People have always tried to understand the surrounding world. By now, many figures and plans have been collected showing the way people have tried to systematize their knowledge, from geographic maps to pure speculative scheme of the Universe ([1], Fig. 1).
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Figure 1. The structure of the Universe according to concepts of the middle of the 14th century [1]. At the bottom, the earth, water, air, and everything created from them are shown. The horizontal layers from bottom to top show: the sphere of fire, the sphere of seven planets' (the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn), the sphere of fixed stars, and divine heavens totally screened from observation by opaque clouds. |
For a very long time, humankind included numerous "fixed" stars, planets, the Sun, and the Moon into its sphere of interests. Probably, the first systematic survey of all that is visible by the naked eye was performed by Hipparchus in the 2nd century BC. He drew up a catalog including about 850 stars. After almost two thousand years, at the end of the 18th century, French astronomer Charles Messier published the first catalog including not stars but stellar clusters and nebulae. As we know know, about a third of these nebulae are extragalactic bodies - external galaxies. However, Messier was not interested in the dim fuzzy spots he discovered. He was primarily interested in comets and compiled this catalog in order to distinguish comets from fixed nebulae.
William Herschel (1738-1822, "Coelorum perrupit claustra") was the first to formulate the problem of global sky surveys to study the structure and evolution of the world outside the solar system [2]. To survey stars in the sky, he applied the original method of "star gauging" (counting the number of stars in selected sky areas 1) and statistical data analysis. This allowed him to establish the general shape of our Galaxy and to estimate correctly its oblatness (~ 1/5). Another great merit of Herschel was the first systematic survey of faint nebulae and an attempt to establish regularities in their large-scale distribution. He discovered more than 2.5 thousand nebulae and star clusters, of which 80% are other galaxies. Herschel was first to attempt to estimate the size of dim nebulae and to measure their distance. His very approximate estimations gave rise to a picture of the Universe where the Milky Way is an ordinary stellar system of an infinite number of other galaxies. In the 19th century, his son John Herschel (1792-1871) continued searches for and studies of `milky nebulae.' John Herschel expanded his study to the southern hemisphere, doubled the number of known faint nebulae, and continued studying their distribution in the sky.
The photographic process discovered in the middle of the 19th century allowed astronomers to abandon visual observations and to proceed to photographic sky surveys. At the end of the 19th century, E. Barnard (1857-1923) started systematic photographic observations of the sky and performed the first photographic survey of the Milky Way. Studies of the structure and dynamics of our Galaxy greatly benefited from the plan of `selected areas' by J. Kapteyn (1851-1922). To execute this project, Kapteyn called upon astronomers from across the world to carry out photographic observations and to carefully study stars (including counting and measuring apparent magnitudes, proper motions, etc.) in 206 areas evenly distributed over the sky. This plan clearly demonstrated the fruitfulness of international collaboration in solving laborious observational problems and anticipated some features of big modern observational projects.
After E. Hubble (1889-1953) discovered the extragalactic nature of faint nebulae, it became clear that the Universe is much larger than had previously been thought. In order to study the large-scale structure of the Universe and to understand the nature, origin, and evolution of its principal `bricks' - galaxies, extensive sets of extragalactic studies had to be compiled and analyzed. This work was started by Hubble himself (see, e.g., [3]), as well as by other astronomers (Shapley, Ames, Humason, Lundmark, Bok, etc.).
At the end of the 1920s - beginning of the 1930s, Hubble performed a laborious survey of more than 40,000 galaxies in 1,283 areas located in both celestial hemispheres [4]. The main results of Hubble's work can be summarized as follows: the number of galaxies continues to grow up to the limiting magnitude of the survey; this growth is in quantitative agreement with a homogeneous distribution of galaxies in space (later, Hubble discovered that integral counts of fainter galaxies increase more slowly than was expected for their homogeneous distribution); there is a strong dependence of the observed number of galaxies per unit area of the sky on the galactic latitude (due to Galactic extinction); it is the logarithm of the number of galaxies per square degree (N) and not the number N itself that, after reducing all plates to standard conditions, follows a Gaussian distribution. This last discovery was one of the first indications that galaxies tend to `crowd' [5]. Hubble's results strongly differed from star counts: in counting stars, we reach the boundaries of our stellar system, while observations of galaxies do not show the presence of a boundary of the extragalactic world.
During the entire subsequent history of the 20th century, the main achievements in sky surveys and deep studies of selected areas were due to `technological' successes, such a the use of new big and specialized telescopes, increasingly sensitive photo emulsions, and then CCD matrices and computers, the elaboration of multi-object spectroscopy, etc. Each such a `technological' step has led to ever deeper penetration into the Universe.
The invention of a new telescopic system, the wide-field reflector, by Estonian optician Bernhard Schmidt (1879-1935) was one of the most important stages in the development of the observational technique. A correcting plate mounted in front of a reflector's objective allows compensating for most aberrations of the main mirror. This opens the possibility to construct candlepower telescopes with a wide (~ 10°) undistorted field of view. The best known Schmidt telescope (the correction plate diameter is 122 cm, the diameter of the objective is 183 cm) is installed at the Mount Palomar Observatory in California. Its field of view is 6.6°. In 1950-1958, this telescope was used to carry out the famous Palomar Sky Survey (see below). The biggest Schmidt telescope (the correction plate diameter is 137 cm, the diameter of the objective is 200 cm) is installed near Jena in Germany. The biggest telescope of this kind on the territory of the former Soviet Union operates at Byurakan Observatory (Armenia) (the correction plate diameter is 102 cm, the diameter of the objective is 132 cm, the field of view is 4°). The Byurakan Schmidt telescope was used to carry out the well-known survey of galaxies with ultraviolet excess (the so-called Markarian galaxies).
In the last 10-15 years, several international projects have been carried out that distinctively changed the aspect of modern astronomy. Observational data on the structure of our and other galaxies were increased by dozens and hundreds of times. For the first time, it became possible to study the evolution of galaxies and their large-scale structure starting almost from the moment of their formation until now. There are statements that a `golden age' of studies of galaxy formation and evolution has begun. The general feeling among astronomers and physicists (especially theoreticians) is partially characterized by the title of a colloquium that took place at Caltech several years ago: "Galaxy formation: End of the Road!" [6].
Time will tell how justified such optimism is, but undoubtedly extragalactic astronomy and observational cosmology are in a period now that can possibly be compared only with the 1920s, when the first galaxies were identified and the expansion of the Universe was discovered. In this paper, I attempt to briefly review selected observational projects of the last years. In view of the immense observational data, I restrict myself to optical and extragalactic surveys.
Throughout the review, I use a cosmological model with
m = 0.3,
=
0.7, and H0 = 70 km/s/Mpc.
1 This was one of a few cases where an astronomer, in full agreement with the commonplace opinion about his kind of work, actually "counted stars" by looking through a telescope. Back.