Free Astronomy Magazine March-April 2016

26 GALACTIC EVOLUTION age’, which, if distant from merging epi- sodes, tells us very little on the past history of these systems. Some years ago, however, were laid the basis for the solution of the problem. In fact, astronomers began to identify in the nuclei of many galaxies some small structu- res in the form of disk, composed by a large number of stars in rapid rotation around the galactic centres. These nuclear disks range in size between a few dozen and a few hundred parsecs (1 parsec = 3.26 light- years), have a brightness greater than 10 million Suns and masses also of the order of tens of millions of solar masses. The nu- clear disks appear to be nesting both, wi- thout distinction, at the centre of spiral galaxies and in the core of the elliptical ones. It is estimated that about 20% of the latter galaxies (of which the major ones are considered the point of arrival of the re- peated mergers between galaxies) host a nuclear disk in their centre. But what is the origin of these relatively small and compact structures? The various mechanisms propo- sed by researchers over the years can be summarised in two different scenarios. In the first, a disruptive event, such as the close passage between two galaxies, would redirect the gas of one of the two towards the core of the most massive one, where with its spiralling movement would form a relatively flat disk, inside which dense local clumpings would trigger the production of new stars. The second scenario sees instead the absorption of different globular clus- ters, whose already-formed stars would pool in the centre of the hosting galaxy, thus ending up forming the disk. As can be imagined, the discriminating fac- tor between the two scenarios is the age of the stars that form the two disks: in the first case it would be significantly lower than that of the galaxy as a whole, while in the second it would be almost identical. Since some numerical simulations have re- cently shown that nuclear disks are fragile structures unable to survive mergers be- tween galaxies (their stars end up dispersed), it appears unlikely that these same disks can be made of stars as old as the galaxy hosting them. If that were so, it should be concluded that galactic mergers ceased sometime in the distant past, which is not T he Very Large Telescope fa- cility of ESO, which has provided to the team of Marc Sarzi the neces- sary material for determining the minimum age of the nuclear disk of NGC 4458. [ESO]