It depends on how you search for them. For decades, the farthest known constituents have been the quasars and radio galaxies because they were either extremely 'blue' objects or very powerful radio sources. Recently, some individual very distant clusters of galaxies have been identified as well. Once infrared technology began to develop in the early 1970's, 'ultra luminous infrared galaxies', and the so-called starburst galaxies were discovered which can be seen to great distances as infrared sources.
For several decades, astronomers looking at the spectra of quasars have detected 'lines' from some form of cloud-like material located between us and the distant quasar. In the most distant quasars, there can be dozens of these 'Lyman-alpha systems' present in the quasar's spectra, each 'system' of lines of carbon, iron and other elements, are found at the same redshift indicating the same cloud is being detected, but there can be many of these systems superimposed in the spectrum at redshifts from nearly zero ( meaning they are nearby) up to the redshift of the quasar itself. Astronomers interpret these 'clouds' as the interstellar gas from individual galaxies too faint to directly see in most cases, which are located between the quasar and us. In a few cases, these galaxies have been directly observed which clinches their identification.
Astronomers have also detected what are called Faint Blue Galaxies, which are thought to be low-mass dwarf galaxies undergoing a 'burst' of star formation activity. These are very distant objects suggesting that active star formation in dwarf galaxies like the Large Magellanic Cloud, was more vigorous long ago.
The most distant objects know are still the quasars. There are only about 5000 - 6000 of them that have been cataloged after very intensive searches using many different techniques. Astronomers estimate that the universe may only have created fewer than 100,000 of these very luminous objects in a total population of galaxies now thought to exceed 70 billion! Quasars are rare, and only a very small minority of all galaxies have gone through this phase; probably limited to the most massive galaxies.
The recent deep image provided by astronomers using the Hubble Space Telescope ( see one of my other questions for a link to it) is spectacular in that so many different types of galaxies seem to be jumbled together along the line of sight. The most distant objects we can see appear to be pieces of galaxies and small irregular clouds containing millions of stars. There are few if any pretty spiral galaxies, and we think we are seeing the smaller building blocks of conventional galaxies, merging together and colliding to form the larger systems we see around us today.
Copyright (C) 1997 Dr. Sten Odenwald
Return to Ask the Astronomer.