Editing Hubble Deep Field (section)

==Scientific results==
[[File:HDF extracts showing many galaxies.jpg|thumb|left|Details from the HDF illustrate the wide variety of galaxy shapes, sizes and colors found in the distant universe.]]
[[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 |access-date=September 24, 2016}}</ref>]]

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) |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|s2cid=17310815 }}</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> One of the first observations planned for the [[James Webb Space Telescope]] was a mid-infrared image of the Hubble Ultra-Deep Field.<ref>{{cite web|url=https://www.stsci.edu/jwst/science-execution/program-information.html?id=1207|title=Program Information – GTO 1207|access-date=2022-01-25}}</ref>

[[File:Webb observes the Hubble Ultra Deep Field (udf-a).jpg|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.<ref>{{cite news |url=https://esawebb.org/images/udf-a/|title=Webb observes the Hubble Ultra Deep Field |date=October 17, 2023}}</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 85% of all matter in the Universe by mass.<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>