How far is far? And, when do you know when you get there? This is not a Dr. Seuss riddle, but the ultimate "final frontier" confronting astronomers. We are on the cusp of seeing nearly as far as we ever can into the universe of stars and galaxies. The question can be better phrased: How young is young? And how do you know when you've seen the earliest objects that ever existed? That's because the farther we look into space the further back into time we see. We're accustomed to instantaneous communications on Earth, but starlight carries a roaming fee. It takes billions of years for information to reach us from the remote universe.
Now astronomers have pushed the Hubble Space Telescope to its limits by finding what they believe is the most distant object ever seen in the universe. Its light traveled 13.2 billion years to reach Hubble. The dim object is a compact galaxy of blue stars that existed 480 million years after the Big Bang, only four percent of the universe's current age. It is tiny. Astronomers were surprised to discover that these observations offer evidence that the rate at which the universe was forming stars grew precipitously in about a 200-million-year time span. This huge change in the rate of star birth means that if astronomers can probe a little further back in time they are going to see even more dramatic changes. This will require the power of the James Webb Space Telescope, the planned successor to Hubble, which will be launched later this decade.
Astronomers have pushed NASA's Hubble Space Telescope to its limits by finding what they believe is the most distant object ever seen in the universe. Its light traveled 13.2 billion years to reach Hubble, roughly 150 million years longer than the previous record holder. The age of the universe is 13.7 billion years.
The dim object, called UDFj-39546284, is a compact galaxy of blue stars that existed 480 million years after the Big Bang, only four percent of the universe's current age. It is tiny. Over one hundred such mini-galaxies would be needed to make up our Milky Way.
Astronomers were surprised to find evidence that the rate at which the universe was forming stars grew precipitously in about a 200-million-year time span.
"We're seeing huge changes in the rate of star birth that tell us that if we go a little further back in time we're going to see even more dramatic changes," says Garth Illingworth of the University of California at Santa Cruz. The rate of star birth increased by about a factor of ten going from 480 million years to 650 million years after the Big Bang.
"These observations provide us with our best insights yet into the earlier primeval objects that have yet to be found," adds Rychard Bouwens of the Leiden University in the Netherlands.
Astronomers don't know exactly when the first stars appeared in the universe, but every step farther from Earth takes them deeper into the early universe's "formative years" when stars and galaxies were just beginning to emerge in the aftermath of the Big Bang. "We're moving into a regime where there are big changes afoot. Another couple of hundred million years toward the Big Bang, that will be the time where the first galaxies really are starting to get built up," says Illingworth.
Bouwens and Illingworth are reporting the discovery in the January 27 issue of the British science journal Nature.
The even more distant proto-galaxies that Illingworth expects are out there will require the infrared vision of NASA's James Webb Space Telescope, which is the successor to Hubble. Planned for launch later this decade, Webb will provide confirming spectroscopic measurements of the object's tremendous distance being reported today.
After over a year of detailed analysis, the object was positively identified in the Hubble Ultra Deep Field – Infrared (HUDF-IR) data taken in the late summer of both 2009 and 2010. This observation was made with the Wide Field Camera 3 (WFC3) starting just a few months after it was installed into the Hubble Space Telescope in May of 2009, during the last NASA space shuttle servicing mission to Hubble.
The object appears as a faint dot of starlight in the Hubble exposures. It is too young and too small to have the familiar spiral shape that is characteristic of galaxies in the local universe. Though its individual stars can't be resolved by Hubble, the evidence suggests that this is a compact galaxy of hot stars that first started to form over 100-200 million years earlier, from gas trapped in a pocket of dark matter.
The proto-galaxy is only visible at the farthest infrared wavelengths observable by Hubble. This means that the expansion of the universe has stretched and thereby reddened its light more than that of any other galaxy previously identified in the HUDF-IR, to the very limit of what Hubble can detect. Webb will go deeper into infrared wavelengths and will be at least an order of magnitude more sensitive than Hubble, allowing it to more efficiently hunt for primeval galaxies at even greater distances, at earlier times, closer to the Big Bang.
Astronomers plumb the depths of the universe, and probe its history, by measuring how much the light from an object has been stretched by the expansion of space. This is called the redshift value or "z." In general, the greater the observed "z" value for a galaxy, the more distant it is in time and space as observed from our own Milky Way. Before Hubble was launched, astronomers could only see galaxies out to a z of approximately 1, corresponding to halfway across the universe.
The original Hubble Deep Field taken in 1995 leapfrogged to z=4, or roughly 90 percent of the way back to the beginning of time. The Advanced Camera for Surveys (ACS) produced the Hubble Ultra Deep Field of 2004, pushing back the limit to z~6. ACS was installed on Hubble during Servicing Mission 3B in 2002. Hubble's first infrared camera, the Near Infrared Camera and Multi-Object Spectrometer, reached out to z=7. The WFC3 first took us back to z~8, and has now plausibly penetrated for the first time to z=10. The Webb Space Telescope is expected to leapfrog to z of approximately 15, 275 million years after the Big Bang, and possibly beyond. The very first stars may have formed between z of 30 and 15.
The hypothesized hierarchical growth of galaxies – from stellar clumps to majestic spirals and ellipticals – didn't become evident until the Hubble Space Telescope deep-field exposures. The first 500 million years of the universe's existence, from z of 1000 to 10, is now the missing chapter in the hierarchical growth of galaxies. It's not clear how the universe assembled structure out of a darkening, cooling fireball of the Big Bang. As with a developing embryo, astronomers know there must have been an early period of rapid changes that would set the initial conditions to make the universe of galaxies what it is today.