Old galaxies shed new light on young universe
Jul 7, 2004
Massive galaxies may have formed much earlier in the history of the universe than previously thought according to new observations. Two teams of astrophysicists have detected massive galaxies with redshifts of more than 1.5, which means that they must have formed before the universe was a quarter of its present age. The results are in direct conflict with the currently favoured "hierarchical" model of galaxy formation.
According to the hierarchical model large galaxies are formed as a result of smaller galaxies merging. The model predicts that massive galaxies should appear rather late in the history of the universe, when it was about half its present age. Astronomers can calculate how old a galaxy is by measuring its redshift: galaxies with large redshifts are older and further away than those with small redshifts.
However, galaxies with high redshifts are difficult to detect because they are very faint and because the radiation they emit is shifted to longer wavelengths that are easily absorbed by the Earth's atmosphere. This means that although astronomers can now routinely detect bright star-forming galaxies at redshifts up to 6.6, the most distant massive galaxy observed to date is still an object with a redshift of 1.552 that was discovered 10 years ago.
To overcome this problem, Andrea Cimatti of the Istituto Nazionale di Astrofisica (INAF) in Italy and co-workers searched for massive galaxies (1011 to 1012 solar masses) at redshifts greater than 1.5 by comparing their spectra with those of massive galaxies found in the present-day universe (Nature 430 184). Working with the Very Large Telescope at the European Southern Observatory in Chile, Cimatti and co-workers performed a survey of an area of the sky known as the Hubble Ultra Deep Field and identified four massive spheroid-shaped galaxies at redshifts between 1.6 and 1.9.
Meanwhile, Karl Glazebrook from John Hopkins University and colleagues, working with the Gemini telescope in Hawaii, studied a new sample of 150 galaxies in the so-called Gemini Deep Deep Survey (Nature 430 181). These galaxies have redshifts of between 0.8 and 2 and date back to just three billion years after the big bang. Glazebrook and co-workers found that a significant fraction of the galaxies with redshifts greater than 1.5 were massive.
About the author
Belle Dumé is Science Writer at PhysicsWeb