The Hubble Ultra-Deep Field 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. Looking back approximately 13 billion years 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. When combined with existing HUDF
data, astronomers were able to identify a new list of potentially very distant galaxies.
Located southwest of Orion in the southern-hemisphere constellation Fornax, the rectangular image
is 2.4 arcminutes to an edge, or 3.4 arcminutes diagonally. This is approximately one tenth
of the angular diameter of a full moon viewed from Earth, smaller than a 1 mm by 1 mm
square of paper held at 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 on the celestial sphere. On September 25, 2012, NASA released a further
refined version of the Ultra-Deep Field dubbed the eXtreme Deep Field. The XDF reveals galaxies
that span back 13.2 billion years in time, revealing a galaxy theorized to be formed
only 450 million years after the big bang event. On June 3, 2014, NASA released the
Hubble Ultra-Deep Field image composed of, for the first time, the full range of ultraviolet
to near-infrared light.
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 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. 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. 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. The earlier observations, using the Wide Field
and Planetary Camera 2 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. 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″. The field is 200 arcseconds to a side, with
a total area of 11 square arcminutes, and lies in the constellation of Fornax.
Observations
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.
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, from 400 orbits, with a typical exposure time of 1200 seconds. 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.
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 follow-up program, HUDF05, was approved and granted
204 orbits to observe the two parallel fields. 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 devoted 192 orbits
to observations of three fields, including HUDF, using the newly available F105W, F125W
and F160W infra-red filters: Contents
The HUDF used to be 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. Several
galaxies in the HUDF are candidates, based on photometric redshifts, to be amongst the
most distant astronomical objects. The red dwarf UDF 2457 at distance of 59,000 light-years
is the furthest star resolved by the HUDF. 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. The NICMOS measurements may have
discovered galaxies at redshifts up to 12. Scientific results
High rates of star formation during the very early stages of galaxy formation, under a
billion years after the Big Bang. Improved characterization of the distribution
of galaxies, their numbers, sizes and luminosities at different epochs, allowing investigation
into the evolution of galaxies. 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.
Successor
The Hubble eXtreme Deep Field is an image of a small part of space in the center of
the Hubble Ultra Deep Field within the constellation Fornax, showing the deepest optical view in
space. Released on September 25, 2012, it took 10 years to compile the images and shows
galaxies from 13.2 billion years ago. The exposure time was two million seconds, or
approximately 23 days. The faintest galaxies are one ten-billionth the brightness of what
the human eye can see. The red galaxies are the remnants of galaxies after major collisions
during their elderly years. Many of the smaller galaxies are very young galaxies that eventually
became the major galaxies, like the Milky Way and other galaxies in our galactic neighborhood.
The Hubble eXtreme Deep Field, or XDF, adds another 5,500 galaxies to Hubble's 2003 and
2004 view into a tiny patch of the farthest universe.
See also List of Deep Fields
Hubble Extreme Deep Field HUDF-JD2
UDF 2457 UDF 423
UDFj-39546284 UDFy-38135539
References
External links 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