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RESEARCH PRODUCT

Observational Cosmology at High Redshift

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Summary. I offer a brief review of the evolution and present status of our observational knowledge of the high-redshift Universe. In particular, I focus on the different methods that have been devised to select distant objects, and the observational evidence in hand to support (or else) the standard evolutionary scenario. 1 How High is High? The study of objects at cosmological distances from us started in the 1960s with the discovery and identification of quasi-stellar radiosources. The explanation of the features observed in the optical spectra of these objects as highly redshifted hydrogen lines opened the door to the very distant Universe. For the next 30 years after the discovery of quasars, the early Universe was the realm of monsters that could only be observed at such large distances because of their peculiar properties. These included powerful radioemission (usually associated to jets), or the presence of a strong x-ray flux. Of course, these selection effects led to strong biases in all the census of high-redshift objects that were produced, a lack that was well known by astronomers. It was suggested by different groups in the eighties that different techniques, based on colour selection, could in fact overcome those selection effects [13, 16]. The idea had been around since, Baum, Peebles and Patridge, and took momentum specially with the work by Steidel and collaborators, who designed a colour-selection technique able to sieve objects at redshift z ≈ 3 almost routinely, via deep imaging through three filters [2, 20, 21]. The next impulse came with the Hubble Space Telescope observations of the Hubble Deep Field in 1995. These observations, designed to be extremely deep and rich in colour information, opened a new era of discovery for high-redshift, normal galaxies at all redshifts out to z ≈ 6 (see, e.g. [14, 7]). Next step, the same selection techniques were applied once more to “monsters” like very luminous quasars, which, being extremely scarce in terms of number density in the sky, can only be detected with very large surveys that