30 May 2012

Milky Way inner halo reveals its age

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The Milky Way inner halo is approximately 11.4 billion years old, almost 2 billion years younger than the oldest stars discovered in the galaxy.
Milky Way halo structure

This illustration shows the Milky Way galaxy’s inner and outer halos. A halo is a spherical cloud of stars surrounding a galaxy. Astronomers have proposed that the Milky Way’s halo is composed of two populations of stars. Credit: NASA / ESA / A. Feild (STScI)

LONDON: The Milky Way inner halo is approximately 11.4 billion years old, almost 2 billion years younger than the oldest stars discovered in the galaxy.

The findings, which are published in Nature, may help astrophysicists to understand how the Milky Way and other galaxies formed and confirm that the inner halo is a distinct population of stars from the outer halo.

“The technique involves studying the cinders of stellar evolution, dead white dwarf stars, and using them as a proxy for the age of the parent population,” explained Jason Kalirai (Space Telescope Science Institute, Baltimore, Maryland) who carried out the research.

“By dating the halo, an important piece of the galaxy assembly puzzle is now in place,” he said.

How old is that star?

The spiral shaped Milky Way galaxy has long been known to consist of a spheroidal central bulge; a rotating disk section, comprised of young and old stars (including the sun) and dust clouds where new stars form; and a much more spread-out halo, comprised of older stars.

Recent research has demonstrated that the halo may be made up of two distinct populations of stars with different attributes.

“What has been missing thus far is knowledge of the precise age of these populations,” explained Kalirai.

“Typical methods to measure the ages (e.g. from stellar luminosities and temperatures), do not work well for low-mass stars. The new findings provide an independent age diagnostic to date the ‘inner’ halo of the Milky Way.”

Cinders of the galaxy

Kalirai used a technique he had previously developed which calculates the time for a star to evolve from its initial fuel-burning state to its current white dwarf state by comparing their masses. Stellar mass governs how long a star is able to burn hydrogen for.

Using this method, he estimated that the stars in the inner halo of the Milky Way are 11.4 billion years old, which is within the range of previous estimates of 10 to 14 billion years. The oldest reliably aged cluster of stars in the outer halo region to date is 13.5 billion years old.

White dwarf stars form when normal stars like the sun have burnt up all their fuel and lost their outer layers. The centre of the star becomes white hot before cooling over many years.

“White dwarfs are remarkable objects,” said Kalirai. “They contain approximately the mass of the Sun in a volume that is the size of the Earth. This means they are very dense. “

He explained that “the extreme conditions of a white dwarf cause their spectra to have a very strange shape, one that can be modeled to yield the temperature and mass of the star.”

The differing distances of halo stars from the Earth make measuring age by more traditional methods such as comparing temperature against luminosity very difficult, commented UK astrophysicist Nick Cross from the Royal Observatory in Edinburgh.

“However, using the spectroscopic widths of the Balmer lines in the atmospheres of white dwarfs give masses without the need for distances,” he noted. “The width depends on the gravity and the pressure, which are both dependent on the mass.”

Kalirai compared the masses of newly formed white dwarf stars located in the inner halo of the Milky Way with those of white dwarfs in the well-known 12.5 billion year old globular cluster known as Messier 4 located in the outer halo. He found that the inner halo white dwarfs were heavier, indicating that their parent stars were also heavier and therefore younger than the white dwarfs in Messier 4.

Birth of the Milky Way

Information obtained using the European Southern Observatory’s Very Large Telescope in Chile was used to estimate the masses of the inner halo white dwarfs and from the Keck I Telescope in Hawaii to measure the masses of the outer halo white dwarfs.

“This new discovery provides important insight on when Milky Way type galaxies built up their inner regions within the chronology of the Universe,” said Kalirai.

Discussing the work, astrophysicist Ata Sarajedini (University of Florida, Gainesville, USA) commented: “The initial mass-final mass relation for white dwarf stars has implications for our understanding of how normal ‘main sequence’ stars like the Sun evolve to become ‘dead’ stellar remnants at the end of their lives as white dwarfs.”

He added that “using white dwarfs to measure the age of the inner Milky Way halo helps us to understand how long it took the halo to form and the chronology of formation.”

Technique has promise, but more testing needed

Cross said that the finding “adds more evidence that the inner halo is a distinct stellar population from the outer halo,” which is a fairly recent discovery.

“Understanding exactly when the stars in these structures formed, combined with detailed measurements of the dynamics and chemical content, allows us to understand whether stars were accreted from dwarf galaxies, as seems to be the case for a significant fraction of the outer halo, or whether they formed within the galaxy, as in the case of the inner halo,” he added.

When asked about his future research, Kalirai said that whilst the technique has promise it needs to be tested on more inner halo white dwarfs to confirm the accuracy of the age estimate.

“For example, if the halo formed over an appreciable age range, we should see a large dispersion in the white dwarf masses. If the outer halo of the Milky Way is indeed as old as the oldest globular clusters, its white dwarf population should be under massive relative to the ones we’ve studied so far.”

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