Free Astronomy Magazine September-October 2014

GALAXIES dynamics in the fusion processes between large elliptical galaxies. One of the first things to comprehend is the origin of the gas (mostly hydrogen) that formed these superclusters, which could originate from the two elliptical galaxies, but it might also directly come from the very hot plasma halo (about 10 million kelvin) that perme- ates the intergalactic regions of SDSS J1531+3414. In the first case, the triggering of star formation would be the result of a strong compression of the gas and hence of an increase in density beyond a critical threshold; in the second case, a flow of rel- atively cool gas would have favoured the birth of accreting nuclei. The result does not substantially change, but the starting conditions and thus the properties of the gas on a different scale and its dynamical response are evidently different and re- vealing for the purposes of understanding the evolution of SDSS J1531+3414. Particu- larly interesting would be a possible source of gas from the inner or immediately pe- ripheral regions of the two elliptical gal- axies, as this would further enhance the rarity of the scenario, given the small mi- nority of galaxies that in a big cluster can retain sufficient free hydrogen to generate new stars. Whatever the origin of the gas flow, the physical processes that give way to the formation of superclusters (as well as individual stars) are well known and are linked to the so-called “Jeans instability”, which occurs when the internal pressure of an interstellar gas cloud is not sufficient to balance the gravitational collapse of one of its regions or, as in this specific case, a mul- T he images below high- light some of the characteristics of the central region of SDSS J1531 +3414. In the “A” frame is shown the oldest stellar component (orange) and the contours of the ultraviolet emis- sion attributable to young super- clusters (green); in “B the super- clusters in the vis- ible band; in “C” a detailed image in the near ultra- violet; in “D” a particularly con- trasted picture, with the number- ing adopted to identify the 19 superclusters. [G. Tremblay, M. Gladders, S. Baum et al.] A B C D