Galaxies in a cosmic spider web made of dark matter
Using ESO’s Very Large Telescope (VLT), astronomers have found six galaxies orbiting a supermassive black hole when the universe was less than a billion years old. The galaxies are in a kind of cosmic «spider web» of gas that covers an area 300 times the size of the Milky Way. «The filaments of the cosmic web are like spider web threads,» explains Marco Mignoli, astronomer at the National Institute of Astrophysics (INAF) in Bologna, Italy, and lead author of the new research published today in Astronomy & Astrophysics. «The galaxies grow where the filaments cross, and gas flows – which are available to both the galaxies and the central supermassive black hole as fuel – can flow along the filaments».
The light from this large reticular structure, with its black hole of one billion solar masses, began its journey to us when the universe was only 900 million years old. «With our work, we have placed an important piece in the largely incomplete puzzle that is the formation and growth of such extreme but abundant objects so soon after the Big Bang,» says co-author Roberto Gilli, also an astronomer at INAF in Bologna, who refers to supermassive black holes.
The very first black holes, which are thought to have been formed by the collapse of the first stars, must have grown very fast to reach masses of one billion suns within the first 900 million years. But astronomers have struggled to explain how sufficient quantities of «black hole fuel» could have been available for these objects to grow to such enormous sizes in such a short time. The newly found structure offers a likely explanation: the «spider web» and the galaxies within it contain enough gas to provide the fuel that the central black hole needs to quickly become a supermassive giant.
But how did such large web-like structures come into being in the first place? Astronomers believe that huge halos of mysterious dark matter are the key. It is believed that these large regions of invisible matter in the early universe attracted huge amounts of gas. Together, the gas and the invisible dark matter form the reticular structures in which galaxies and black holes can develop.
«Our results support the idea that the most distant and massive black holes in halos of massive dark matter form and grow in large-scale structures, and that the lack of earlier evidence of such structures is probably due to observational limitations,» says Colin Norman of Johns Hopkins University in Baltimore, USA, who also collaborated on the study.
The galaxies now discovered are among the faintest that modern telescopes can observe. This discovery required several hours of observations with the largest optical telescopes available, including the VLT at ESO. Using the MUSE and FORS2 instruments on the VLT at ESO’s Paranal Observatory in the Chilean Atacama Desert, the team confirmed the connection between four of the six galaxies and the black hole. «We believe that we have just seen the tip of the iceberg, and that the few galaxies discovered so far around this supermassive black hole are only the brightest,» said co-author Barbara Balmaverde, astronomer at INAF in Turin, Italy.
These results contribute to our understanding of how supermassive black holes and large cosmic structures have formed and evolved. ESO’s Extremely Large Telescope, currently under construction in Chile, will be able to build on this research by using its powerful instruments to observe many more fainter galaxies around massive black holes in the early universe.