Free Astronomy Magazine March-April 2016

24 In the more central re- gions of many galaxies, there are small and com- pact disk-shaped struc- tures that can be used as timepieces for measuring the time elapsed since their last merge with other galaxies. For the first time, two teams of researchers were able to read the ‘time’ marked by these structures. Unveiled the two nuclear T he evolution of galaxies is largely go- verned by merging events with other galaxies. This mechanism seems to be at the base of the formation of the large galaxies that we observe in various eras of the universe. Astronomers are in broad agreement that as from 500-600 million years after the Big Bang the first small gal- axies that formed in that remote past be- gan to collide with each other, generating bigger and bigger structures. With the prog- ressing expansion of the universe, this pro- cess became increasingly less effective, even though it remained moderately active in the most populous galaxy clusters, where the probability of close encounters is greater. In this general framework, which appears to accurately reflect the observational re- ality, there are still, however, some unclear points, on which researchers are trying to shed light. One of them is, for example, the by Michele Ferrara GALACTIC EVOLUTION MARCH-APRIL 2016 I n the back- ground, centre, the elliptical gal- axy NGC 4458, surrounded by other field gal- axies. A team of researchers has succeeded in cal- culating the mini- mum age of the nuclear disk nest- ing in NGC 4458, thus determining the possible date of the last merger undergone by the galaxy. [NASA, ESA, and E. Peng (Peking Univer- sity, Beijing)]