Read out Hubble's deepest view to the south. With a ten-day observation campaign, the Hubble Space Telescope has drilled a hole that is about twelve billion light-years deep into the unknown, exposing many hundreds of ancient galaxies. HDF-S will become the best-explored sky region of the next few years, "predicts Robert Williams, director of the Space Telescope Science Institute (STScI) in Baltimore, Maryland. The man knows what he's talking about. Once before, he has invested his personal time budget, which he is free to use as director of the institute, for a deep look into space. That was in December 1995. At that time, the Hubble Space Telescope had been photographing a nondescript region of the Big Dipper again and again for ten days.

The result, called Hubble Deep Field (HDF), provided a huge boost in the exploration of the universe. Never before had a more distant heavenly region been explored. And since a glance out into the depths of the room is at the same time a look back in time, astronomers were able to lay a trail in the early phase of the cosmos.

Now HDF has gotten a brother in the southern starry sky: Hubble Deep Field-South, short HDF-S. Williams and his 50 employees from the STScI and NASA's Goddard Space Flight Center in Greenbelt, Maryland, had selected a tiny nondescript region in the Tukan constellation, in the vicinity of the Sky South. After several test shots in October 1997, the Hubble Space Telescope was on September 28, 1998 aligned to this place. Until October 10 it looked almost uninterrupted there and took admission for admission - 150 earth orbits from the central area, and 27 more from the environment. There are 995 images in total of ultraviolet, optical and near infrared radiation - each lasting on average 30 to 45 minutes.

In the computer, all these images were then combined into a single image. The result, published at the end of November 1998, shows a gallery of around 2500 galaxies. The faintest have an apparent brightness of the size class 30 - that is a six-billionth of the brightness that we can barely see with the naked eye. As faint as light the glow of a cigarette would appear to us in lunar range. display

"Soon after HDF, I wanted a second deep field shot, " Williams recalls. "After several months of realizing that HDF holds valuable information about the epoch of galaxy formation, I knew we needed to investigate a second region in another part of the sky."

The reason is simple: From a sample alone can hardly draw far-reaching conclusions. Because coincidentally, the space telescope could catch an atypical line of sight, which nobody would notice, because the borehole is so narrow. Relying on a single sample would be as risky as, for example, characterizing the European population by questioning a small group of people in a Sicilian village.

Since the release of HDF data in January 1996, numerous other high-powered telescopes on Earth and in space have been exploring the tiny sky region: from infrared to visible light to X-ray. With the help of the 10-meter Keck telescope in Hawaii, the distances of 125 of the approximately 2500 HDF-S galaxies could be determined. They amount to about twelve billion light-years. The light of these galaxies comes from a time when the universe was at most a billion or two billion years old. Many of them have strange shapes that suggest cosmic billiards. These were earlier than the galaxies were even closer together, more common than today and have apparently strongly influenced the evolution of galaxies.

Meanwhile, many astronomers believe that galaxies initially consisted of smaller components - gas clouds and star clusters - and only grew over time by incorporating neighboring dwarf galaxies into what they are today. The speed of star formation in the early Universe was also better assessed by scientists using HDF. One to three billion years after the Big Bang, the birth of stars was over ten times more common than it is today. However, most of these young stars hide behind dust clouds in the optical and ultraviolet regions of the spectrum. Only their infrared radiation penetrates to us.

HDF-S provides a very similar picture. In addition to disk-shaped spiral galaxies and reddish elliptical galaxies, there are also many oddly shaped objects to see - live shots of galactic collisions. The first analysis of HDF-S thus confirmed the image obtained by HDF and thus the basic assumption of the astronomers that the universe is the same in all directions. Further insights are expected in the coming months and years when the powerful Southern Hemisphere telescopes come into operation - in Australia and especially in Chile, where the European Southern Observatory (ESO) is currently building its new Very Large Telescope.

HDF-S has even more to offer than HDF. So is in the HDF-S a quasar. With a redshift of z = 2.2, this ultra-luminous center of a young galaxy is approximately 9.5 billion light-years from Earth. Astronomers at the Anglo-Australian Observatory in Siding Spring, Australia had discovered the quasar in 1996. He was also one of the reasons to choose this sky cutout for HDF-S. Its light has gone through about three quarters of the observable universe in this direction.

Invisible clouds of hydrogen swallowed some of the light. These absorption lines in the quasar spectrum are like astronomers - cosmic commissioners looking for clues - like "fingerprints". You can draw conclusions about the distribution of intergalactic matter.

HDF-S has a second advantage over the older HDF. At that time, only the Wide Field Camera of the Space Telescope was available. During the last service mission of Space Shuttle astronauts in February 1997, however, two more instruments were installed in the space telescope, which were now also used: STIS (Space Telescope Imaging Spectrograph) and NICMOS (Near Infrared Camera and Multi-Object Spectrometer).

STIS has absorbed the ultraviolet radiation from the HDF-S region. It was sent by, among other things, the hottest stars. In addition, STIS collected spectral data that provides information about the composition of the distant galaxies. NICMOS registered infrared radiation. Part of it comes from objects that are wrapped in dust and therefore not visible in the optical area. Overall, HDF-S contains information from eight wavelength ranges. This wealth of data already provides an initial estimate of galaxy distance, temperature, and velocity.

Recently, NICMOS had recorded an HDF cut in the infrared for 36 hours. It holds perhaps the farthest ancient galaxies in existence, as astronomers from the University of Arizona at Tucson announced last October. "NICMOS has opened the curtain that has so far blocked our view of the most remote objects and reveals new actors on the cosmic stage, " says Rodger I Thompson, head of the astronomy team, the NICMOS Recording has been evaluated for months. "Now we have to find out who, what and where they are. There are still new limits. "

Thompson managed to associate faint red galaxies with compact blue radiation known from HDF imaging. "This means that some objects that appear as separate galaxies are actually hot star-forming regions in much larger, older galaxies, " he explains.

In addition, the astronomers found numerous objects in the NICMOS image that have no optical counterpart in the HDF. They are probably even farther away than the original HDF galaxies. Their infrared "color" suggests that some may be among the remotest star islands we know. Their radiation comes from a time when the universe was only a few hundred million years old. NICMOS is particularly well-suited for the search for such distant objects, since the expansion of the universe has pulled the light of the first galaxies so far apart - moved into the red, longer-wave range - that it is from Earth viewed only in the invisible infrared shine.

"Perhaps we are seeing the first stage of galaxy formation here, " says Alan Dressler of Carnegie Observatories in Pasadena, California. "But the objects are so faint that their nature can only be explored by even more powerful telescopes in the future - such as the Next Generation Space Telescope, which will be launched in 2007."

=== R diger Vaas

science.de

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