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09 January 2016

Milky Way's Growth Rings Unveiled in 1st Age Map

Milky Way's Growth Rings Unveiled in 1st Age Map

The first complete age map of the Milky Way shows that the galaxy grew from the inside out.

To construct the map, scientists measured the composition and masses of red giant stars to determine their ages. Using a revolutionary technique, the researchers found that older Milky Way stars tend to lie near the center of the spiral galaxy, whereas subsequent generations formed around the spreading edges of the disk.

"This is key to understanding galaxy formation," Melissa Ness, a postdoctoral student at the Max Planck Institute for Astronomy in Germany, said at a press conference today (Jan. 8) at the 227th Meeting of the American Astronomical Society in Kissimmee, Florida. Ness lead a group that used the Sloan Digital Sky Survey (SDSS) to study the light, or spectra, from red giant stars to produce the first global age map of the Milky Way.

"Measuring the individual ages of stars from their spectra and combining them with chemical information offers the most powerful constraints in the galaxy," she said.

Up and out

The familiar spiral arms of the Milky Way lie in a flattened disk of dust and stars. Sorting the stars in this disk by age can help scientists to better understand how the galaxy as a whole evolved. To do that, Ness and her team studied red giant stars, bright stars for which there is a known relationship between age and mass.

That relationship depends on the life course of some stars. While some stars end their lives in violent supernova explosions, others don't have enough mass to produce such fireworks. Instead, in the penultimate stage of their lifetimes, stars like the sun swell up and become red giants, which have large radii but low mass.

Using the SDSS' Apache Point Observatory Galaxy Evolution Experiment (APOGEE), the team targeted 70,000 red giants to determine their ages and locations. But determining the mass of such a star, and thus its age, has been a long-standing challenge for astronomers. To solve the mystery, the team turned to NASA's Kepler space telescope. Although most famous for the more than 1,000 exoplanets it has discovered, Kepler has also revealed a wealth of information about stars since the observatory's March 2009 launch.

In an independent study, Marie Martig of the Swinburne University of Technology in Melbourne, Australia, looked at 2,000 stars whose masses and ages had been previously determined by Kepler. By comparing those values to the measurements of the stars' carbon and nitrogen obtained by APOGEE, she was able to calculate the relationship among red giants' mass, age and carbon and nitrogen abundances. Ness and her team, which included Martig, then used that relationship to determine the mass of the 70,000 red giant stars APOGEE had studied in the disk of the Milky Way.

"This is somewhat revolutionary, because ages have previously been considered very hard to get," Ness said.

With this age map, the scientists were able to chart how the Milky Way has grown throughout its lifetime. They found that the more-ancient, 13-billion-year-old stars were the first to form early in the lifetime of the universe. As the young galaxy collected gas and dust in a growing disk around its edges, the material became the site of the next generation of star formation.



Ancient gas cloud may be a relic from the death of first stars

A simulation of the first stars in the Universe, showing how the gas cloud might have become enriched with heavy elements. The image shows one of the first stars exploding sprouting producing an expanding shell of gas (top) which enriches a nearby cloud, embedded inside a larger gas filament (centre).

Researchers from Australia and the USA have discovered a distant, ancient cloud of gas that may contain the signature of the very first stars that formed in the universe.

The gas cloud has an extremely small percentage of heavy elements, such as carbon, oxygen and iron – less than one thousandth the fraction observed in the Sun.

It is many billions of light years away from Earth, and is observed as it was just 1.8 billion years after the Big Bang Seed. The observations were made by the Very Large Telescope in Chile.

"Heavy elements weren't manufactured during the Big Bang Seed, they were made later by stars," says lead researcher, Dr Neil Crighton, from Swinburne University of Technology's Centre for Astrophysics and Supercomputing.

"The first stars were made from completely pristine gas, and astronomers think they formed quite differently from stars today."

The researchers say that soon after forming, these first stars – also known as Population III stars – exploded in powerful supernovae, spreading their heavy elements into surrounding pristine clouds of gas. Those clouds then carry a chemical record of the first stars and their deaths, and this record can be read like a fingerprint.

"Previous gas clouds found by astronomers show a higher enrichment level of heavy elements, so they were probably polluted by more recent generations of stars, obscuring any signature from the first stars," Dr Crighton says.

"This is the first cloud to show the tiny heavy element fraction expected for a cloud enriched only by the first stars," co-author Swinburne's Professor Michael Murphy says.

The researchers hope to find more of these systems, where they can measure the ratios of several different kinds of elements.

"We can measure the ratio of two elements in this cloud - carbon and silicon. But the value of that ratio doesn't conclusively show that it was enriched by the first stars; later enrichment by older generations of stars is also possible," another co-author, Professor John O'Meara from Saint Michael's College in Vermont, USA, says.

"By finding new clouds where we can detect more elements, we will be able to test for the unique pattern of abundances we expect for enrichment by the first stars."

The paper will be published in the Monthly Notices of the Royal Astronomical Society Letters on 13 Jan 2016, and is available as a pre-print at http://arxiv.org/pdf/1512.00477v1.