View source for Hubble Deep Field

{{Sky|12|36|49.4|+|62|12|58|100000000000}}
[[File:HubbleDeepField.800px.jpg|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 [[arcminute]]s 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&nbsp;metres.<ref>{{cite book |title=The Big Questions The Universe |first1=Stuart |last1=Clark |publisher=Hachette UK |year=2011 |isbn=978-1-84916-609-6 |page=69 |url=https://books.google.com/books?id=PkVhBQAAQBAJ}}</ref> 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.<ref name="Ferguson1998" /><ref name="Hubble_image" />

The field is so small that only a few foreground [[star]]s in the [[Milky Way]] lie within it; thus, almost all of the 3,000 objects in the image are [[galaxy|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 [[physical cosmology|study of the early universe]].

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]]). A wider but shallower survey was also made as part of the [[Great Observatories Origins Deep Survey]]. In 2004 a deeper image, known as the [[Hubble Ultra-Deep Field]] (HUDF), was constructed from a few months of light exposure. The HUDF image was at the time the most sensitive [[astronomy|astronomical]] image ever made at visible wavelengths, and it remained so until the [[Hubble eXtreme Deep Field]] (XDF) was released in 2012.

==Conception==
[[File:Improvement in Hubble images after SMM1.jpg|thumb|300px|left|The dramatic improvement in Hubble's imaging capabilities after corrective [[optics]] were installed encouraged attempts to obtain very deep images of distant [[Galaxy|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 light|visible]] and [[ultraviolet light]] images than can be obtained with seeing-limited ground-based telescopes (when good [[adaptive optics]] correction at visible wavelengths becomes possible, 10&nbsp;m 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 [[Speed of light|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.<ref name=Ferguson1998>Ferguson et al. (1999), p.84</ref>

After the spherical aberration was corrected during [[Space Shuttle]] mission [[STS-61]] in 1993,<ref name="Trauger1994">Trauger et al. (1994)</ref> 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.<ref>Abraham et al. (1996)</ref>

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 [[supernova]]e. Once Hubble's corrective optics were shown to be performing well, [[Robert Williams (astronomer)|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 filter]]s. A [[working group]] was set up to develop and implement the project.<ref name="Williams1996">Williams et al. (1996)</ref>

==Target selection==
[[File:Hubble Deep Field location.gif|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.]]
[[File:Hubble Ultra Deep Field diagram.jpg|thumb|Diagram illustrating comparative sampling distance of the HDF and the 2004 Hubble Ultra-Deep Field]]

The field selected for the observations needed to fulfill several criteria. It had to be at a high galactic latitude, because [[interstellar dust|dust]] and obscuring matter in the plane of the [[Milky Way]]'s disc prevents observations of distant galaxies at low galactic latitudes. 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|infrared 'cirrus']], the diffuse, wispy infrared emission believed to be caused by warm dust grains in cool clouds of [[hydrogen]] gas ([[H I region]]s).<ref name="Williams1996" />

These criteria restricted the field of potential target areas. It was decided that the target should be in Hubble's 'continuous viewing zones' (CVZs)—the areas of sky which are not [[occultation|occulted]] by the Earth or the [[moon]] during Hubble's orbit.<ref name="Williams1996" /> The working group decided to concentrate on the northern CVZ, so that northern-hemisphere telescopes such as the [[Keck telescopes]], the [[Kitt Peak National Observatory]] telescopes and the [[Very Large Array]] (VLA) could conduct follow-up observations.<ref name="north_cvz">
{{cite web
 | author = Ferguson, H.
 | date = 1996
 | url = http://www.stsci.edu/ftp/science/hdf/project/field.html
 | title = The Hubble Deep Field—field selection
 | publisher = Space Telescope Science Institute
 | accessdate = December 26, 2008
}}</ref>

Twenty fields satisfying these criteria were initially identified, from which three optimal candidate fields were selected, all within the constellation of [[Ursa Major]]. [[Radio]] snapshot observations with the [[Very Large Array|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 {{RA|12|36|49.4}} and a [[declination]] of {{DEC|+62|12|58}};<ref name="Williams1996" /><ref name="north_cvz" /> it is approximately 2.6 [[arcminute]]s in width,<ref name="Ferguson1998" /><ref name="Ferguson2000a">Ferguson (2000a)</ref> or 1/12 the width of the Moon. The area is approximately 1/28,000,000 of the total area of the sky.<ref name=Curious>{{cite web|url=http://curious.astro.cornell.edu/about-us/98-the-universe/galaxies/observing-galaxies/535-how-big-is-the-hubble-ultra-deep-field-image-intermediate|title=How big is the Hubble Ultra Deep Field image|date=April 2016|publisher=Curious about astronomy? Ask an astronomer.|accessdate=January 7, 2009|author=Anderson, Ryan}}</ref>

==Observations==
[[File:Hubble Deep Field observing geometry.svg|thumb|200px|left|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 [[filter (optics)|filter]]s the observations would use; WFPC2 is equipped with forty-eight filters, including [[narrowband]] filters isolating particular [[emission line]]s 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 [[bandpass]]es overlapping as little as possible were desirable.<ref name="Williams1996" />

In the end, four broadband filters were chosen, centred at [[wavelength]]s of 300 [[Nanometre|nm]] (near-[[ultraviolet]]), 450&nbsp;nm (blue light), 606&nbsp;nm (red light) and 814&nbsp;nm (near-[[infrared]]). Because the [[quantum efficiency]] of Hubble's detectors is quite low at 300&nbsp;nm, the noise in observations at this wavelength is primarily due to [[charge-coupled device|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.<ref name="Williams1996" />

Between December 18 and December 28, 1995—during which time Hubble orbited the Earth about 150 times—342 images of the target area in the chosen filters were taken. The total exposure times at each wavelength were 42.7 hours (300&nbsp;nm), 33.5 hours (450&nbsp;nm), 30.3 hours (606&nbsp;nm) and 34.3 hours (814&nbsp;nm), divided into 342 individual exposures to prevent significant damage to individual images by [[cosmic ray]]s, which cause bright streaks to appear when they strike CCD detectors. A further 10 Hubble orbits were used to make short exposures of flanking fields to aid follow-up observations by other instruments.<ref name="Williams1996" />

==Data processing==
[[File:Galaxy in each of the four wavelengths comprising the HDF.jpg|thumb|300px|A section of the HDF about 14 [[arcseconds]] across in each of the four [[wavelength]]s used to construct the final version: 300 [[Nanometre|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 [[pixel]]s 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 ray]]s in one exposure but not the other. Trails of [[space debris]] and [[artificial satellite]]s were present in the original images, and were carefully removed.<ref name="Williams1996" />

Scattered light from the Earth was evident in about a quarter of the data frames, creating a visible "X" pattern on the images. 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.<ref name="Williams1996" />

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 '[[Drizzle (image processing)|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 [[arcsecond]]s across, 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.<ref name="Williams1996" />

The data processing yielded four [[monochrome]] images (at 300&nbsp;nm, 450&nbsp;nm, 606&nbsp;nm and 814&nbsp;nm), one at each wavelength.<ref name=Ferguson1/> One image was designated as red (814&nbsp;nm), the second as green (606&nbsp;nm) and the third as blue (450&nbsp;nm), and the three images were combined to give a colour image.<ref name=Hubble_image>{{cite web|publisher=NASA|date=1995|title=Hubble's Deepest View of the Universe Unveils Bewildering Galaxies across Billions of Years|url=http://hubblesite.org/newscenter/archive/releases/1996/01/image/a/|accessdate=12 January 2009}}</ref> 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.<ref name=Ferguson1>Ferguson et al. (1999), p.88</ref>

==Contents of the Deep Field==
[[File:ALMA probes the Hubble Ultra Deep Field.jpg|thumb|Deep field image taken by [[Atacama Large Millimeter Array|ALMA]] and Hubble.<ref>{{cite web|title=ALMA Explores the Hubble Ultra Deep Field - Deepest ever millimetre observations of early Universe|url=http://www.eso.org/public/news/eso1633/|website=www.eso.org|accessdate=24 September 2016}}</ref>]]

The final images were released at a meeting of the [[American Astronomical Society]] in January 1996,<ref name="key_findings">
{{cite web
 | date = 1997
 | url = http://oposite.stsci.edu/pubinfo/PR/97/hdf-key-findings.html
 | title = Summary of Key Findings From the Hubble Deep Field
 | accessdate = December 26, 2008
 | publisher = Space Telescope Science Institute
 | archiveurl=https://archive.is/20110701011536/http://oposite.stsci.edu/pubinfo/PR/97/hdf-key-findings.html|archivedate=July 1, 2011}}</ref> and revealed a plethora of distant, faint galaxies. About 3,000 distinct galaxies could be identified in the images,<ref name="Ferguson2000b">Ferguson et al. (2000b)</ref> with both [[irregular galaxy|irregular]] and [[spiral galaxy|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.<ref name="Flynn1996" />

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 [[quasar]]s. Astronomers initially ruled out the possibility that some of the point-like objects are [[white dwarf]]s, 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.<ref name="Hansen1998">Hansen (1998)</ref>

==Scientific results==
[[File:HDF extracts showing many galaxies.jpg|thumb|250px|left|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 by late 2014 the associated scientific paper for the image had received over 900 citations.<ref name="nasa_ads"><!-- Referencing the _citation count_, not the article -->{{cite journal | title=NASA ADS entry for Williams et al. (1996) | publisher=The SAO/NASA Astrophysics Data System | bibcode=1996AJ....112.1335W |author1=Williams, Robert E. |author2=Blacker, Brett |author3=Dickinson, Mark |author4=Dixon, W. Van Dyke |author5=Ferguson, Henry C. |author6=Fruchter, Andrew S. |author7=Giavalisco, Mauro |author8=Gilliland, Ronald L. |author9=Heyer, Inge |author10=Katsanis, Rocio |author11=Levay, Zolt |author12=Lucas, Ray A. |author13=McElroy, Douglas B. |author14=Petro, Larry |author15=Postman, Marc |author16=Adorf, Hans-Martin |author17=Hook, Richard | volume=112 | date=1996 | page=1335 | journal=Astronomical Journal | doi=10.1086/118105|arxiv = astro-ph/9607174 }}</ref> 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 [[Redshift#Expansion of space|cosmological redshift]]. While [[quasar]]s with high redshifts were known, very few galaxies with redshifts greater than one were known before the HDF images were produced.<ref name="key_findings" /> The HDF, however, contained many galaxies with redshifts as high as six, corresponding to distances of about 12 billion [[light-year]]s. Due to redshift the most distant objects in the HDF ([[Lyman-break galaxy|Lyman-break galaxies]]) 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.<ref name=Ferguson2>Ferguson et al. (1999), p.105</ref>

The HDF galaxies contained a considerably larger proportion of disturbed and irregular galaxies than the local universe;<ref name="key_findings" /> galaxy collisions and mergers were more common in the young universe as it was much smaller than today. It is believed that giant [[elliptical galaxy|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.<ref name="Connolly1997">Connolly et al. (1997)</ref>

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 90% of the mass of the Universe.<ref name="Trimble1987">Trimble (1987)</ref> One theory was that dark matter might consist of Massive Astrophysical Compact Halo Objects ([[MACHO]]s)—faint but massive objects such as [[red dwarf]]s and [[planet]]s in the outer regions of galaxies.<ref name="Alcock1992">Alcock et al. (1992)</ref> The HDF showed, however, that there were not significant numbers of red dwarfs in the outer parts of our galaxy.<ref name="key_findings" /><ref name="Flynn1996">Flynn et al. (1996)</ref>

==Multifrequency followup==
[[File:Hubble Deep Field by Spitzer.jpg|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 astronomy|infrared]] or [[Submillimetre astronomy|submillimetre]] wavelength surveys of the HDF instead.<ref name=Ferguson2/> Observations with the [[Infrared Space Observatory]] (ISO) indicated infrared emission from 13 galaxies visible in the optical images, attributed to large quantities of dust associated with intense star formation.<ref name="RowanRobinson1997">Rowan-Robinson et al. (1997)</ref> Infrared observations have also been made with the [[Spitzer Space Telescope]].<ref>{{cite web
 | title = GOODS Spitzer and Ancillary Data
 | url = http://irsa.ipac.caltech.edu/data/SPITZER/GOODS/
 | publisher = NASA/IPAC Infrared Science Archive
 | accessdate = January 7, 2009
}}</ref> Submillimeter observations of the field have been made with [[James Clerk Maxwell Telescope#Continuum detectors|SCUBA]] on the [[James Clerk Maxwell Telescope]], initially detecting 5 sources, although with very low resolution.<ref name="Ferguson2000b" /> Observations have also been made with the [[Subaru (telescope)|Subaru]] telescope in Hawaii.<ref name="hdf_clearinghouse">
{{cite web
 | last = Ferguson | first = H.
 | date = 2002
 | url = http://www.stsci.edu/ftp/science/hdf/clearinghouse/clearinghouse.html
 | title = HDF Clearinghouse
 | publisher = Space Telescope Science Institute
 | accessdate = December 27, 2008
}}</ref>

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.<ref name="Hornschemeier2000">Hornschemeier et al. (2000)</ref>

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.<ref name="Kellerman1998">Kellerman et al. (1998)</ref> The field has also been surveyed with the [[Westerbork Synthesis Radio Telescope]] and the [[MERLIN]] array of radio telescopes at 1.4&nbsp;GHz;<ref name="wsrt">Garratt et al. (2000)</ref><ref name="merlin">
{{cite web
 | url=http://www.merlin.ac.uk/topics/deepfield/index.html
 | title=Preliminary MERLIN Observations of the HST Deep Field
 | publisher=[[Jodrell Bank Observatory]]
 | accessdate=December 27, 2008
}}<!-- DEADLINK --></ref> the combination of VLA and MERLIN maps made at wavelengths of 3.5 and 20&nbsp;cm have located 16 radio sources in the HDF-N field, with many more in the flanking fields.<ref name="Ferguson2000b" /> Radio images of some individual sources in the field have been made with the [[European VLBI Network]] at 1.6&nbsp;GHz with a higher resolution than the Hubble maps.<ref name="evn">Garrett et al. (2001)</ref>

==Subsequent HST observations==
<div style="float:left;margin-right:1em;">
{|
|- valign="top"
|[[File:Hubble Deep Field South full mosaic.jpg|thumb|The [[Hubble Deep Field South]] looks very similar to the original HDF, demonstrating the [[cosmological principle]].]]
|[[File:Hubble ultra deep field high rez edit1.jpg|thumb|The [[Hubble Ultra-Deep Field]] further corroborates this.]]
|}
</div>

An HDF counterpart in the southern celestial hemisphere was created in 1998: the [[Hubble Deep Field South|HDF-South]].<ref name="Williams2000">Williams et al. (2000)</ref> Created using a similar observing strategy,<ref name="Williams2000" /> the HDF-S was very similar in appearance to the original HDF.<ref name="Casertano2000">Casertano et al. (2000)</ref> This supports the [[cosmological principle]] that at its largest scale the Universe is [[Homogeneity (physics)|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 Hubble Deep Field has since been re-observed several times using WFPC2, as well as by the NICMOS and STIS instruments.<ref name="Ferguson2000a" /><ref name="Ferguson2000b" /> Several [[supernova]] events were detected by comparing the first and second epoch observations of the HDF-N.<ref name="Ferguson2000b" />

A wider survey, but less sensitive, was carried out as part of the [[Great Observatories Origins Deep Survey]]; a section of this was then observed for longer to create the [[Hubble Ultra-Deep Field]], which was the most sensitive optical deep field image for years<ref name="Beckwith2006">Beckwith et al. (2006)</ref> until the [[Hubble eXtreme Deep Field]] was completed in 2012.<ref>{{cite news|title=Hubble goes to the eXtreme to assemble the deepest ever view of the Universe|url=http://www.spacetelescope.org/news/heic1214/|accessdate=25 September 2012|newspaper=Hubble press release}}</ref> Images from the Extreme Deep Field, or XDF, were released on 26 September 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.<ref>[http://hubblesite.org/newscenter/archive/releases/2012/37/image/a/ Hubble Site News Center]</ref><ref>[https://www.theguardian.com/science/2012/sep/26/hubble-astronomers-deepest-view-night-sky|Hubble Astronomers Release Deepest View of the Night Sky]</ref>
{{clear}}

==See also==
{{Portal|Astronomy|Cosmology}}
* [[List of Deep Fields]]

==Notes and references==
{{Reflist|30em}}

==Bibliography==
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* {{cite journal
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   | volume = 289
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  |arxiv = astro-ph/9707030
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{{refend}}

==External links==
{{Commonscat-inline|Hubble Deep Field}}
* {{cite web | url=http://www.stsci.edu/ftp/science/hdf/hdf.html | title=The Hubble Deep Field | publisher=STScI}} Main Hubble Deep Field website.
* {{cite web | url=http://hubblesite.org/newscenter/archive/1996/01 | title=Hubble's Deepest View of the Universe Unveils Bewildering Galaxies across Billions of Years | date=January 15, 1996}} NASA's original press release.
* {{cite web| url=http://www.salon.com/comics/opus/2007/08/05/opus/| title=Opus Cartoon| publisher=salon| deadurl=yes| archiveurl=https://web.archive.org/web/20090414051143/http://www.salon.com/comics/opus/2007/08/05/opus/| archivedate=2009-04-14| df=}} Opus Cartoon.

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