The Hubble Space Telescope has revealed the most distant galaxies ever seen thanks to its concentrated exposure on a tiny spot of space.
Astronomers using the telescope have released an image that looks deeper into the universe than ever before. Called the eXtreme Deep Field, or XDF, the picture was assembled by combining 10 years of Nasa Hubble Space Telescope (HST) photographs taken of a patch of sky at the centre of the original Hubble Ultra Deep Field. The XDF is a small fraction of the angular diameter of the full Moon.
The Hubble Ultra Deep Field is an image of a small area of space in the constellation of Fornax (The Furnace), created using HST data from 2003 and 2004. By collecting faint light over one million seconds of observation, the resulting image revealed thousands of galaxies, both nearby and very distant, making it the deepest image of the universe ever taken at that time.
The new full-colour XDF image is even more sensitive than the original Hubble Ultra Deep Field image, thanks to the additional observations, and contains about 5,500 galaxies, even within its smaller field of view. The faintest galaxies are one ten-billionth the brightness that the unaided human eye can see.
Hubble was pointed at a tiny patch of southern sky in repeat visits made over the past decade with a total exposure time of two million seconds – the equivalent of more than 500 hours or 23 days. Because Hubble can only observe for about 45 minutes of every 97-minute orbit, the observations that make up the XDF represent 50 days of telescope time.
More than 2,000 images of the same field were taken with Hubble's two primary cameras: the Advanced Camera for Surveys and the Wide Field Camera 3, which extends Hubble's vision into near-infrared light. These were then combined to form the XDF.
In the newly released picture, spiral galaxies similar in shape to the Milky Way and its neighbour the Andromeda galaxy appear, as do large, fuzzy red galaxies in which the formation of new stars has ceased. These red galaxies are the remnants of dramatic collisions between galaxies and are in their declining years as the stars within them age.
Peppered across the field are tiny, faint, and yet more distant galaxies that astronomers note are like the seedlings from which today's magnificent galaxies grew. Nasa says that the history of galaxies – from soon after the first galaxies were born to the great galaxies of today, like the Milky Way
– is laid out in this one image.
Garth Illingworth, of the University of California at Santa Cruz and principal investigator of the Hubble Ultra Deep Field 2009 (HUDF09) programme, said: "The XDF is the deepest image of the sky ever obtained and reveals the faintest and most distant galaxies ever seen. XDF allows us to explore further back in time than ever before."
The Universe is 13.7 billion years old, and the XDF reveals galaxies that span back 13.2 billion years in time. Most of the galaxies in the XDF are seen when they were young, small, and growing, often violently as they collided and merged together.
The early Universe was a time of dramatic birth for galaxies containing brilliant blue stars far brighter than our Sun. The light from those past events is just arriving at Earth now, and so the XDF is a time tunnel into the distant past when the universe was just a fraction of its current age.
The youngest galaxy found in the XDF existed just 450 million years after the universe's birth in the Big Bang.
Before Hubble was launched in 1990, astronomers were able to see galaxies up to about seven billion light-years away, half way back to the Big Bang.
Observations with telescopes on the ground were not able to establish how galaxies formed and evolved in the early Universe.
Hubble gave astronomers their first view of the actual forms of galaxies when they were young. This provided direct visual evidence that the universe is truly changing as it ages. Like watching individual frames of a motion picture, the Hubble deep surveys reveal the emergence of structure in the infant Universe and the subsequent dynamic stages of galaxy evolution.
Now Nasa hopes to look even further into the XDF with the planned launch of its James Webb Space Telescope, which will find even fainter galaxies that existed when the universe was just a few hundred million years old.
Because of the expansion of the universe, light from the distant past is stretched into longer, infrared wavelengths. The Webb telescope's infrared vision is ideally suited to push the XDF even deeper, into a time when the first stars and galaxies formed and filled the early 'dark ages' of the universe with light.
Because of the expansion of the universe, light from the distant past is stretched into longer, infrared wavelengths. The Webb telescope's infrared vision is ideally suited to push the XDF even deeper, into a time when the first stars and galaxies formed and filled the early "dark ages" of the universe with light.