Hubble Deep Field
As Hubble Deep Field (HDF ) is called the image of a small portion of the sky which was photographed extensively in December 1995 with the Hubble Space Telescope.
Background
For the Hubble Deep Field, an area in the Great Bear was selected, which is relatively free from disturbing influences perspective and surrounding bright stars. The area has an edge length of 144 arcsec, which is approximately the angle at which a tennis ball at 100 m distance appears. The image is an overlay of 342 individual images that have been with the Wide Field and Planetary Camera 2 ( WFPC2 ) of the Hubble Space Telescope in the course of ten days, between 18 and 28 December 1995, was added.
The area is so small that it houses only a few stars of the Milky Way. All other objects are galaxies, including the youngest and most distant, which had been observed until then. Since so many very young galaxies were found, the HDF is a milestone in the study of the early universe and became the source of nearly 400 scientific articles.
Three years after the HDF observations were made, a region was selected in the southern hemisphere and evaluated in the same way. The picture was called the Hubble Deep Field South. From the similarities between the two regions, it was concluded that the universe on a large scale is homogeneous and isotropic and that the Earth is not in any particular area of the universe ( the cosmological principle ). 2004 an additional image was published, called the Hubble Ultra Deep Field, which looks into with an exposure time of 11.3 days in the visible light again deeper into the universe. History's deepest view of the universe granted the 2012 issued Picture Hubble Extreme Deep Field with an exposure time of 23.1 days.
Concept
One of the main objectives of the Hubble Space Telescope (HST ) was to make high-resolution images of distant galaxies, which are not possible from the ground. About the atmosphere, the HST observed free of atmospheric disturbances, which can see in the ultraviolet region is much more sensitive than telescopes on Earth (once good adaptive optical corrections in the visible range are possible, can telescopes in the 10 -meter class on the earth with the be the Hubble space Telescope competitive ). Although the telescope mirror showed spherical aberration at the beginning, the telescope was able to include galaxies in previously unattainable distance since early 1990. Since light takes billions of years to come from distant galaxies to us, we see this in the condition in which they were billions of years ago. By extending the observation capabilities to increasingly distant galaxies can therefore better understand how they develop.
Since the mirror correction during Space Shuttle mission STS -61 in 1993 excellent images can be made , so as to examine more distant and fainter galaxies. The Medium Deep Survey ( MDS), which used the Wide Field and Planetary Camera 2 ( WFPC2 ), took on deep images of randomly selected regions, while other instruments were used for planned observations. At the same time, studies of nearby galaxies have been carried out, the people you've seen from the observation with telescopes on Earth. All these studies showed that there are important differences between the characteristics of today's galaxies and the galaxies that existed billions of years ago.
Up to 10 percent of the observation time of the HST is reported as "Director's Discretionary (DD) Time." You will be awarded to astronomers who want to study unexpected transient phenomena such as a supernova. After Hubble had received its optical correction, Robert Williams, director of the Space Telescope Science Institute, in 1995, decided to use a significant part of DD time to study distant galaxies. An advisory committee recommended the use of the WFPC2 to far from the galactic disk, select a typical area of the sky and map with multiple optical filters. A working group was formed to develop the project and to realize.
Target selection
The area should be selected to meet several criteria. It should be far away from the galactic plane of the Milky Way, for the present there prevent dust and other obscuring matter that the weak light can reach the Earth from distant galaxies. Furthermore, the target area could contain objects that transmit visible light (such as nearby stars ), infrared, ultraviolet and X-rays emit, later to be able to more easily examine the objects of the HDF in other wavelength regions, and the region had in an area with thin Infrarotzirrus lie. The latter refers to a diffuse infrared emission, presumably from warm dust in cold hydrogen clouds (HI region ) dates.
These criteria reduced the regions that came as a target area in question considerably. It was further decided that the area in Hubble's constant observation zones (continuous viewing zones, CVZs ) should lie. The sky are regions which are temporarily not covered by the earth or the moon. They opted for the northern CVZ, because here were able to carry telescopes in the northern hemisphere, such as the Keck Observatory and the Very Large Array, follow-up observations.
There were originally identified twenty areas that met all these criteria and from which three optimal candidates were selected. They are all located in the constellation of the Great Bear. Through a radio snapshot into a box fell 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 this area. The Hubble Space Telescope requires two stars, to which his Teleskopleitsensoren can orient during the observation. Because of the importance of the observation of the HDF would you still have an extra pair of stars for emergencies. The decision was made on a region of right ascension 12h 36m 44s and declination 62 ° 12 ' 58.000 "
Observation
After the observed region was selected, the observation sequence has been developed. An important decision was the choice of filter. The WFPC2 is equipped with 48 filters, including filters, which pass only a few interesting for astrophysics emission lines, and broadband filters, with which the colors of the stars and galaxies can be studied. The decision depended on the passage from each filter, ie the amount of light it lets through, and the wavelength range that could be covered by the total observations. They tried to avoid overlap of the wavelength ranges of the filters used as much as possible.
In the end they decided on four broad-band filters, centered at 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's detectors at 300 nm is very low. The noise in the observations in this wavelength region comes mainly from the noise of the CCD and less of the starry sky. Therefore, the observations were carried out in this wavelength range, if high level of background noise, the other band passes could be affected.
The images were taken over a period of ten days, during Hubble orbited the Earth 150 times. The exposure times for each wavelength ranges are 42.7 hours for 300 nm, 450 nm for 33.5 hours, 30.3 hours for 606 nm and 34.3 hours for 814 nm, spread over 342 individual observations, so the pictures not the severe damage by the cosmic rays are exposed. This would lead to bright stripes on the CCD detectors.
Data processing
The 342 individual images make it possible to detect occurring in the addition to a complete picture only in individual images artifacts automatically and remove. These artifacts include bright pixels that have arisen during recording with goals from cosmic-ray particles, and traces of space debris and artificial satellites, which are also shown on each original images.
Scattered light from the earth could be seen in a quarter of the images. This was removed by the hosted influenced by the light image on a non -influenced image and subtracted from the unaffected influenced image. The resulting image was smoothed and could then be subtracted from the affected image. By this operation, almost all the disturbing light is removed from the affected images.
After these adjustments, the 342 individual images were aligned with each other and superimposed. For this, a technique was called ' drizzling ' used. For this purpose, the direction for each shot was minimally changed in the recordings. Each pixel of the CCD chip WFPC2 corresponds to an angular range of 0.09 arc second edge length. However, since changing the direction by less than 0.09 arc seconds, you can achieve a higher resolution. By means of appropriate image processing algorithms could be so achieve a resolution of 0.04 arc seconds in the final result.
The images recorded with four different color filters black and white original photographs were combined in image processing to a final, somewhat arbitrary color- color image, which was then published. Three of the output images were taken in the red, green and blue light, and adjust the color components of the color image dar. Since the transmission curves of the filters, together with the spectral sensitivity curve of the camera, not exactly match the spectral sensitivity curve of the human eye to red, green match, and blue light, the colors shown are only an approximation. The choice of filters for the HDF ( and a variety of Hubble's images ) is basically intended to bring the greatest scientific benefits, and less to show colors perceived by the human eye.
Content of the final image
The final image shows a variety of distant, faint galaxies. Over 3,000 clearly visible galaxies could be identified on the image. There are both irregular and spiral galaxies exists also a few galaxies with only a few pixels in diameter. Overall, the HDF contains less than 10 stars in the foreground. The far greater rest are distant galaxies.
There are around 50 on the HDF objects available that look like blue dots. Many are associated with nearby galaxies in connection which together form chains and arcs. They are regions of intense star formation. Others may be distant quasars. Astronomers initially included the possibility that they are white dwarfs in the point-like objects because they are too blue to be consistent with the theories of white dwarfs in unison. However, subsequent work has shown that white dwarfs with age are blue, which makes it possible that the HDF could still contain white dwarfs.
Results
The HDF provided much material for cosmologists. By 2005, nearly 400 products on the basis of the HDF in the astronomical literature published. One of the most fundamental discoveries was the large number of galaxies with high redshift.
Due to the expansion of the universe, the distance of distant galaxies from earth increases. Also takes the wavelength of light from galaxies, the further they are away from the Earth, too. While quasars at high redshift were already known, it took a long time, very few galaxies with redshifts greater than 1 have been found. The HDF contains many distant galaxies with redshifts of 6, corresponding to the distance of 12 billion light years. ( The redshift of more distant objects in the HDF leads to such long wavelengths that they are not visible on the images from Hubble. , You can only be observed with telescopes on the ground. )
The galaxies in the HDF have a higher proportion of disturbed and irregular galaxies in the local universe, as galaxies collisions and mergers occurred in the early universe much more common than today. From the state that have the galaxies in different stages of development, astronomers can estimate the changes in the star formation rate over the lifetime of the universe. While estimates of the redshift of HDF galaxies are inaccurate because of their faintness, astronomers assume that the star formation had its maximum before 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 that have been found in the foreground. For years, astronomers try to find out, from which the so-called dark matter exists. It is mass that is not visible in direct observations, but contains 90 % of the mass of the universe. One hypothesis is that a part of the dark matter in massive astrophysical compact halo objects ( MACHOs ) exists. There are faint but massive objects such as red dwarfs or planets in the outer regions of the galaxy. Through the HDF has been shown, however, that there is no greater number 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 still not finished by far. Since 1995 this area has been studied by many telescopes on the ground and some other space telescopes at wavelengths from radio to X-rays.
Very highly redshifted objects were discovered in the HDF with some telescopes on the ground, especially with the James Clerk Maxwell Telescope. Due to the high redshift of these objects can not be seen in the spectrum of visible light. They are looking the other hand, in the infrared or sub-millimeter wavelengths.
Important space-based observations have been made, among others, with the Chandra X-ray Observatory and the Infrared Space Observatory ( ISO). Investigations in the field of X-rays revealed six sources in the HDF, which are associated with three elliptical galaxies, a spiral galaxy, an active galactic nucleus and an extremely red object. In the latter case it is assumed that it is a distant galaxy, wherein the dust absorbs the blue part of the emitted light.
ISO observations show infrared emission from 13 galaxies that can be observed on the optical image. You are most likely places of intense star formation, which are surrounded by a large amount of dust, which is heated thereby and radiates in the infrared. Radio telescopes on Earth have revealed seven radio sources in the HDF. All of these can be assigned to galaxies visible in the optical range.
1998, a similar photograph as the HDF has been made in the southern hemisphere: The Hubble Deep Field South. There were strong similarities between the HDF - S and the original HDF. This confirms the cosmological principle that the universe is in the greatest distances is homogeneous and isotropic.