White Dwarf Stars in the Milky Way Bulge
Credit for Hubble Images: NASA, ESA, A. Calamida and K. Sahu (STScI),
and the SWEEPS Science Team. Credit for Ground-based Image: A. Fujii
Object Name: SWEEPS Field
and the SWEEPS Science Team. Credit for Ground-based Image: A. Fujii
Object Name: SWEEPS Field
About this image: NASA's Hubble Space Telescope has detected for the first time a population of white dwarfs embedded in the hub of our Milky Way galaxy. The Hubble images are the deepest, most detailed study of the galaxy's central bulge of stars. The smoldering remnants of once-vibrant stars can yield clues to our galaxy's early construction stages that happened long before Earth and our sun formed.
[Left] — This is a ground-based view of the Milky Way’s central bulge, seen in the direction of the constellation Sagittarius. Giant dust clouds block most of the starlight coming from the galactic center. Hubble, however, peered through a region (marked by the arrow) called the Sagittarius Window, which offers a keyhole view into the galaxy's hub.
[Upper right] — This is a small section of Hubble's view of the dense collection of stars crammed together in the galactic bulge. The region surveyed is part of the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) field and is located 26,000 light-years away.
[Lower right] — Hubble uncovered extremely faint and hot white dwarfs. This is a sample of 4 out of the 70 brightest white dwarfs spied by Hubble in the Milky Way's bulge. Astronomers picked them out based on their faintness, blue-white color, and motion relative to our sun. The numbers in the inset images correspond to the white dwarfs' location in the larger Hubble view.
Hubble's Advanced Camera for Surveys made the observations in 2004 and 2011-2013.
Using NASA's Hubble Space Telescope to conduct a "cosmic archaeological dig" at the very heart of our Milky Way galaxy, astronomers have uncovered the blueprints of our galaxy's early construction phase.
Peering deep into the Milky Way's crowded central hub of stars,
Hubble researchers have uncovered for the first time a population of
ancient white dwarfs — smoldering remnants of once-vibrant stars that
inhabited the core. Finding these relics at last can yield clues to how
our galaxy was built, long before Earth and our sun formed.
The observations are the deepest, most detailed study of the galaxy's foundational city structure — its vast central bulge that lies in the middle of a pancake-shaped disk of stars, where our solar system dwells.
As with any archaeological relic, the white dwarfs contain the
history of a bygone era. They 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 the
Space Telescope Science Institute (STScI) in Baltimore, Maryland, the
science paper's lead author. "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 observations were so sensitive that the astronomers also used the
data to pick out the feeble glow of white dwarfs. 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 among tens of thousands of stars.
These stellar relics are small and extremely dense. They are about
the size of Earth but 200,000 times denser. A teaspoon of white dwarf
material would weigh about 15 tons. Their tiny stature makes them so
dim that it would be as challenging as looking for the glow of a pocket
flashlight located on the moon. Astronomers used the sharp Hubble
images to separate the bulge stars from the myriad stars in the
foreground of our galaxy's disk by tracking their movements over time.
The team accomplished this task by analyzing 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 and is located 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.
"Comparing the positions of the stars from now and 10 years ago we
were able to measure accurate motions of the stars," said Kailash Sahu
of STScI, the study's leader. "The motions allowed us to tell if they
were disk stars, bulge stars, or halo stars."
The astronomers identified the white dwarfs by analyzing the colors
of the bulge stars and comparing them with theoretical models. 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.
The team's results appeared in the Sept. 1, 2015, issue of The
Astrophysical Journal. A companion paper appeared in The Astrophysical
Journal in 2014.
CONTACT
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Annalisa Calamida / Kailash Sahu
Space Telescope Science Institute, Baltimore, Maryland
410-338-4341 / 410-338-4930
calamida@stsci.edu / ksahu@stsci.edu
