How massive galaxy clusters assemble in universe revealed

Astronomers have recently discovered the likely precursors of the massive galaxy clusters that are seen today, with the help of combined observations of the distant Universe made with ESA's Herschel and Planck space observatories.

Galaxies like our Milky Way with its 100 billion stars are usually not found in isolation. In the Universe today, 13.8 billion years after the Big Bang, many are in dense clusters of tens, hundreds or even thousands of galaxies.

Astronomers found objects in the distant Universe, seen at a time when it was only three billion years old, which could be precursors of the clusters seen in universe today.

Planck's main goal was to provide the most precise map of the relic radiation of the Big Bang, the cosmic microwave background. To do so, it surveyed the entire sky in nine different wavelengths from the far-infrared to radio, in order to eliminate foreground emission from our galaxy and others in the Universe.

But those foreground sources can be important in other fields of astronomy, and it was in Planck's short wavelength data that scientists were able to identify 234 bright sources with characteristics that suggested they were located in the distant, early Universe.

Herschel then observed these objects across the far-infrared to submillimetre wavelength range, but with much higher sensitivity and angular resolution.

Herschel revealed that the vast majority of the Planck-detected sources are consistent with dense concentrations of galaxies in the early Universe, vigorously forming new stars.

Each of these young galaxies is seen to be converting gas and dust into stars at a rate of a few hundred to 1500 times the mass of our Sun per year. By comparison, the Milky Way galaxy today was producing stars at an average rate of just one solar mass per year.

While the astronomers have not yet conclusively established the ages and luminosities of many of these newly discovered distant galaxy concentrations, they are the best candidates yet found for "proto-clusters," precursors of the large, mature galaxy clusters that are seen in the Universe today.