thumb|300px|The Hubble Deep Field

The Hubble Deep Field (HDF) is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope. It covers an area about 2.6 arcminutes on a side, about one 24-millionth of the whole sky, which is 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 28, 1995.

After the spherical aberration was corrected during Space Shuttle mission STS-61 in 1993, the improved imaging capabilities of the telescope were used to study increasingly distant and faint galaxies. The Medium Deep Survey (MDS) used the Wide Field and Planetary Camera 2 (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

thumb|300px|The HDF is at the centre of this image of one [[degree (angle)|degree of sky. The Moon as seen from Earth would fill roughly one quarter of this image.]]

The field selected for the observations needed to fulfill 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 at low galactic latitudes (see Zone of Avoidance). 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).

Twenty fields satisfying these criteria were identified, from which three optimal candidate fields were selected, all within the constellation of Ursa Major. Radio snapshot observations with the VLA 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 and a declination of ; or 1/12 the width of the Moon. The area is about 1/24,000,000 of the total area of the sky.

Observations

thumb|left|The HDF was in Hubble's northern Continuous Viewing Zone, as shown by this diagram.

thumb|Diagram illustrating comparative sampling distance of the HDF and the 2004 [[Hubble Ultra-Deep Field]]

Once a field was selected, an observing strategy was developed. An important decision was to determine which filters the observations would use; WFPC2 is equipped with 48 filters, including narrowband filters isolating particular emission lines of astrophysical interest, and broadband filters useful for the study of the colors 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. Because the wavelengths at which the images were taken do not correspond to the wavelengths of red, green and blue light, the colors in the final image only give an approximate representation of the actual colors 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 colors corresponding to what the human eye would actually perceive.

Contents

The final images were released at a meeting of the American Astronomical Society in January 1996, and 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 twenty galactic foreground stars; by far the majority of objects in the field are distant galaxies.

Scientific results

thumb|left|Details from the HDF illustrate the wide variety of galaxy shapes, sizes and colors found in the distant universe.

thumb|Deep field image taken by [[Atacama Large Millimeter Array|ALMA and Hubble.]]

The HDF data provided extremely rich material for cosmologists to analyse and by late 2014 the associated scientific paper for the image had received over 900 citations. 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 the cosmological redshift. While quasars with high redshifts were known, very few galaxies with redshifts greater than one were known before the HDF images were produced. One of the first observations planned for the James Webb Space Telescope was a mid-infrared image of the Hubble Ultra-Deep Field.

thumb|left|200px|On 11 October 2022, the [[James Webb Space Telescope spent over 20 hours observing the long-studied Ultra Deep Field of the NASA/ESA Hubble Space Telescope for the first time.]]

The HDF galaxies contained a considerably larger proportion of disturbed and irregular galaxies than the local universe;

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 dark matter, mass which seems to be undetectable but which observations implied made up about 85% of all matter in the Universe by mass. 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.

Multifrequency followup

thumb|The HDF imaged by the [[Spitzer Space Telescope. The top segment shows the foreground objects in the field; the bottom shows the background with the foreground objects removed.]]

Very-high redshift objects (Lyman-break galaxies) cannot be seen in visible light and generally are detected in infrared or submillimetre wavelength surveys of the HDF instead. Infrared observations have also been made with the Spitzer Space Telescope. Submillimeter observations of the field have been made with SCUBA on the James Clerk Maxwell Telescope, initially detecting 5 sources, although with very low resolution.

X-ray observations by the Chandra X-ray Observatory 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.

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. The field has also been surveyed with the Westerbork Synthesis Radio Telescope and the MERLIN array of radio telescopes at 1.4 GHz; the combination of VLA and MERLIN maps made at wavelengths of 3.5 and 20 cm have located 16 radio sources in the HDF-N field, with many more in the flanking fields.

Subsequent HST observations

An HDF counterpart in the southern celestial hemisphere was created in 1998: the HDF-South (HDF-S). Created using a similar observing strategy, This supports the cosmological principle that at its largest scale the Universe is homogeneous. The HDF-S survey used the Space Telescope Imaging Spectrograph (STIS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instruments installed on the HST in 1997; the region of the original Hubble Deep Field (HDF-N) has since been re-observed several times using WFPC2, as well as by the NICMOS and STIS instruments. until the Hubble eXtreme Deep Field was completed in 2012. Images from the Extreme Deep Field, or XDF, were released on September 26, 2012, to a number of media agencies. Images released in the XDF show galaxies which are now believed to have formed in the first 500 million years following the Big Bang.

See also

  • List of deep fields

Notes and references

Bibliography

  • ; also published in Nature 394: 860 .
  • Main Hubble Deep Field website.
  • NASA's original press release.