Hubble Uncovers One of the Smallest and Farthest Galaxies in the Universe
Credit: NASA, ESA, A. Zitrin (California Institute of Technology), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)
Artist's Illustration of a Giant Cosmic Magnifying Glass
Illustration Credit: NASA, ESA, and Z. Levay (STScI). Science Credit: NASA, ESA, A. Zitrin (Caltech), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)
Peering through a giant cosmic magnifying glass, NASA's Hubble Space Telescope has spotted one of the farthest, faintest, and smallest galaxies ever seen. The diminutive object is estimated to be over 13 billion light-years away.
This new detection is considered one of the most reliable distance
measurements of a galaxy that existed in the early universe, said the
Hubble researchers. They used two independent methods to estimate its
distance.
The galaxy appears as a tiny blob that is only a small fraction of
the size of our Milky Way galaxy. But it offers a peek back into a time
when the universe was only about 500 million years old, roughly 3
percent of its current age of 13.7 billion years. Astronomers have
uncovered about 10 other galaxy candidates at this early era. But this
newly found galaxy is significantly smaller and fainter than most of
those other remote objects detected to date.
"This object is a unique example of what is suspected to be an
abundant, underlying population of extremely small and faint galaxies
at about 500 million years after the big bang," explained study leader
Adi Zitrin of the California Institute of Technology in Pasadena. "The
discovery is telling us that galaxies as faint as this one exist, and we
should continue looking for them and even fainter objects so that we
can understand how galaxies and the universe have evolved over time."
The galaxy was detected as part of the Frontier Fields program, an
ambitious three-year effort, begun in 2013, that teams Hubble with
NASA's other Great Observatories — the Spitzer Space Telescope and the
Chandra X-ray Observatory — to probe the early universe by studying
large galaxy clusters. These clusters are so massive that their
gravity deflects light passing through them, magnifying, brightening,
and distorting background objects in a phenomenon called gravitational
lensing. These powerful lenses allow astronomers to find many dim,
distant structures that otherwise might be too faint to see.
In this new discovery, the lensing power of the mammoth galaxy
cluster Abell 2744, nicknamed Pandora's Cluster, produced three
magnified images of the same galaxy. Each magnified image makes the
galaxy appear as much as 10 times larger and brighter than it would
look without the intervening lens.
An analysis of the distant galaxy shows that it measures merely 850
light-years across, 500 times smaller than the Milky Way, and is
estimated to have a mass of only 40 million suns. The galaxy's star
formation rate is about one star every three years (one-third the star
formation rate in the Milky Way). Although this may seem low, Zitrin
said that given its small size and low mass, the tiny galaxy is in fact
rapidly evolving and efficiently forming stars.
"Galaxies such as this one are probably small clumps of matter that
are starting to form stars and shine light, but they don't have a
defined structure yet," Zitrin said. "Therefore, it's possible that we
only see one bright clump magnified due to the lensing, and this is one
possibility as to why it is smaller than typical field galaxies of that
time."
Zitrin's team spotted the galaxy's gravitationally multiplied images
using near-infrared and visible-light photos of the galaxy cluster
taken by Hubble's Wide Field Camera 3 and Advanced Camera for Surveys.
But at first they didn't know how far away it was from Earth.
Normally, astronomers use spectroscopy to determine an object's
distance. The farther away a galaxy, the more its light has been
stretched by the universe's expansion. Astronomers can precisely
measure this effect through spectroscopy, which characterizes an
object's light.
But the gravitationally lensed galaxy and other objects found at this
early epoch are too far away and too dim for astronomers to use
spectroscopy. Astronomers instead analyze an object's color to estimate
its distance. The universe's expansion reddens an object's color in
predictable ways, which scientists can measure.
Members of Zitrin's team not only performed the color-analysis
technique, but they also took advantage of the multiple images produced
by the gravitational lens to independently confirm their distance
estimate. The astronomers measured the angular separation between the
three magnified images of the galaxy in the Hubble photos. The greater
the angular separation due to lensing, the farther away the object is
from Earth. To test this concept, the astronomers compared the three
magnified images with the locations of several other multiply imaged
objects lensed by Abell 2744 that are not as far behind the cluster.
The angular distance between the magnified images of the closer
galaxies was smaller.
"These measurements imply that, given the large angular separation
between the three images of our background galaxy, the object must lie
very far away," Zitrin explained. "It also matches the distance
estimate we calculated, based on the color-analysis technique. So we
are about 95 percent confident that this object is at a remote distance,
at redshift 10 (a measure of the stretching of space since the big
bang). The lensing takes away any doubt that this might be a heavily
reddened, nearby object masquerading as a far more distant object."
Astronomers have long debated whether such early galaxies could have
provided enough radiation to warm the hydrogen that cooled soon after
the big bang. This process, called "reionization," is thought to have
occurred 200 million to 1 billion years after the birth of the
universe. Reionization made the universe transparent to light, allowing
astronomers to look far back into time without running into a "fog" of
cold hydrogen.
"We tend to assume that galaxies ionized the universe with their
ultraviolet light," Zitrin said. "But we do not see enough galaxies or
light that could do that. So we need to look at fainter and fainter
galaxies, and the Frontier Fields and galaxy cluster lensing can help
us achieve this goal."
The team's results appeared in the Sept. 5 online edition of The Astrophysical Journal Letters.
CONTACT
Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou@nasa.gov
Donna Weaver
Space Telescope Science Institute, Baltimore, Md.
410-338-4493
dweaver@stsci.edu
Source: HubbleSite
