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Hubble Deep Field

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   The Hubble Deep Field
   Enlarge
   The Hubble Deep Field

   The Hubble Deep Field (HDF) is an image of a small region in the
   constellation Ursa Major, based on the results of a series of
   observations by the Hubble Space Telescope. It covers an area 144
   arcseconds across, equivalent in angular size to a tennis ball at a
   distance of 100 metres. The image was assembled from 342 separate
   exposures taken with the Space Telescope's Wide Field and Planetary
   Camera 2 over ten consecutive days between December 18 and December 28,
   1995.

   The field is so small that only a few foreground stars in the Milky Way
   lie within it; thus, almost all of the 3,000 objects in the image are
   galaxies, some of which are among the youngest and most distant known.
   By revealing such large numbers of very young galaxies, the HDF has
   become a landmark image in the study of the early universe, and it has
   been the source of almost 400 scientific papers since it was created.

   Three years after the HDF observations were taken, a region in the
   south celestial hemisphere was imaged in a similar way and named the
   Hubble Deep Field South. The similarities between the two regions
   strengthened the belief that the universe is uniform over large scales
   and that the Earth occupies a typical region in the universe (the
   cosmological principle). In 2004 a deeper image, known as the Hubble
   Ultra Deep Field, was constructed from a total of eleven days of
   observations. This image is the deepest (most sensitive) astronomical
   image ever made at visible wavelengths.

Conception

   The dramatic improvement in Hubble's imaging capabilities after
   corrective optics were installed encouraged attempts to obtain very
   deep images of distant galaxies
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   The dramatic improvement in Hubble's imaging capabilities after
   corrective optics were installed encouraged attempts to obtain very
   deep images of distant galaxies

   One of the key aims of the astronomers who designed the Hubble Space
   Telescope was to use its high optical resolution to study distant
   galaxies to a level of detail that was not possible from the ground.
   Positioned above the atmosphere, Hubble avoids atmospheric airglow
   allowing it to take more sensitive visible and ultraviolet light images
   than can be obtained with seeing-limited ground-based telescopes (when
   good adaptive optics correction becomes available in the visible, 10m
   ground-based telescopes may become competitive). Although the
   telescope's mirror suffered from spherical aberration when the
   telescope was launched in 1990, it could still be used to take images
   of more distant galaxies than had previously been obtainable. Because
   light takes billions of years to reach Earth from very distant
   galaxies, we see them as they were billions of years ago; thus,
   extending the scope of such research to increasingly distant galaxies
   allows a better understanding of how they evolve.

   After the spherical aberration was corrected during Space Shuttle
   mission STS-61 in 1993, the now excellent imaging capabilities of the
   telescope were used to study increasingly distant and faint galaxies.
   The Medium Deep Survey (MDS) used the WFPC2 to take deep images of
   random fields while other instruments were being used for scheduled
   observations. At the same time, other dedicated programs focused on
   galaxies that were already known through ground-based observation. All
   of these studies revealed substantial differences between the
   properties of galaxies today and those that existed several billion
   years ago.

   Up to 10% of the HST's observation time is designated as Director's
   Discretionary (DD) Time, and is typically awarded to astronomers who
   wish to study unexpected transient phenomena, such as supernovae. Once
   Hubble's corrective optics were shown to be performing well, Robert
   Williams, the then director of the Space Telescope Science Institute,
   decided to devote a substantial fraction of his DD time during 1995 to
   the study of distant galaxies. A special Institute Advisory Committee
   recommended that the WFPC2 be used to image a "typical" patch of sky at
   a high galactic latitude, using several optical filters. A working
   group was set up to develop and implement the project.

Target selection

   The HDF field is at the centre of this image, one degree across, which
   shows the unremarkable nature of this patch of sky.
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   The HDF field is at the centre of this image, one degree across, which
   shows the unremarkable nature of this patch of sky.

   The field selected for the observations needed to fulfil several
   criteria. It had to be at a high galactic latitude, because dust and
   obscuring matter in the plane of the Milky Way's disc prevents
   observations of distant galaxies. The target field had to avoid known
   bright sources of visible light (such as foreground stars), and
   infrared, ultraviolet and X-ray emissions, to facilitate later studies
   at many wavelengths of the objects in the deep field, and also needed
   to be in a region with a low background infrared ' cirrus', the
   diffuse, wispy infrared emission believed to be caused by warm dust
   grains in cool clouds of hydrogen gas ( H I regions).

   These criteria considerably restricted the field of potential target
   areas. It was further decided that the target should be in Hubble's
   'continuous viewing zones' (CVZs)—the areas of sky which are not
   occulted by the Earth or the moon during Hubble's orbit. The working
   group decided to concentrate on the northern CVZ, so that
   northern-hemisphere telescopes, such as the Keck telescopes and the
   Very Large Array, could conduct follow-up observations.

   Twenty fields satisfying all of these criteria were initially
   identified, from which three optimal candidate fields were selected,
   all within the constellation of Ursa Major. Radio snapshot observations
   ruled out one of these fields because it contained a bright radio
   source, and the final decision between the other two was made on the
   basis of the availability of guide stars near the field: Hubble
   observations normally require a pair of nearby stars on which the
   telescope's Fine Guidance Sensors can lock during an exposure, but
   given the importance of the HDF observations, the working group
   required a second set of back-up guide stars. The field that was
   eventually selected is located at a right ascension of 12h 36m 49.4s
   and a declination of +62° 12′ 48″ ^.

Observations

   The HDF was located in Hubble's northern Continuous Viewing Zone, as
   shown by this diagram
   Enlarge
   The HDF was located in Hubble's northern Continuous Viewing Zone, as
   shown by this diagram

   Once a field had been selected, an observing strategy had to be
   developed. An important decision was to determine which filters the
   observations would use; WFPC2 is equipped with forty-eight filters,
   including narrowband filters isolating particular emission lines of
   astrophysical interest, and broadband filters useful for the study of
   the colours of stars and galaxies. The choice of filters to be used for
   the HDF depended on the ' throughput' of each filter— the total
   proportion of light that it allows through— and the spectral coverage
   available. Filters with bandpasses overlapping as little as possible
   were desirable.

   In the end, four broadband filters were chosen, centred at wavelengths
   of 300 nm (near-ultraviolet), 450 nm (blue light), 606 nm (red light)
   and 814 nm (near- infrared). Because the quantum efficiency of Hubble's
   detectors is quite low at 300 nm, the noise in observations at this
   wavelength is primarily due to CCD noise rather than sky background;
   thus, these observations could be conducted at times when high
   background noise would have harmed the efficiency of observations in
   other passbands.

   Images of the target area in the chosen filters were taken over ten
   consecutive days, during which Hubble orbited the Earth about 150
   times. The total exposure times at each wavelength were 42.7 hours (300
   nm), 33.5 hours (450 nm), 30.3 hours (606 nm) and 34.3 hours (814 nm),
   divided into 342 individual exposures to prevent significant damage to
   individual images by cosmic rays, which cause bright streaks to appear
   when they strike CCD detectors.

Data processing

   A section of the HDF about 14 arcseconds across in each of the four
   wavelengths used to construct the final version: 300 nm (top left), 450
   nm (top right), 606 nm (bottom left) and 814 nm (bottom right).
   Enlarge
   A section of the HDF about 14 arcseconds across in each of the four
   wavelengths used to construct the final version: 300 nm (top left), 450
   nm (top right), 606 nm (bottom left) and 814 nm (bottom right).

   The production of a final combined image at each wavelength was a
   complex process. Bright pixels caused by cosmic ray impacts during
   exposures were removed by comparing exposures of equal length taken one
   after the other, and identifying pixels that were affected by cosmic
   rays in one exposure but not the other. Trails of space debris and
   artificial satellites were present in the original images, and were
   carefully removed.

   Scattered light from the Earth was evident in about a quarter of the
   data frames. This was removed by taking an image affected by scattered
   light, aligning it with an unaffected image, and subtracting the
   unaffected image from the affected one. The resulting image was
   smoothed, and could then be subtracted from the bright frame. This
   procedure removed almost all of the scattered light from the affected
   images.

   Once the 342 individual images were cleaned of cosmic-ray hits and
   corrected for scattered light, they had to be combined. Scientists
   involved in the HDF observations pioneered a technique called '
   drizzling', in which the pointing of the telescope was varied minutely
   between sets of exposures. Each pixel on the WFPC2 CCD chips recorded
   an area of sky 0.09 arcseconds in diameter, but by changing the
   direction in which the telescope was pointing by less than that between
   exposures, the resulting images were combined using sophisticated
   image-processing techniques to yield a final angular resolution better
   than this value. The HDF images produced at each wavelength had final
   pixel sizes of 0.03985 arcseconds.

   The data processing yielded four monochrome images, one at each
   wavelength. The combining of these into the full colour images released
   to the public was a fairly arbitrary process, with one image designated
   as each of red, green and blue, and the three images combined to give a
   colour image. Because the wavelengths at which the images were taken do
   not correspond to the wavelengths of red, green and blue light, the
   colours in the final image only give an approximate representation of
   the actual colours of the galaxies in the image; the choice of filters
   for the HDF (and the majority of Hubble images) was primarily designed
   to maximize the scientific utility of the observations rather than to
   create colours corresponding to what the human eye would actually
   perceive.

Contents of the Deep Field

   The final images revealed a plethora of distant, faint galaxies. About
   3,000 distinct galaxies could be identified in the images, with both
   irregular and spiral galaxies clearly visible, although some galaxies
   in the field are only a few pixels across. In all, the HDF is thought
   to contain fewer than ten galactic foreground stars; by far the
   majority of objects in the field are distant galaxies.

   There are about fifty blue point-like objects in the HDF. Many seem to
   be associated with nearby galaxies, which together form chains and
   arcs: these are likely to be regions of intense star formation. Others
   may be distant quasars. Astronomers initially ruled out the possibility
   that some of the point-like objects are white dwarfs, because they are
   too blue to be consistent with theories of white dwarf evolution
   prevalent at the time. However, more recent work has found that many
   white dwarfs become bluer as they age, lending support to the idea that
   the HDF might contain white dwarfs .

Scientific results

   Details from the HDF illustrate the wide variety of galaxy shapes,
   sizes and colours found in the distant universe
   Enlarge
   Details from the HDF illustrate the wide variety of galaxy shapes,
   sizes and colours found in the distant universe

   The HDF data provided extremely rich material for cosmologists to
   analyse and as of 2005, almost 400 papers based on the HDF have
   appeared in the astronomical literature. One of the most fundamental
   findings was the discovery of large numbers of galaxies with high
   redshift values.

   As the universe expands, more distant objects recede from the Earth
   faster, in what is called the Hubble Flow. The light from very distant
   galaxies is significantly affected by doppler shifting, which reddens
   the radiation that we receive from them. While quasars with high
   redshifts were known, very few galaxies with redshifts greater than 1
   were known before the HDF images were produced. The HDF, however,
   contained many galaxies with redshifts as high as 6, corresponding to
   distances of about 12 billion light years . (Due to redshift the most
   distant objects in the HDF are not actually visible in the Hubble
   images; they can only be detected in images of the HDF taken at longer
   wavelengths by ground-based telescopes.)

   The HDF galaxies contained a considerably larger proportion of
   disturbed and irregular galaxies than the local universe; galaxy
   collisions and mergers were more common in the young universe as it was
   much smaller than today. It is believed that giant elliptical galaxies
   form when spirals and irregular galaxies collide.

   The wealth of galaxies at different stages of their evolution also
   allowed astronomers to estimate the variation in the rate of star
   formation over the lifetime of the universe. While estimates of the
   redshifts of HDF galaxies are somewhat crude, astronomers believe that
   star formation was occurring at its maximum rate 8–10 billion years
   ago, and has decreased by a factor of about 10 since then .

   Another important result from the HDF was the very small number of
   foreground stars present. For years astronomers had been puzzling over
   the nature of so-called dark matter, mass which seems to be
   undetectable but which observations implied made up about 90% of the
   mass of the universe. One theory was that dark matter might consist of
   Massive Astrophysical Compact Halo Objects ( MACHOs) — faint but
   massive objects such as red dwarfs and planets in the outer regions of
   galaxies. The HDF showed, however, that there were not significant
   numbers of red dwarfs in the outer parts of our galaxy.

Subsequent observations

   The Hubble Deep Field South looks very similar to the original HDF,
   demonstrating the Cosmological Principle.
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   The Hubble Deep Field South looks very similar to the original HDF,
   demonstrating the Cosmological Principle.
   The Hubble Ultra Deep Field further corroborates this. The smallest,
   redest galaxies, about 100, are some of the most distant to have been
   imaged in an optical telescope.
   Enlarge
   The Hubble Ultra Deep Field further corroborates this. The smallest,
   redest galaxies, about 100, are some of the most distant to have been
   imaged in an optical telescope.

   The HDF is a landmark in observational cosmology and much still remains
   to be learned from it. Since 1995, the field has been observed by many
   ground-based telescopes as well as some further space telescopes, at
   wavelengths from radio to X-ray.

   Very-high redshift objects were discovered within the HDF using a
   number of groundbased telescopes, in particular via the James Clerk
   Maxwell Telescope. The high redshift of these objects means that they
   cannot be seen in visible light and generally are detected in infrared
   or submillimetre wavelength surveys of the HDF instead.

   Important space-based observations have included those by the Chandra
   X-ray Observatory and the Infrared Space Observatory (ISO). X-ray
   observations revealed six sources in the HDF, which were found to
   correspond to three elliptical galaxies: one spiral galaxy, one active
   galactic nucleus and one extremely red object, thought to be a distant
   galaxy containing a large amount of dust absorbing its blue light
   emissions .

   ISO observations indicated infrared emission from 13 galaxies visible
   in the optical images, attributed to large quantities of dust
   associated with intense star formation. Ground-based radio images taken
   using the VLA revealed seven radio sources in the HDF, all of which
   correspond to galaxies visible in the optical images.

   1998 saw the creation of an HDF counterpart in the southern celestial
   hemisphere: the HDF-South. Created using a similar observing strategy,
   the HDF-S was very similar in appearance to the original HDF. This
   supports the cosmological principle that at its largest scale the
   universe is homogenous.

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