Free Astronomy Magazine September-October 2014

GALAXIES markable event. It is in fact estimated that similar chains of superclusters can survive approximately ten million years, which is only a fraction of the time that host gal- axies will take to merge with each other. The rarity of those structures is even more surprising when considering that they were formed (and perhaps they are still being formed) as a result of the encounter between two large el- liptical galaxies (typical of the cen- tral regions of clusters of galaxies); events less frequently observed than those involving spiral or mixed-type galaxies. It is not sur- prising, therefore, that this is the first occurrence in which it has been possible to simultaneously observe two elliptical interacting galaxies associated with young stellar superclusters. So far, the pres- ence of these latter was believed to be exclusive to the arms of spi- ral galaxies and to the tidal bridges of material created in strongly inter- acting galactic systems. Having now discovered them in a new scenario will allow astrono- mers to better understand the gas other than stellar superclusters, composed of stars on average more massive than the Sun, and thus with a short lifespan. As pointed out by the same Tremblay, this suggests that we are in the presence of rare and transient structures, and just being able to observe them is already a re- S ome details of the complex structure studied by Tremblay's team: in the top- left are shown some arcs produced by gravitational lensing and the two interacting el- liptical galaxies; in the top-right are instead shown the nuclei of the two galaxies and the “pearl necklace” formed by the 19 stellar superclus- ters. [NASA, ESA, G. Tremblay/ESO] The graph on the left shows the pro- jected separation between adjacent superclusters in accordance with their number (1 kpc = 3,260 light- years). [G. Trem- blay, M. Gladders, S. Baum et al.]