First Images Of The "Cosmic Web" Hint At Billions Of Undiscovered Dwarf Galaxies

One of the hydrogen filaments (in blue) superimposed to the Hubble image of the Ultra Deep Field. The filament stretches for 15 million light-years. Image Credit: Roland Bacon, David Mary, ESO and NASA

On a large scale, much bigger than galaxies, matter in the universe is distributed in filamentary structures we call the "cosmic web". It’s a vast network of clustered galaxies linked together by intergalactic gas. Observing the gas filaments is not easy and only recently have we been able to study them directly. Now, astronomers have observed several filaments of the cosmic web at once for the first time.

Reported in Astronomy & Astrophysics, the observations required an incredible 140 hours of observations of a single small area in the sky using the Very Large Telescope (VLT) and its MUSE (Multi-Unit Spectroscopic Explorer) instrument. The furthest filaments observed were in place when the universe was just 1.3 billion years old. The closest was 800 million years later. That was a crucial time for the cosmos as galaxies were forming stars and growing at an incredible pace.

The area observed in this ambitious campaign is already famous. It’s part of the Hubble Ultra Deep Field. The region was observed by the Hubble Space Telescope between late 2003 and early 2004. Since then, it has been refined over and over again, and it has remained the deepest image of the universe ever taken. The new observations massively expand on it.

cosmic web
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.  Image credit: © Roland Bacon, David Mary, ESO and NASA

Astronomers have previously estimated that about 10,000 galaxies are visible in this region of the sky. The number is destined to grow. In the parts of the Ultra Deep Field studied by the team, the number is 40 percent higher. And that's not all. There are likely to be many more galaxies that we can’t see.

The team employed a simulation to work out what it is illuminating the gas filaments of the cosmic web. The gas in these intergalactic connections is extremely diffused so if we can see it something may be illuminating it. The simulation had an answer for the team. There is a humongous number of galaxies there. The study suggests that a population of billions of dwarf galaxies is illuminating these filaments. But they are too small and faint for us to see them.

The detection of the cosmic web
The filaments of the cosmic web seen in this study show here according to the age of the universe at the time they emitted light. Image Credit: Roland Bacon / David Mary

"We think that the light we are seeing comes mainly from young galaxies, each containing millions of times fewer stars than our own Milky Way. Such tiny galaxies were likely responsible for the end of the cosmic 'dark ages', when less than a billion years after the Big Bang, the universe was illuminated and heated by the first generations of stars," co-author Joop Schaye from Leiden Observatory, said in a statement.

This finding will certainly start a big debate in the field. The most common hypothesis for the source of the illumination is believed to be a diffuse cosmic ultraviolet background produced by young stars in galaxies. This would heat up the filaments, making them glow. However, more observations will be needed to better understand the mysteries of the cosmic web.


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