Astronomers captured images of the cosmic web for the first time

Cosmological simulation of the distant Universe. The image shows the light emitted by hydrogen atoms in the cosmic web in a region roughly 15 million light years across. In addition to the very weak emission from intergalactic gas, a number of point sources can be seen: these are galaxies in the process of forming their first stars. © Jeremy Blaizot / projet SPHINX


The Big Bang model and galaxy formation have predicted the gas filaments in which galaxies form, called the cosmic web. But, until now, we don’t have an image of such an object.


Using the MUSE1 instrument installed on ESO’s Very Large Telescope in Chile, astronomers now captured the cosmic web images in the early Universe for the first time.


Astronomers pointed the telescope at a single region of the sky for over 140 hours. The region selected forms part of the Hubble Ultra-Deep Field, which was the deepest image of the cosmos ever obtained.


The 2250 galaxies in the ‘cone’ of the Universe observed by MUSE are shown here according to the age of the Universe (in billions of years). The period of the early Universe (0.8 to 2.2 billion years after the Big Bang) explored in this study is shown in red. The 22 regions with galaxy over-density are indicated by grey rectangles. The 5 regions where filaments have been identified most prominently are shown in blue. © Roland Bacon / David Mary


It took them eight months to carry out this exceptional observing campaign. This was trailed by a year of data processing which interestingly uncovered light from the hydrogen filaments and images of several filaments as they were one to two billion years after the Big Bang.


However, the team’s biggest surprise was when simulations showed that the light from the gas came from a hitherto invisible population of billions of dwarf galaxies spawning a host of stars.


One of the hydrogen filaments (in blue) discovered by MUSE in the Hubble Ultra-Deep Field. It is located in the constellation Fornax at a distance of 11.5 billion light years, and stretches across 15 million light years. The image in the background is from Hubble. © Roland Bacon, David Mary, ESO and NASA


Although these galaxies are too faint to be detected individually with current instruments, their existence will have significant consequences for galaxy formation models, with implications that scientists are only just beginning to explore.


Senior author Roland Bacon, a scientist at the Centre for Astrophysics Research in Lyon, told AFP, “After an initial period of darkness, the Universe erupted with light and produced a huge number of stars.”


“One of the big questions is what ended that period of darkness, leading to a phase in the early Universe known as re-ionization.”


Cosmological simulation of a filament made up of hundreds of thousands of small galaxies. The image on the left shows the emissions produced by all the galaxies as it might be observed in situ. The image on the right shows the filament as it would be seen by MUSE. Even with a very long exposure time, the vast majority of the galaxies cannot be detected individually. However, the light from all these small galaxies is detected as a diffuse background, rather like the Milky Way when seen with the naked eye. © Thibault Garel and Roland Bacon


Until now, astronomers know that the web was limited to a few specific regions, particularly in the direction of quasars, whose powerful radiation acts like car headlights, revealing gas clouds along the line of sight. However, these regions do not disclose the entire network of filaments- where galaxies form.


Emanuele Daddi, a researcher at Atomic Energy Commission who did not take part in the study, said, “These findings are fundamental. We have never seen a discharge of gases on this scale, which is essential for understanding how galaxies form.”

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