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 at the time shortly after the big bang.

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 24, 2003 through January 16, 2004. It is the deepest image of the universe ever taken in visible light, looking back approximately 13 billion years, and it will be used to search for galaxies that existed between 400 and 800 million years after the Big Bang.The HUDF image was taken in a section of the sky with a low density of bright stars in the near-field, allowing much better viewing of dimmer, more distant objects. The image contains an estimated 10,000 galaxies.

Located southwest of Orion in the Southern-Hemisphere constellation Fornax, the image covers 11.0 square arcminutes. This is just one-tenth the diameter of the full moon as viewed from Earth, 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.

[edit] Planning

Location of the Hubble Ultra Deep Field on the sky.

In the years since the original Hubble Deep Field, the Hubble Deep Field South and the GOODS sample were analysed, 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 realised 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).^[1]

Unlike the Deep Fields, the HUDF does not lie in Hubble's Continuous Viewing Zone (CVZ). The earlier observations, using the 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.^[1]

As with the earlier fields, this one was required to contain very little emission from our galaxy, with little Zodiacal dust. The field was also required to be in a range of declinations such that it could be observed both by southern hemisphere instruments, such as the Atacama Large Millimeter Array, and northern hemisphere ones, such as those located on Hawaii. It was ultimately decided to observe a section of the Chandra Deep Field South, due to existing deep X-ray observations from Chandra and two interesting objects already observed in the GOODS sample at the same location: a redshift 5.8 galaxy and a supernova. The coordinates of the field are right ascension 3^h 32^m 39.0^s, declination -27° 47′ 29.1″ (J2000). The field is 200 arcseconds to a side, with a total area of 11 square arcminutes,^[1] and lies in the constellation of Fornax.^[2]

[edit] Observations

Four filters were used on the ACS, centred on 435, 606, 775 and 850 nm, with exposure times set to give equal sensitivity in all filters. These wavelength ranges match those used by the GOODS sample, allowing direct comparison between the two. As with the Deep Fields, the HUDF used Directors Discretionary Time. In order to get the best resolution possible, the observations were "dithered" by pointing the telescope at slightly different positions for each exposure—a process trialled with the Hubble Deep Field—so that the final image has a higher resolution than the pixels on their own would normally allow.^[1]

The observations were done in two epochs, between September 23 and October 28, 2003, and December 4, 2003, to January 15, 2004. The total exposure time is just under 1 million seconds, from 400 orbits, with a typical exposure time of 1200 seconds.^[1] In total, 800 ACS exposures were taken over the course of 11.3 days, 2 every orbit, and NICMOS observed for 4.5 days. To observe the whole sky to the same sensitivity, the HST would need to observe continuously for a million years.^[2]

[edit] Contents

Part of the Hubble Ultra Deep Field

The HUDF is the deepest image of the universe ever taken in visible light, looking back approximately 13 billion years, and it will be used to search for galaxies that existed between 400 and 800 million years after the Big Bang (Redshifts between 7 and 12).^[2] The star near the center of the field is USNO-A2.0 0600-01400432 with apparent magnitude of 18.95.^{[citation needed]}

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.^[1] The NICMOS measurements may have discovered galaxies at redshifts up to 12.^[2]

[edit] Scientific results

• High rates of star formation during the very early stages of galaxy formation, under a billion years after the Big Bang.^[1]
• Improved characterization of the distribution of galaxies, their numbers, sizes and luminosities at different epochs, allowing investigation into the evolution of galaxies.^[1]
• Confirmation that galaxies at high redshifts are smaller and less symmetrical than ones at lower redshifts, showing the rapid evolution of galaxies in the first couple of billion years after the Big Bang.^[1]

[edit] Follow-up observations

The HUDF will likely remain as the deepest view of the universe until observations are made by the James Webb Space Telescope.^[2]

[edit] References

[edit] Bibliography

[edit] External links

Wikimedia Commons has media related to: Hubble Ultra Deep Field

• NASA site with animations
• Exploring galaxy formation in the early universe - How did the first galaxies get so large so quickly?
• Scalable interactive UDF with 10,000 galaxies mapped. Wikisky.org