The universe is expanding and as it does so the distance between any two points increases. This has the effect of redshifting electromagnetic radiation emitted from those points: the wavelength of the radiation is stretched out as the Universe expands, making it appear redder. The amount of this shift is proportional to the distance between the emitter and observer, and thus we can use this effect to measure distances to objects in the Universe.
High-redshift refers to objects that are very far away and thus have a very large redshift. These are some of the most distant objects in the Universe, and studying them can give us insight into its early history.
The study of high-redshift objects began in earnest in the 1960s with quasars, which are extremely luminous galaxies powered by supermassive black holes. Quasars can be detected at enormous distances, and their spectra (the intensity of light versus wavelength) contain many emission lines from different atoms and ions. By measuring these emission lines we can determine how much each element has been shifted by cosmological expansion, and hence calculate how far away the quasar is.
In recent years our view of high-redshift objects has been transformed by new observations made possible by advances in technology. In particular, sensitive detectors on board satellites such as NASA’s Hubble Space Telescope (HST) have allowed us to see further into space than ever before. At present, HST can detect galaxies at redshifts up to z~10 (i.e., 10 times farther away than what was previously possible), although ground-based telescopes have pushed this limit even higher with detections at z~12.5 using gravitational lensing techniques (more on this later).
These new observations have revealed a wealth of information about galaxy formation and evolution in the early Universe. For example, we now know that there were many more star-forming galaxies at high redshift than there are today; indeed, it seems that almost all galaxies were undergoing vigorous star formation during this epoch. This period of intense star formation lasted for several hundred million years until it abruptly stopped around z~2-3; since then galaxy growth has largely been governed by “passive” processes such as accretion onto existing stellar populations (rather than new stars being formed). We also know that galaxies were much smaller at high redshift; they have since undergone massive growth through mergers with other galaxies to become the giant ellipticals and spirals that we see today.”