Hubble Ultra Deep Field

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The Hubble Ultra Deep Field or HUDF is an image of a small region of space 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 in time more than 13 billion years. The HUDF contains an estimated 10,000 galaxies. The patch of sky in which the galaxies reside (just one-tenth the diameter of the full moon) was chosen because it had a low density of bright stars. 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 constellation Fornax at right ascension 3h 32m 40.0s, declination -27° 48' 00" (J2000), the image covers 3 arcminutes square, which is smaller than a grain of sand held at arm's length, and is tilted about 45° such that the upper left corner points toward north on the celestial sphere.

The million-second-long exposure reveals the first galaxies to emerge from the so-called "dark ages," the time shortly after the Big Bang when the first stars reheated the cold, dark universe. The view is actually two separate images taken by Hubble's Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). 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.

The Big Bang theory suggests that the Universe has a finite age, and if this were true 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 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). The Hubble Ultra Deep Field also shows more evidence for galaxy formation and merging than in local studies, as expected for the early Universe.

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