Coordinates: 3h 32m 39.0s, −27° 47′ 29.1″
The Hubble Ultra-Deep Field (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 to January 16, 2004. It is the deepest image of the universe ever taken,[1] looking back approximately 13 billion years (between 400 and 800 million years after the Big Bang), and it will be used to search for galaxies that existed at that time. 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. In August and September 2009, the Hubble's Deep Field was expanded using the infrared channel of the recently attached Wide Field Camera 3 (WFC3). When combined with existing HUDF data, astronomers were able to identify a new list of potentially very distant galaxies.[2]
Located southwest of Orion in the southern-hemisphere constellation Fornax, the image covers 11.0 square arcminutes. This is just one-seventieth the solid angle subtended by 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 so that the upper left corner points toward north (−46.4°) on the celestial sphere.
Contents |
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).[3]. A workshop on how to best carry out surveys with the ACS was held at STScI in late 2002. At the workshop Massimo Stiavelli advocated an Ultra Deep Field as a way to study the objects responsible for the reionization of the Universe[4]. 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.[3]
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 X-ray Observatory 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 3h 32m 39.0s, declination -27° 47′ 29.1″ (J2000). The field is 200 arcseconds to a side, with a total area of 11 square arcminutes,[3] and lies in the constellation of Fornax.[1]
Four filters were used on the ACS, centered 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.[3]
The observations were done in two epochs, 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, from 400 orbits, with a typical exposure time of 1200 seconds.[3] In total, 800 ACS exposures were taken over the course of 11.3 days, 2 every orbit, and NICMOS observed for 4.5 days. All the individual ACS exposures were processed and combined by Anton Koekemoer into a single set of scientifically useful images, each with a total exposure time ranging from 134,900 seconds to 347,100 seconds. To observe the whole sky to the same sensitivity, the HST would need to observe continuously for a million years.[1]
| Camera | Filter | Wavelength | Total exposure time | Exposures |
|---|---|---|---|---|
| ACS | F435W | 435 nm | 134,900 s (56 orbits) | 116 |
| ACS | F606W | 606 nm | 135,300 s (56 orbits) | 116 |
| ACS | F775W | 775 nm | 347,100 s (144 orbits) | 288 |
| ACS | F850LP | 850 nm | 346,600 s (144 orbits) | 288 |
The sensitivity of the ACS limits its capability of detecting galaxies at high redshift to about 6. The deep NICMOS fields obtained in parallel to the ACS images could in principle be used to detect galaxies at redshift 7 or higher but they were lacking visible band images of similar depth. These are necessary to identify high redshift objects as they should not be seen in the visible bands. In order to obtain deep visible exposures on top of the NICMOS parallel fields a followup program, HUDF05, was approved and granted 204 orbits to observe the two parallel fields GO-10632). 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):[5]
| 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 |
The HUDF is the deepest image of the universe ever taken 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).[1] The furthest object located as of 2011 was UDFj-39546284 at a time of 600 million years after the Big Bang.[6][7] The red dwarf UDF 2457 at distance of 59,000 light-years is the furthest star resolved by the HUDF.[8] The star near the center of the field is USNO-A2.0 0600-01400432 with apparent magnitude of 18.95.[9]
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.[3] The NICMOS measurements may have discovered galaxies at redshifts up to 12.[1]
| Physical cosmology |
|---|
| Universe · Big Bang Age of the universe Timeline of the Big Bang Ultimate fate of the universe |
|
Early universe
|
|
Expanding universe
|
|
Components
|
|
||||||||||||||