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Home , Hubble Deep Field, Hubble Deep Field South

The most distant of the

Domingos Soares

In April 1990, the American space agency (NASA) and the European space agency (ESA) placed into ’s orbit a reflecting telescope with a 2.4 m diameter mirror, that has revolutionized since then. The telescope was, much appropriately, called “” (HST, hereinafter). It is a homage to the father of , the American astronomer (1889-1953). Free of the effects of opti- cal degradation in Earth’s , the HST reaches, in its observations, a resolution 10 times as large as telescopes on the ground. The accomplishments of HST are innumerable and frequently spectacular. Many examples of them can be seen at http://hubblesite.org. The observations we are interested in — of the most distant galaxies of the universe — were carried out from September 2003 to January 2004 and were called “Hubble Ultra Deep Field”, HUDF. The deepness here refers to the distance of the objects — in their majority galaxies in various stages of evolution. The region of the sky that was observed, i.e., the field, is located in the Fornax , very close to our familiar Orion constellation (the Hunter), where the famous three also known as the “Three Kings” or “Three Sisters”, which form the Orion’s Belt, are located. The field is very small and its area on the sky corresponds, for the effect of comparison, to about one hundredth of the area of the Full . This area is absolutely dark, when observed through telescopes on the ground! Nothing can be seen on it, not even any of our own , the . But what HST “saw” was simply daunting! Nothing more, nothing less than approximately 10,000 galaxies, with the most exquisites shapes.

1 “The Hubble Ultra Deep Field” (HUDF), an image obtained by the Hubble Space Telescope in 2003 and 2004. The area of the sky that appears in the image has a length of 10% of the Full Moon diameter, and is located in the Furnace (For- nax) constellation, close to well-known Orion constellation. It is estimated that the HUDF has more than 10,000 galaxies. The larger galaxies are the closest. The ma- jority of the galaxies appear as small shapeless spots (Image: S. Beckwith/STScI- NASA/ESA).

2 How HST achieved that feat? Simply receiving the very faint from the selected region during more than 11 days! Two HST’s instruments were used to record simultaneous images. The ACS (“Advanced Camera for Sur- veys”) obtained images in visible light — blue, green and red filters —, and the NICMOS (“Near Camera and Multi-object Spectrometer”) ob- tained images in infrared light. The HST covered 400 orbits during the 11.3 days, which amount to a little less than 1 million seconds of exposure to the light coming from the chosen field. Two exposures per orbit were made, one for each one of the cameras. The most distant galaxies in the HUDF are at an estimated distance of 13 billion light-years! The light from those galaxies began their voyage in our direction much before the existence of the Earth and of the Sun itself, whose age is calculated at approximately 5 billion years. The galaxies appearing on the HUDF are in different stages of evolution, according to its larger or smaller distance from us. It is worthwhile remembering that, in the immense cosmos, a great distance in space represents also a great dive into the past. This is so because the light has a finite speed and, consequently, a great distance will be traveled in a great period of time as well. But HUDF did not represent the first try of HST in search of the most distant galaxies of the cosmos. Before it two similar observations were made. They were called “ North” (HDF-N) and “” (HDF-S). HDF-N was the pioneer of HST’s deep observations. They were under- taken in December 1995. The selected field sits on the circumpolar northern constellation of (the Great Bear). The field exhibits about 1,500 galaxies, also in various stages of evolution. HDF-N did not reach as deeply as HUDF — the exposure time was approximately 10 days —, but the max- imum distance reached by HDF-N is not much different, about 12 billion light-years.

3 “The Hubble Deep Field North” (HDF-N), an image obtained by the Hubble Space Telescope in 1995. In this image, more than 1,500 galaxies appear on an area of the sky whose length is equivalent to 3% of the Full Moon diameter, and is located in the northern Ursa Major constellation, close to the north celestial pole. Like in the HUDF, the majority of the 1,500 galaxies appearing here show up as small deformed spots against the sky background (Image: R. Williams/STScI-NASA).

HDF-S was observed in October 1998, and it is the equivalent to HDF- N for the sky region close to the south celestial pole. The field sits in the constellation (the Toucan). Like in HDF-N, the most distant galaxies are seen at the distance of approximately 12 billion light-years.

4 “The Hubble Deep Field South” (HDF-S), an image obtained by the Hubble Space Telescope in 1998. The area of the sky is the same as for HDF-N, but is located close to the south celestial pole, in the Toucan (Tucana) constellation (Image: R. Williams/STScI-NASA).

The deep fields of the HST are being investigated in order to obtain a better knowledge of galaxy evolution, i.e., of the way galaxies born, live and eventually disintegrate in the great universe we inhabit. Some immediate

5 conclusions can be drawn. Regardless of the direction we observe, the cos- mos presents itself essentially in the same way, dotted with immense stellar systems, the galaxies. Another impressive conclusion is that if we had ob- served the whole sky — not only the minuscule celestial regions of the deep fields — we would find more than 125 billion galaxies! In numbers: the whole sky represents an area equal to 12.7 million times the field of the HUDF. HST’s astronomers estimate that to observe all of this area, the HST would take 1 million years of continuous observation!

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