thumb|300px|The original NASA release, containing about 10,000 galaxies of various ages, sizes, shapes, and colors. The smallest, reddest ones are some of the most distant galaxies to have been imaged by an optical telescope, probably existing shortly after the [[Big Bang.]]
The Hubble Ultra-Deep Field (HUDF) is a deep-field image of a small region of space in the constellation Fornax, containing an estimated 10,000 galaxies. The original data for the image was collected by the Hubble Space Telescope from September 2003 to January 2004 and the first version of the image was released on March 9, 2004. or 3.4 arcminutes diagonally. This is about one-tenth of the angular diameter of a full moon viewed from Earth (less than 34 arcminutes), smaller than a 1 mm<sup>2</sup> piece of paper held 1 m away, and equal to roughly one twenty-six-millionth of the total area of the sky. The image is oriented so that the upper left corner points toward north (−46.4°) on the celestial sphere.
In August and September 2009, the HUDF field was observed at longer wavelengths (1.0 to 1.6 μm) using the infrared channel of the recently fitted Wide Field Camera 3 (WFC3). This additional data enabled astronomers to identify a new list of potentially very distant galaxies.
On September 25, 2012, NASA released a new version of the Ultra-Deep Field dubbed the eXtreme Deep Field (XDF).<!-- title now redirects to this one --> The XDF reveals galaxies from 13.2 billion years ago, including one thought to have formed only 450 million years after the Big Bang.
On June 3, 2014, NASA released the Hubble Ultra Deep Field 2014 image, the first HUDF image to use the full range of ultraviolet to near-infrared light. A composite of separate exposures taken in 2002 to 2012 with Hubble's Advanced Camera for Surveys and Wide Field Camera 3, it shows some 10,000 galaxies.
On January 23, 2019, the Instituto de Astrofísica de Canarias released an even deeper version of the infrared images of the Hubble Ultra Deep Field obtained with the WFC3 instrument, named the ABYSS Hubble Ultra Deep Field. The new images improve the previous reduction of the WFC3/IR images, including careful sky background subtraction around the largest galaxies on the field of view. After this update, some galaxies were found to be almost twice as big as previously measured.
Planning
In the years since the original Hubble Deep Field, the Hubble Deep Field South and the GOODS sample were analyzed, providing increased statistics at the high redshifts probed by the HDF. When the Advanced Camera for Surveys (ACS) detector was installed on the HST, it was realized that an ultra-deep field could observe galaxy formation out to even higher redshifts than had currently been observed, as well as providing more information about galaxy formation at intermediate redshifts (z~2). Following the workshop, the STScI Director Steven Beckwith decided to devote 400 orbits of Director's Discretionary time to the UDF and appointed Stiavelli as the lead of the Home Team implementing the observations.
Unlike the Deep Fields, the HUDF does not lie in Hubble's Continuous Viewing Zone (CVZ). The earlier observations, using the Wide Field and Planetary Camera 2 (WFPC2) camera, were able to take advantage of the increased observing time on these zones by using wavelengths with higher noise to observe at times when earthshine contaminated the observations; however, ACS does not observe at these wavelengths, so the advantage was reduced.
The observations were done in two sessions, from September 23 to October 28, 2003, and December 4, 2003, to January 15, 2004. The total exposure time is just under 1 million seconds (~11 days), from 400 orbits, with a typical exposure time of 1200 seconds.
{| class="wikitable"
|+ Observations made of the HUDF with ACS. The orientation of the HST was chosen so that further NICMOS parallel images would fall on top of the main UDF field.
After the installation of WFC3 on Hubble in 2009, the HUDF09 programme (GO-11563) devoted 192 orbits to observations of three fields, including HUDF, using the newly available F105W, F125W and F160W infra-red filters (which correspond to the Y, J and H bands):
{| class="wikitable"
|+ Observations made of the HUDF with WFC3
|-
! Camera !! Filter !! Wavelength !! Exposure time
|-
| WFC3 || F105W || 1050 nm ± 150 || 16 orbits, 14 usable
|-
| WFC3 || F125W || 1250 nm ± 150 || 16 orbits
|-
| WFC3 || F160W || 1600 nm ± 150 || 28 orbits
|}
Contents
The HUDF is the deepest image of the universe ever taken and has been used to search for galaxies that existed between 400 and 800 million years after the Big Bang (redshifts between 7 and 12). The star near the center of the field is USNO-A2.0 0600–01400432 with apparent magnitude of 18.95.
The field imaged by the ACS contains over 10,000 objects, the majority of which are galaxies, many at redshifts greater than 3, and some that probably have redshifts between 6 and 7. or about 80% of the area of the HUDF. This represents about one thirty-two millionth of the sky.
The HXDF contains about 5,500 galaxies, the oldest of which are seen as they were 13.2 billion years ago. The faintest galaxies are one ten-billionth the brightness of what the human eye can see. The red galaxies in the image are the remnants of galaxies after major collisions during their elderly years. Many of the smaller galaxies in the image are very young galaxies that eventually developed into major galaxies, similar to the Milky Way and other galaxies in our galactic neighborhood.