Hubble Ultra Deep Field

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This high-resolution image of the HUDF includes galaxies of various ages, sizes, shapes, and colors. The smallest, reddest galaxies, about 100, are some of the most distant galaxies to have been imaged by an optical telescope, existing when the universe was just 800 million years old.

The Hubble Ultra Deep Field, or HUDF, is an image of a small region of space in the constellation Fornax, composited from Hubble Space Telescope data accumulated over a period from September 3, 2003 through January 16, 2004. It is the deepest image of the universe ever taken in visible light, looking back (to when the universe is thought to have been 800 million years old) more than 13 billion years ago.^{[citation needed]} The HUDF contains an estimated 10,000 galaxies^[1]. The patch of sky in which the galaxies reside (just one-tenth the diameter of the full moon as viewed from Earth) was chosen because it had a low density of bright stars in the near-field. Although most of the targets visible in the Hubble image can also be seen at infrared wavelengths by ground-based telescopes, Hubble is the only instrument which can make observations of these distant targets at visible wavelengths. Located southwest of Orion in the Southern-Hemisphere constellation Fornax at right ascension 3h 32m 40.0s, declination -27° 47' 29" (J2000), the image covers 36.7 square arcminutes^[2]. This is smaller than a 1 mm by 1 mm square of paper held 1 meter away, and equal to roughly one thirteen-millionth of the total area of the sky. The image is oriented such that the upper left corner points toward north (-46.4°) on the celestial sphere. The star near the center of the field is USNO-A2.0 0600-01400432 with apparent magnitude of 18.95.

In total, the image required 800 exposures taken over the course of 400 Hubble orbits around Earth. The total amount of exposure time was 11.3 days for the ACS and 4.5 days for the NICMOS.

According to the Big Bang theory, the universe has a finite age, so we might expect very distant (and hence very young) galaxies to look different from the typical older galaxies we see today. This is indeed seen in the HUDF, although some argue^{[citation needed]} that the difference is partly a result of the unusual wavelength used for the HUDF (corresponding to ultraviolet light from the rest-frame of the most distant galaxies)^{[citation needed]}. The Hubble Ultra Deep Field also shows more evidence for galaxy formation and merging than in local studies, as expected for the early universe^{[citation needed]}.