White dwarf stars in the early Milky Way. Credit: NASA, ESA, A. Calamida and K. Sahu (STScI), and the SWEEPS Science Team; Credit for Ground-based Image: A. Fujii
Peering deep into the Milky Way’s crowded central hub of stars, Hubble researchers have uncovered for the 1st time a population of ancient white dwarfs, smoldering remnants of once-vibrant stars that inhabited the core. This can yield clues to how our galaxy was built, long before Earth/sun formed.
The white dwarfs contain information about the stars that existed about 12 billion years ago that burned out to form the white dwarfs. As these dying embers of once-radiant stars cool, they serve as multi-billion-year-old time pieces that tell astronomers about the Milky Way’s groundbreaking years.
An analysis of the Hubble data supports the idea that the Milky Way’s bulge formed first and that its stellar inhabitants were born very quickly – in less than roughly 2 billion years. The rest of the galaxy’s sprawling disk of second- and third-generation stars grew more slowly in the suburbs, encircling the central bulge like the brim of a giant sombrero.
“It is important to observe the Milky Way’s bulge because it is the only bulge we can study in detail,” explained Annalisa Calamida of STScI. “You can see bulges in distant galaxies, but you cannot resolve the very faint stars, such as the white dwarfs. The Milky Way’s bulge includes almost a quarter of the galaxy’s stellar mass. Characterizing the properties of the bulge stars can then provide important information to understanding the formation of the entire Milky Way galaxy and that of similar, more distant galaxies.”
The Hubble survey also found slightly more low-mass stars in the bulge, compared to those in the galaxy’s disk population. “This result suggests that the environment in the bulge may have been different than the one in the disk, resulting in a different star-formation mechanism,” Calamida said. The team based its results on an analysis of 70 of the hottest white dwarfs detectable by Hubble in a small region of the bulge.
These stellar relics are small, dim, about the size of Earth but 200,000X denser. A teaspoon of white dwarf material would weigh ~15 tons. They analyzed Hubble images of the same field of 240,000 stars, taken 10 years apart. The long timespan allowed the astronomers to make very precise measurements of the stars’ motion and pick out 70,000 bulge stars. The bulge’s stellar inhabitants move at a different rate than stars in the disk, allowing the astronomers to identify them.
The region surveyed is part of the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) field, 26,000 light-years away. The unusually dust-free location on the sky offers a keyhole view into the “downtown” bulge. Hubble’s Advanced Camera for Surveys made the observations in 2004 and 2011-2013. The motions allowed us to tell if they were disk stars, bulge stars, or halo stars.” The extremely hot white dwarfs appear bluer relative to sun-like stars. As white dwarfs age, they become cooler and fainter, becoming difficult even for sharp-eyed Hubble to detect.
“These 70 white dwarfs represent the peak of the iceberg,” Sahu said. “We estimate that the total number of white dwarfs is about 100,000 in this tiny Hubble view of the bulge. Future telescopes such as NASA’s James Webb Space Telescope will allow us to count almost all of the stars in the bulge down to the faintest ones, which today’s telescopes, even Hubble, cannot see.”
The team next plans to increase their sample of white dwarfs by analyzing other portions of the SWEEPS field. This should ultimately lead to a more precise estimate of the age of the galactic bulge. They might also determine if star formation processes in the bulge billions of years ago were different from what’s seen in the younger disk of our galaxy. http://hubblesite.org/newscenter/archive/releases/2015/38
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