NASA's Hubble Space Telescope has uncovered the long-suspected underlying population of galaxies that produced the bulk of new stars during the universe's early years. They are the smallest, faintest, and most numerous galaxies ever seen in the remote universe, captured by Hubble deep exposures taken in ultraviolet light.
The 58 young, diminutive galaxies spied by Hubble were photographed
as they appeared more than 10 billion years ago, during the heyday of
star birth. The newly discovered galaxies are 100 times more numerous
than their more massive cousins. But they are 100 times fainter than
galaxies typically detected in previous deep-field surveys of the early
universe.
The results are being presented at the 223rd meeting of the American Astronomical Society in Washington, D.C.
These galaxies would normally be too faint for Hubble to see. To
detect them, astronomers teamed Hubble with a natural zoom lens in
space, produced by the gravity of a giant foreground galaxy cluster,
Abell 1689. The cluster is so massive that it magnifies the light from
faraway galaxies behind it due to a phenomenon called gravitational
lensing, where the curvature of space acts like a giant funhouse mirror
to stretch and brighten distant objects.
"There's always been a concern that we've only found the brightest of
the distant galaxies," said study leader Brian Siana of the University
of California, Riverside. "The bright galaxies, however, represent the
tip of the iceberg. We believe most of the stars forming in the early
universe are occurring in galaxies we normally can't see at all. Now we
have found those 'unseen' galaxies, and we're really confident that
we're seeing the rest of the iceberg."
Siana's team believes it has completed the census of galaxies at an
epoch when the universe was roughly 3.4 billion years old. If this
sample of galaxies is representative of the entire population at this
early time, then the majority of new stars formed in these small
galaxies. "Though these galaxies are very faint, their increased numbers
mean that they account for the majority of star formation during this
epoch," said team member Anahita Alavi, also of the University of
California, Riverside, and first author on the science paper describing
the results.
Uncovering these galaxies also helps bolster claims that hot stars in
small galaxies pumped out enough radiation to ionize hydrogen by
stripping off electrons. This process, called "reionization," occurred
about 13 billion years ago, within the first billion years after the
big bang. Reionization made the universe transparent to light, allowing
astronomers to look far back into time. "Although the galaxies in our
sample existed a few billion years after reionization, it's presumed
that galaxies like these, or possibly some of these galaxies, did play a
big role in reionization," Siana said.
These objects do not look like the majestic spiral and elliptical
galaxies seen in our galactic neighborhood. "The gravitational lensing
stretches out the apparent shape of the distant galaxies, resolving
them. Without the lensing, some of the galaxies would be just point
sources to Hubble. We now have an idea about their sizes that previously
were impossible to measure because the galaxies were unresolved," said
Alavi. The Hubble analysis shows they are small, irregularly shaped
objects measuring just a few thousand light-years across. Even when
fully mature, these galaxies will be about one-tenth to one
one-hundredth the mass the Milky Way. Because they are undergoing a
firestorm of star birth, their light is dominated by the ultraviolet
glow of fledgling stars.
The research team used Hubble's Wide Field Camera 3 to search for
faint, star-forming galaxies in ultraviolet light, a reliable tracer of
star birth. The galaxies existed when the universe was undergoing a
"baby boom" of star formation, estimated to have peaked between 9
billion and 12 billion years ago.
"Our goal with these observations was not to find a large number of
galaxies, but to find much fainter galaxies," Alavi explained.
This strategy of surveying large numbers of background galaxies with
deep observations of lensing clusters is being used in a new three-year
Hubble survey, called the Frontier Fields. Hubble astronomers are
using Hubble to exploit the magnification powers of six massive galaxy
clusters in a hunt for small galaxies that existed more than 12 billion
to 13 billion years ago.
The galaxies discovered in these lensing surveys will be prime
targets for NASA's James Webb Space Telescope, an infrared observatory
scheduled to launch in 2018. Through spectroscopy, Webb will be able to
divide the light from each galaxy into its constituent colors. This
analysis yields information on the star birth and chemical content of
each galaxy.
CONTACT
Donna Weaver / Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
