Quasar Halos

An unexpected discovery has been made about some of the most distant objects in the Universe — quasars.

Their name stemming from “quasi-stellar object”, these phenomena are the active cores of distant galaxies, their extremely bright central point of light bearing a visual similarity to stars in telescope images. Each quasar’s light is coming from a supermassive black hole, which are thought to sit at the heart of most galaxies, as it consumes the material from a separate galaxy with which it is colliding and being sucked in by the black hole’s immense gravitational pull.

By studying some of these quasars with the MUSE instrument on the European Southern Observatory’s Very Large Telescope (VLT) in Chile, astronomers stumbled upon something intriguing: around every quasar in the sample there appeared to be a giant cloud of surrounding gas. The immense energy output from the central quasar illuminates these clouds, which extend far outside the host galaxies, causing them to glow and become visible to astronomers.

In this new study, a total of 19 quasars were observed. Previous efforts by astronomers have shown that only around 10% of such objects have shown the presence of such huge surrounding clouds of gas. The detection of clouds around every single quasar in the study throws up some problems for astronomers, who may now have to rethink some of what they thought they knew about how galaxies formed in the early Universe. Because these quasars are so distant, the light has taken many billions of years to reach us, meaning we are seeing how galaxies interacted and grew near the very beginning of the Universe.

On the largest scales, matter in the Universe appears to form a spindly frame of threads, a so-called “cosmic web”. The trouble is, the distribution of this web is usually incredibly difficult to detect and study. Quasars are very useful because they are part of the web and form bright points in it — like cities at night seen from above — which allow astronomers to piece together the web’s shape.

A further surprise discovery was the temperature of the giant gas clouds. Analysis showed that the gas was around 10,000 degrees Celsius, which, in astronomical terms, is quite cold. In fact, if the current theories of galaxy formation used by astronomers were correct, these gas clouds should be much hotter, upwards of a million degrees.

Image credit: P. Marenfeld and NOAO/AURA/NSF.


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