Using NASA's Hubble Space Telescope, astronomers have seen further back in time than ever before and have uncovered a previously unseen population of seven primitive galaxies that formed more than 13 billion years ago, when the universe was less than 3 percent of its present age.
The deepest images to date from Hubble yield the first statistically robust sample of galaxies that tells how abundant they were close to the era when galaxies first formed.
“We found the most distant galaxies yet identified and were able to reliably determine their age,” said Brant Robertson
, an assistant professor of astronomy at the University of Arizona’s Steward Observatory
and one of the main contributors to the research.
The results show a smooth decline in the number of galaxies with increasing look-back time to about 380 million years after the big bang. The observations support the idea that galaxies assembled continuously over time and also may have provided enough radiation to reheat, or re-ionize, the universe a few hundred million years after the big bang, lending supportive evidence to a critical but still poorly understood milestone in the history of the universe.
These pioneering observations blaze a trail for future exploration of this epoch by NASA's next-generation spacecraft, the James Webb Space Telescope
, for which two UA teams have designed camera systems
. Looking deeper into the universe also means peering farther back in time. The universe is now 13.7 billion years old.
The newly discovered galaxies are seen as they looked 380 to 600 million years after the big bang. Their light is just arriving at Earth now.
Astronomers study the distant universe in near-infrared light because the expansion of space stretches ultraviolet and visible light from galaxies into infrared wavelengths, a phenomenon called “redshift.” The more distant a galaxy, the higher its redshift.
The greater depth of the new Hubble images, together with a carefully designed survey strategy, allows this work to go further than previous studies, thereby providing the first reliable census of this epoch, say the researchers. Notably, one of the galaxies may be a distance record breaker, observed 380 million years after the birth of our universe in the big bang, corresponding to a redshift of 11.9.
The results are from an ambitious Hubble survey of an intensively studied patch of sky known as the Ultra Deep Field, or UDF
. In the new 2012 campaign, called UDF 2012, a team of astronomers led by Richard Ellis of the California Institute of Technology in Pasadena, Calif. used Hubble's Wide Field Camera 3 to peer deeper into space in near-infrared light than any previous Hubble observation. The observations were made over a period of six weeks during August and September, and the first scientific results are now appearing in a series of scientific papers. The UDF 2012 team is publicly releasing these unique data, after preparing them for other research groups to use.
A major goal of the new program was to determine how rapidly the number of galaxies increases over time in the early universe. This measure is the key evidence for how quickly galaxies build up their constituent stars.
“We see those galaxies in the ultraviolet light, which almost entirely comes from newly born stars within those young galaxies, and so the light is a measure of the rate at which those stars were forming at that time,” explained Robertson, who works at the interface of observation and theory to understand how the first galaxies formed and what impact they had on the universe.
“By measuring how many galaxies there are and how bright they are, we can figure out on average how rapidly they form in a given volume of cosmos,” he said. “Based on that, we can estimate star formation rates per unit volume at those early times. This estimate is an important scientific result of this paper.”
The team estimated the galaxy distances by studying their colors through a carefully chosen set of four filters at specific near-infrared wavelengths.
“We added an additional filter and undertook much deeper exposures in some filters than in earlier work in order to convincingly reject the possibility that some of our galaxies might be foreground objects,” said team member James Dunlop of the Institute for Astronomy at the University of Edinburgh.
“The highest redshift we could detect with the current filters in place on Hubble is 13,” Robertson said. “We suspect galaxies started forming around redshift 20, which corresponds to about 200 million years after the big bang.”
For galaxies whose light has been shifted to infrared wavelengths, Dunlop said, the intervening hydrogen will have absorbed all of the light that was originally emitted as visible light and most of the light initially released at near-infrared wavelengths. Therefore, these galaxies will not be detected in most of Hubble's filters. They will only be seen in Hubble's longer-wavelength infrared filters, which hold the key to discovering the earliest galaxies.
“Our study has taken the subject forward in two ways,” explained Richard Ellis, the study’s lead author. “First, we have used Hubble to make longer exposures than previously. The added depth is essential to reliably probe the early period of cosmic history. Second, we have used Hubble's available color filters very effectively to more precisely measure galaxy distances.”
The results from the UDF 2012 campaign mean there may be many undiscovered galaxies even deeper in space waiting to be uncovered by the James Webb telescope.
“With the James Webb Telescope, we’ll be able to look at even more distant galaxies than with Hubble,” Robertson said. “We don’t really know what is out there, and our observations indicate there might be galaxies out there even that far back in time.”
Together with Daniel Stark, a Hubble Fellow at the UA’s department of astronomy, and graduate student Evan Schneider, Robertson has worked on applying the observations to another big question about the universe: an important, but poorly understood milestone in the universe’s history known as re-ionization.
Astronomers have long debated whether hot stars in such early galaxies could have provided enough radiation to warm the cold hydrogen that formed soon after the big bang.
This process, called “re-ionization,” is thought to have occurred 200 million to a billion years after our universe's birth. This process made the universe transparent to light, allowing astronomers to look far back into time. The galaxies in the new study are seen in this early epoch.
“Observations of the microwave afterglow from the big bang tell us that re-ionization happened more than about 13 billion years ago,” Robertson said. “Our data confirms that re-ionization was a drawn-out process occurring over several hundred million years with galaxies slowly building up their stars and chemical elements. There wasn't a single dramatic moment when galaxies formed; it was a gradual process.”