto de Astrofísica de Canarias and Departamento de Astrofísica, Universidad de La Laguna), Aldo Serenelli (Instituto de Ciencias del Espacio (ICE/CSIC-IEEC) Campus UAB), and Victor Silva Aguirre (Aarhus University)

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Additional images and download area

<p>Figure 2: Relationship between the abundances of certain chemical elements in stellar atmospheres (where they can be detected using spectroscopy) and the mass of the star for red giants with known masses and observed spectra in the APOKASC sample. On the horizontal axis, the abundance of metals (in astronomy, all elements heavier than hydrogen) as compared to hydrogen, on the vertical axis the abundance of carbon C compared with that of nitrogen N. Color-coding shows the median age of stars in each region.</p> Zoom Image

Figure 2: Relationship between the abundances of certain chemical elements in stellar atmospheres (where they can be detected using spectroscopy) and the mass of the star for red giants with known masses and observed spectra in the APOKASC sample. On the horizontal axis, the abundance of metals (in astronomy, all elements heavier than hydrogen) as compared to hydrogen, on the vertical axis the abundance of carbon C compared with that of nitrogen N. Color-coding shows the median age of stars in each region.

Figure 3: Examples for differences in the spectra of red giants of different mass, as encoded by the flexible model produced by Ness et al. using <em>The Cannon</em>. Shown are the differences between a red giant with 0.7 solar masses and one with 3.3 solar masses. From differences like these, Ness et al. deduce the masses of red giants. Zoom Image
Figure 3: Examples for differences in the spectra of red giants of different mass, as encoded by the flexible model produced by Ness et al. using The Cannon. Shown are the differences between a red giant with 0.7 solar masses and one with 3.3 solar masses. From differences like these, Ness et al. deduce the masses of red giants.
<p>Figure 4: Age distribution for a sample of red giant stars ranging from the galactic center to the outskirts of the Milky Way. The horizontal axis shows distances from the galactic center, the vertical axis distance above or below the galactic plane (not to scale). Color-coding shows the median age of the red giants observed at that particular position by Ness et al. The concentration of young stars towards the plane of the disk is clearly visible, as is the age distribution that confirms our galaxy has been growing from the inside out.</p> Zoom Image

Figure 4: Age distribution for a sample of red giant stars ranging from the galactic center to the outskirts of the Milky Way. The horizontal axis shows distances from the galactic center, the vertical axis distance above or below the galactic plane (not to scale). Color-coding shows the median age of the red giants observed at that particular position by Ness et al. The concentration of young stars towards the plane of the disk is clearly visible, as is the age distribution that confirms our galaxy has been growing from the inside out.

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