Unprecedented Images Show The "Dim Matter" That Connects All Galaxies

Using the Cosmic Web Imager, astronomers from Caltech have captured the first three-dimensional images of the intergalactic medium (IGM) — the diffuse gas that connects galaxies throughout the universe.

Above: The Lyman alpha blob in emerging galaxy cluster showing gas filaments flowing into blob (as shown by arrows).


Astronomers have theorized about the presence of these filaments since the 1980s. Computer simulations have suggested that as the universe cooled after the Big Bang, most of its matter, including dark matter, congealed into a network of filaments that now span the cosmos. This primordial gas from the Big Bang shouldn't be spread uniformly throughout space, but rather through channels, or filaments, that extend across galaxies and flow between them. Earlier this year, astronomers were able to capture their first glimpse of this cosmic web using the Keck Telescope in Hawaii, which revealed a network of smaller and larger filaments zigzagging through intergalactic space.

Now, scientists from Caltech have used the Cosmic Web Imager (CWI) — a spectrographic imager that takes pictures at many different wavelengths simultaneously — to obtain their first three-dimensional images of the IGM. It's an opportunity for astronomers to get a better sense of the structure of these filaments, and to improve their understanding of galactic and intergalactic dynamics. And in fact, the CWI has already detected a possible spiral-galaxy-in-the-making that's about three times bigger than the Milky Way.

Comparison of Lyman alpha blob and a simulation of the ICW based on theoretical predictions.


The CWI was developed by Caltech professor Christopher Martin. He describes the diffuse gas of the IGM as "dim matter" as a way to discern it from the bright matter of stars and galaxies, along with the dark matter and energy that comprises most of the universe.

"Not only does it comprise most of the normal matter in the universe, it is also the medium in which galaxies form and grow," he noted in a statement.


A quasar showing surrounding gas (in blue) and direction of filamentary gas inflow.


The CWI allowed the astronomers to visualize these filaments, while also allowing them to measure composition, mass, and velocity. The first cosmic filaments observed by the imager reside near two very bright objects, a quasar and a so-called Lyman alpha blob in an emerging galaxy cluster. These objects were chosen because they're bright, lighting up the surrounding IGM and boosting its detectable signal.

Observations revealed a narrow filament — about a million light-years long — flowing into the quasar, possibly even fueling the growth of the galaxy that hosts the quasar. They also detected three filaments surrounding the Lyman alpha blob, with a measured spin that shows that the gas from these filaments is flowing into the blob and affecting its dynamics. These objects both appeared about two billion years after the Big Bang, a time of rapid star formation in galaxies.


"In the case of the Lyman alpha blob," added Martin, "I think we're looking at a giant protogalactic disk. It's almost 300,000 light-years in diameter, three times the size of the Milky Way."

Read the entire study at Astrophysical Journal: "Intergalactic Medium Observations with the Cosmic Web Imager: I. The Circum-QSO Medium of QSO 1549+19, and Evidence for a Filamentary Gas Inflow" and "Intergalactic Medium Observations with the Cosmic Web Imager: II. Discovery of Extended, Kinematically-linked Emission around SSA22 Lyα Blob 2." Supplementary information via Caltech.


Images: Christopher Martin, Robert Hurt.

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