Free Astronomy Magazine September-October 2014

The light of background galaxies retains in- formation on their distance even after the distortion caused by lensing and conse- quently the reference lines appear signifi- cantly more redshifted –primary indicator of greater distance, and hence of greater recessional velocity. The same reasoning applies to any object lying superimposed –in perspective view– to a cluster of gal- axies, but actually closer to the observer: in this case the reference spectral lines are less redshifted than those of the cluster –pri- mary indicator of smaller distance, and hence of smaller recessional velocity. There is no doubt that both galaxies are lying at the same distance, given that they are sufficiently bright to produce two dif- ferent spectral lines systems even if exam- ined as a single object, when in reality the two spectra lay perfectly superimposed, creating only one. If any difference exist between their recessional velocity, they are beyond instrumental capabilities and do not exceed 300 km/s, which is far too small for not considering them as interacting gal- axies, whose halos currently overlap by ap- proximately 65,000 light-years –a figure surely destined to grow given that they are moving toward each other. After having confirmed that all players are located at the same distance, the only thing left to do for Tremblay’s team was to understand the nature of the about twenty brightest structures (19, to be precise). From the amount of ultraviolet radiation emitted, their colour and their rather com- pact shape, they could not be anything parable to the average of the cluster and in particular to that of the two galaxies that it seems to link. As al- ways in these cases, it is suf- ficient to locate the position of certain spectral lines in the light of the structures examined to derive the dis- tance of these latter. T he side video begins with a view of the famous Corona Bo realis asterism before zoom- ing in toward the centre of SDSS J1531+3414. [NASA, ESA, ESO]