Streams of gas, akin to rivers feeding vast oceans, nurture galaxies scattered across the cosmos. These streams, part of the enigmatic cosmic web, have remained elusive due to their faintness. While astronomers have been aware of the cosmic web for decades, its direct imaging, particularly in the darkest cosmic regions, has eluded them—until now.
The groundbreaking findings stem from the Keck Cosmic Web Imager, or KCWI, crafted by Caltech’s Edward C. Stone Professor of Physics, Christopher Martin, and his team. They have unveiled the first-ever direct light emitted by the most concealed segment of the cosmic web: the delicate, interwoven filaments stretching across the cosmos’s shadowy realms between galaxies. KCWI is stationed at the W. M. Keck Observatory atop Maunakea in Hawaiʻi.
The instrument’s name, Keck Cosmic Web Imager, reflects their hope to directly observe the cosmic web, a hope that has been realized. Galaxies in our universe take shape from swirling clouds of gas, which eventually coalesce into the stars illuminating these galaxies. Astronomers hypothesize that these cold, dark filaments traverse the cosmos to supply galaxies with the gas necessary to birth new stars.
In 2015, Martin and his team discovered compelling evidence for this cold-flow model of galaxy formation—a lengthy filament channeling gas into a massive galaxy. They employed a prototype of KCWI, the Cosmic Web Imager, at Caltech’s Palomar Observatory for this research. However, most of the cosmic web remains concealed in the void between galaxies, making it challenging to observe.
Martin compares their previous observations to glimpsing filamentary structures beneath a lamppost, while their recent breakthrough allows them to perceive these structures without the need for external illumination.
This noteworthy discovery is detailed in a paper published in Nature Astronomy.
Martin’s pursuit to unveil the cosmic web in its entirety dates back to his graduate studies. He believes that this comprehensive imaging of the cosmic web will equip astronomers with vital insights required to comprehend the intricate processes behind galaxy formation and evolution. Moreover, it can aid in mapping the distribution of enigmatic dark matter, which constitutes a significant portion of the universe’s matter composition yet remains shrouded in mystery.
“The cosmic web serves as the blueprint of our universe,” Martin asserts. “It houses the majority of normal, or baryonic, matter in our galaxy and serves as a direct indicator of dark matter’s whereabouts.”
The feeble glow of filaments
To directly observe the cosmic web, scientists employ sophisticated instruments known as spectrometers, which dissect light into a spectrum of various wavelengths. Within these spectra, hydrogen gas, the primary component of the cosmic web, can be identified through its most prominent emission line called the Lyman alpha line.
Christopher Martin and his team meticulously designed KCWI to detect these faint Lyman alpha signatures while creating a two-dimensional (2D) image of the cosmos, thus earning its title as an imaging spectrometer. This instrument’s initial iteration covers the “blue” section of the visible-light spectrum, encompassing wavelengths ranging from 350 to 560 nanometers. (The second component of this instrument, the Keck Cosmic Reionization Mapper or KCRM, specializes in the red, longer-wavelength portion of the visible spectrum and was recently integrated into the Keck Observatory.)
KCWI’s precision spectrometers can search for Lyman alpha signatures emanating from the cosmic web across a spectrum of wavelengths. Due to the universe’s expansion, which stretches light to longer wavelengths, gas located farther from Earth exhibits a redder Lyman alpha signature. The 2D images captured by KCWI at different light wavelengths can be superimposed to construct a three-dimensional (3D) map depicting the emission from the cosmic web. In this particular observation, KCWI scrutinized a region of space situated between 10 and 12 billion light-years away.
“In essence, we’re crafting a 3D blueprint of the cosmic web,” elucidates Martin. “We collect spectra for every point within an image across various wavelengths, and these wavelengths correspond to distance.”
Confusion with the diffuse light of space
Detecting the cosmic web poses a unique challenge due to its faint illumination, which can easily blend with the surrounding background light above Maunakea. This includes atmospheric glows, zodiacal light generated by sunlight scattering off interplanetary dust, and even the radiance from our own Milky Way galaxy.
To overcome this hurdle, Christopher Martin devised an innovative strategy for subtracting this unwanted background light from the target images. He explains, “We examine two different sections of the sky, labeled A and B. The filament structures appear at distinct distances in these two directions, enabling us to extract the background light from image B and subtract it from A, and vice versa, leaving only the structures. I conducted extensive simulations of this approach in 2019 to ensure its effectiveness.”
The outcome is a groundbreaking method that opens up “a completely new avenue for exploring the universe,” as Martin highlights.
Senior instrument scientist Mateusz Matuszewski adds, “With KCRM, the newly integrated red channel of KCWI, we can peer even further into the past. We’re incredibly excited about the insights this new tool will provide regarding distant filaments and the era when the first stars and black holes emerged.”
In an intriguing fusion of science and art, Martin collaborated with artist Matt Schumaker to translate cosmic web data into music for a project titled “Spiral, supercluster, filament, wall (after Michael Anderson).” This project pays tribute to the late astronaut Michael Anderson, who tragically lost his life in the Space Shuttle Columbia accident in 2003. Martin, envisioning the cosmic filaments as colossal violin strings, converted their masses into frequencies centered around the musical note of middle C. The result is a captivating auditory piece that can be experienced here.
Source: W. M. Keck Observatory
