Hubble Deep Field
Hubble Deep Field ( HDF ), even Hubble Deep Field is an image of a small portion of the night sky , which in December 1995 with the Hubble Space Telescope with maximum time technically possible resolution was taken. The selected area of the sky contains only a few nearby stars and other nearby objects, so distant galaxies could be observed up to a distance of about 12 billion light years . The Hubble Deep Field enables the study of the evolution of galaxies in the early universe.
background
For the Hubble Deep Field, an area in the Great Bear was selected that is relatively free from disturbing visual influences and surrounding bright stars. The area has an edge length of 144 arc seconds , which is roughly the angle at which a tennis ball appears 100 m away. The image is an overlay of 342 individual images captured with the Wide Field and Planetary Camera 2 (WFPC2) of the Hubble Space Telescope over ten days between December 18 and 28, 1995.
The area is so small that there are only a few stars of the Milky Way in it . All other objects are galaxies, including the youngest after the Big Bang and therefore the most distant ones that had been observed up until then. With so many very young galaxies found, the HDF is a landmark in early universe exploration and has become the source of nearly 400 scientific articles.
Three years after the HDF observations were made, a region in the southern hemisphere was selected and evaluated in the same way. The picture was named Hubble Deep Field South . From the similarities between the two regions, it was concluded that the universe is homogeneous and isotropic on a larger scale and that the earth is not located in any particular area of the universe (the cosmological principle ). In 2004 another picture was published, called Hubble Ultra Deep Field , which looks deeper into the universe with an exposure time of 11.3 days in visible light. The Hubble Extreme Deep Field image published in 2012 with an exposure time of 23.1 days provides the deepest view of the universe to date .
concept
One of the goals of the Hubble Space Telescope (HST) was to make high-resolution images of distant galaxies that are impossible from the ground. The HST observes above the atmosphere free of atmospheric disturbances, which means that it can see much more sensitively in the ultraviolet range than telescopes on Earth (as soon as good adaptive optical corrections are also possible in the visible range, telescopes of the 10-meter class on Earth can use the Hubble Space Telescope to become competitive). Although the telescope mirror initially showed spherical aberration , the telescope had been able to record galaxies in previously unreachable distances since the beginning of 1990. Since light takes billions of years to come to Earth from distant galaxies, they can be seen in the state they were in the early universe. By expanding the observation possibilities to galaxies further and further away, one can better understand how these develop.
Since the mirror correction during the space shuttle mission STS-61 in 1993, excellent images have been made in order to examine galaxies that are increasingly distant and fainter. The Medium Deep Survey (MDS), which used the Wide Field and Planetary Camera 2 (WFPC2), took deep images of randomly selected regions while other instruments were used for planned observations. At the same time, studies of nearby galaxies were carried out, which were already known from observations with telescopes on Earth. All of these studies showed that there are important differences between the properties of galaxies today and those that existed billions of years ago.
Up to 10 percent of the observation time of the HST is shown as "Director's Discretionary (DD) Time". It is given to astronomers who want to study unexpected transient phenomena, such as a supernova. After Hubble received his optical correction, Robert Williams , the director of the Space Telescope Science Institute , decided in 1995 to use a significant portion of his DD time studying distant galaxies. An advisory committee recommended the use of the WFPC2 to select a typical area of the sky far from the galactic disk and to image it with several optical filters. A working group was formed to develop and implement the project.
Target selection
The area to be selected should meet several criteria. It should be far away from the galactic disk of the Milky Way , because the dust and other darkening matter located there prevent the weak light from distant galaxies from reaching the earth. Furthermore, the target area could not contain objects that emit visible light (such as nearby stars), infrared , ultraviolet and X- rays in order to later be able to more easily examine the objects of the HDF in other wavelength ranges, and the region should be in an area with only thin Infrared cirrus lying. The latter denotes a diffuse infrared emission, which probably originates from warm dust in cold hydrogen clouds ( HI area ).
These criteria significantly reduced the regions that could be targeted. It was also decided that the area should be in Hubble's continuous viewing zones (CVZs). These are areas of the sky that are not temporarily covered by the earth or the moon . The northern CVZ was chosen because telescopes from the northern hemisphere, such as the Keck Observatory and Very Large Array , could carry out follow -up observations .
Twenty areas were originally identified that met all these criteria and from which three optimal candidates were selected. They are all in the constellation of the Great Bear . A radio snapshot caused a field to fall out because it contained a strong radio source, and the final decision between the last two was made based on the availability of guide stars around that area. The Hubble Space Telescope needs two stars that its telescope guide sensors can use for orientation during observation. Because of the importance of observing the HDF, they wanted to have an additional pair of stars for emergencies. The decision was made on a region with the right ascension 12h 36 m 44s and the declination + 62 ° 12 ′ 58,000 ″.
observation
After the region to be observed had been selected, the observation procedure was worked out. An important decision was the choice of filters . The WFPC2 is equipped with 48 filters, including filters that allow only a few emission lines of interest to astrophysics to pass, and broadband filters that can be used to study the colors of stars and galaxies. The decision depended on the transmission for each filter, i.e. the amount of light that it transmits and the wavelength range that could be covered by the observations. Attempts were made to avoid overlapping of the wavelength ranges of the filters used as much as possible.
In the end, it was decided to use four broadband filters, centered around the wavelengths 300 nm (near ultraviolet ), 450 nm (blue light), 606 nm (red light) and 814 nm ( near infrared ). However, the quantum efficiency of Hubble detectors is very low at 300 nm. The noise in the observations at this wavelength range comes mainly from the noise of the CCD and less from the starry sky. Therefore, the observations in this wavelength range were carried out when strong background noise would have impaired the other bandpass filters.
The images were taken over a ten-day period while Hubble orbited the earth 150 times. The exposure times for the individual wavelength ranges are 42.7 hours for 300 nm, 33.5 hours for 450 nm, 30.3 hours for 606 nm and 34.3 hours for 814 nm, spread over 342 individual observations so that the individual images are not the exposed to severe damage from the cosmic rays . This would lead to light stripes on the CCD detectors.
Data processing
The 342 individual images make it possible to automatically detect and remove artefacts that only appear in individual images when added to a total image. These artifacts include bright pixels that were created during the recording by hits from particles of cosmic rays , as well as traces of space debris and artificial satellites , which can also be seen in individual original images .
Scattered light from the earth was seen in a quarter of the images. This was removed by aligning the image influenced by the light with an unaffected image and subtracting the unaffected image from the influenced image. The resulting image was smoothed and then could be subtracted from the affected image. This process removed almost all disturbing light from the affected images.
After these adjustments, the 342 individual images were aligned and superimposed on one another. A technique called 'drizzling' was used for this. For this purpose, the direction was changed minimally for each recording. Each pixel of the WFPC2 CCD chip corresponds to an angular range of 0.09 arc seconds edge length. However, by changing the direction by less than 0.09 arc seconds, a higher resolution can be achieved. Using the appropriate image processing algorithms, a resolution of 0.04 arc seconds could be achieved.
The original black-and-white images, taken with four different color filters, were combined in the image processing to form a final, somewhat arbitrary color image, which was then published. Three of the output images were recorded in the area of red, green and blue light and represent the color components of the colored image. Since the transmission curves of the filters, together with the spectral sensitivity curve of the camera, do not exactly match the spectral sensitivity curve of the human eye for red, green and blue light match, the colors shown are only an approximation. The choice of filters for the HDF (and a variety of Hubble's images) is basically meant to be of the greatest scientific benefit, and less so to show colors that the human eye can see.
Content of the finished image
The final image shows a multitude of distant, faintly glowing galaxies. Over 3,000 clearly recognizable galaxies could be made out in the picture. There are both irregular and spiral galaxies , plus a few galaxies only a few pixels in diameter. Overall, the HDF contains less than 10 stars in the foreground. The far larger remainder are distant galaxies.
There are around 50 objects on the HDF that look like blue dots. Many are associated with nearby galaxies that together form chains and arcs. They are regions of intense star formation . Others may be distant quasars . Astronomers initially ruled out the possibility that the point-like objects were white dwarfs because they are too blue to be consistent with white dwarf theories. However, later work has shown that white dwarfs become bluer with age, making it possible that the HDF could contain white dwarfs.
Results
The HDF provided a lot of material for cosmologists . By 2005 almost 400 articles based on the HDF had appeared in astronomical literature. One of the most fundamental discoveries was the multitude of galaxies with large redshifts .
The expansion of the universe increases the distance of distant galaxies from the earth. Likewise, the wavelength of light from galaxies increases the further away they are from Earth. While quasars with a large redshift were already known, for a long time only very few galaxies with a redshift greater than 1 were found. The HDF contains many distant galaxies with redshifts of 6, corresponding to a distance of 12 billion light years. (The redshift of even more distant objects in the HDF results in wavelengths so long that they are not visible in the images from Hubble. They can only be observed with telescopes on Earth.)
The galaxies in the HDF have a higher proportion of disturbed and irregular galaxies than in the local universe, because galaxy collisions and mergers occurred much more frequently in the young universe than today. From the state of the galaxies in the various stages of development, astronomers can estimate the changes in the rate of star formation over the lifetime of the universe. While estimates of the redshift of HDF galaxies are imprecise due to their weakness of light, astronomers assume that star formation peaked 8–10 billion years ago and has since declined by a factor of 10.
Another important result of the HDF was the small number of stars found in the foreground. For years, astronomers have been trying to find out what so-called dark matter is made of. It is mass that is not visible in direct observations, but contains 90% of the mass of the universe. One hypothesis is that some dark matter consists of massive astrophysical compact halo objects ( MACHOs ). They are weak but massive objects like red dwarfs or planets in the outer regions of the galaxy. However, the HDF has shown that there are no large numbers of red dwarfs in the outer layers of the galaxy.
Subsequent observations
The HDF is a milestone in observational cosmology, and the evaluation of the data is far from over. Since 1995 this area has been examined by many telescopes on Earth and several other space telescopes at wavelengths from radio to X-rays.
Very strongly redshifted objects were discovered in the HDF with some telescopes on Earth, in particular with the James Clerk Maxwell telescope . Because of the high redshift of these objects, they cannot be seen in the spectrum of visible light. On the other hand, one looks for them in the infrared or at sub-millimeter wavelengths.
Important space-based observations have been made with the Chandra X-ray Observatory and the Infrared Space Observatory (ISO) , among others . X-ray studies revealed six sources in the HDF associated with three elliptical galaxies, a spiral galaxy, an active galaxy core, and an extremely red object. The latter is believed to be a distant galaxy where the dust absorbs the blue portion of the emitted light.
ISO observations show infrared emission from 13 galaxies that can be observed on the optical image. They are most likely places of intense star formation that are surrounded by a large amount of dust, which is heated up and radiates in the infrared. Radio telescopes on Earth revealed seven radio sources in the HDF. All of these can be assigned to galaxies that are visible in the optical range.
In 1998, a recording similar to that of the HDF was made in the southern hemisphere: The Hubble Deep Field South . There were great similarities between the HDF-S and the original HDF. This reinforces the cosmological principle that the universe is homogeneous and isotropic at the greatest distances.
literature
- Daniel Fischer, Hilmar Duerbeck : The Hubble Universe: New Images and Insights . Licensed edition of Weltbildverlag, Augsburg, 2000, copyright Kosmos Verlagsanstalt Basel (formerly Birkhäuser), ISBN 3-8289-3407-2 .
- RE Williams et al .: The Hubble Deep Field: Observations, data reduction, and galaxy photometry , Astronomical Journal, 1996, Vol. 112, p. 1335.
- HC Ferguson: The Hubble Deep Fields , Astronomical Data Analysis Software and Systems IX, ASP Conference Proceedings, N. Manset, C. Veillet, D. Crabtree (Eds.). Astronomical Society of the Pacific , 2000, Vol. 216, p. 395, ISBN 1-58381-047-1 .
- BMS Hansen: Observational signatures of old white dwarfs , 19th Texas Symposium on Relativistic Astrophysics and Cosmology, 1998, J. Paul, T. Montmerle, E. Aubourg (Eds.)
- A. Hornschemeier et al .: X-Ray sources in the Hubble Deep Field detected by Chandra , Astrophysical Journal , 2000, Vol. 541, pp. 49-53.
- AJ Connolly et al .: The evolution of the global star formation history as measured from the Hubble Deep Field , Astrophysical Journal Letters, 1997, Vol. 486, p. 11.
