Free Astronomy Magazine September-October 2014

GALAXIES titude of its regions. Each collapsed region forms one or more stars, which depending on their initial conditions gather in large or small clusters. Similar processes take place also in areas much closer to our daily life, as for example when it rains, with the water that instead of falling in the form of contin- uous strands takes the form of droplets. Although the scales here are manifestly dif- ferent, the temperature, pressure and den- sity play in both cases a determining role. But let us briefly examine more in detail those 19 superclusters connected to the two interacting ellipticals. In the HST images they all appear at least marginally resolved into stars and show a rounded shape, with slight asymmetries which in some cases re- semble sketched out spiral arms. On average they are spaced from each other by 3,000- 6,000 light-years, and overall they occupy an area extending up to 90,000 light-years (not much less than the Milky Way’s diameter). All appear projected toward the region where the two galactic halos overlap, con- firming that the interaction of the two el- lipticals is by no means unrelated to the exis- tence of those bright structures. The length of the hydrogen filaments winding through the superclusters reaches an average of 30,000 light-years. It is estimated that the rate of star formation within the 19 struc- tures is currently 5 solar masses per year, a figure comparable to that of the entire Milky Way. The uniqueness of that superclu- sters chain has motivated researchers to car- ry out new observations, using instruments capable of operating in spectral bands other than those already investigated with HST – i.e., ultraviolet and visible bands. For a bet- ter understanding of the processes taking place in the environment hosting those structures are in fact necessary multiband observations, both from the ground and from space, and this is precisely the reason why the centre of SDSS J1531+3414 will be shortly investigated with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Chandra X-ray space telescope. n A lthough seem- ingly com- plex, this diagram shows with sim- plicity the equidis- tance of the two galaxies hosting the superclusters. In the left column it can be seen that despite having taken a spectrum which includes both components (blue circle), the resulting set of lines is unique. In the right column, two separate spectra (taken with the Andalu- cia Faint Object Spectrograph and Camera) show maximum velocity offsets of about 280 km/s, insuffi- cient to consider the two galaxies as unrelated. [G. Tremblay, M. Gladders, S. Baum et al.]