By analyzing the light from a distant quasar, a team of astronomers has made the first direct observation ever of one of the largest of structures in the Universe: a cosmic web filament.
Decades of observations have shown the Universe’s structure to be hierarchical in nature. Stars are grouped together to form galaxies, which in turn form clusters and superclusters, and those superclusters are grouped together to create filaments—long threads of matter running through intergalactic space, interspersed with gigantic spaces of empty void. All the matter and energy in the Universe can be traced inside these filaments, creating a structure that observed from afar can be described as a cosmic “web.”
“A consequence of the Big Bang and the dominance of dark matter, is that ordinary matter is driven like foam on the crest of a wave, into vast interconnected sheets and filaments stretched over enormous cosmic voids—much like the structure of a kitchen sponge,” says Dr. Stefan Keller, a research fellow at the Australian National University’s Research School of Astronomy and Astrophysics. “Unlike a sponge, however, gravity draws the material over these interconnecting filaments, towards the largest lumps of matter.”
All the ordinary matter that we can observe in the Universe (like stars, planets, and people) is thought to account only for 4.9 percent of the Universe’s total mass-energy content. Another 26.8 percent is believed to consist of a different type of matter of completely unknown nature, called “dark matter,” with “dark energy” making up for the other 68.3 percent. Dark matter is a hypothetical, but as of yet unobserved, type of matter. Having no electromagnetic interaction with the rest of the Universe at all, its presence can be inferred only by its gravity.
Although the existence of cosmic filaments had been confirmed by many different observing campaigns during the past three decades, the three-dimensional distribution of matter inside them hadn’t been observed before. In a fascinating new paper published in the scientific journal Nature, a team of astronomers, led by Sebastiano Cantalupo, postdoctoral fellow at the University of California Santa Cruz, is detailing just such a study of a cosmic filament by observing the light coming from a distant quasar.
Quasars are the centers of very distant active galaxies, usually many billions of light-years away, and can be characterised as the most luminous and energetic objects in the entire Universe. With a diameter no bigger than that of our Solar System, they emit up to thousands of times more energy than our own Milky Way Galaxy, throughout the whole electromagnetic spectrum. Such huge amounts of energy coming from such relatively small objects indicate the centers of quasars are home to supermassive black holes, whose accretion of nearby material is driving all this massive energy output.
Cantalupo and his team made a series of observations during November 2013 of quasar UM 287, which is located approximately 9.5 billion light-years away. Using the 10-m Keck I telescope’s Low Resolution Imaging Spectrometer at the Keck Observatory on Mauna Key in Hawaii, the team imaged the quasar with a custom-built narrow-band filter. The filter was sensitive to Lyman-alpha light, which is the light emitted by ionised hydrogen when excited by a luminous, outside source. The astronomers wanted to specifically see if the light from UM 287 would reveal the presence of any nearby cosmic filaments of gas. What they found was exciting and unexpected. “This quasar is illuminating diffuse gas on scales well beyond any we’ve seen before, giving us the first picture of extended gas between galaxies. It provides a terrific insight into the overall structure of our universe,” says Dr. J. Xavier Prochaska, professor of Astronomy & Astrophysics at the University of California, Santa Cruz, and co-author of the study.
The team’s study unveiled the presence of a vast intergalactic nebula of ionised hydrogen approximately 1.5 million light-years wide—more than 10 times larger than our own Milky Way Galaxy. Light from the quasar was exciting the hydrogen inside the nebula, forcing it to glow by emitting ultraviolet light, which astronomers observed as Lyman-alpha emission lines on UM 287’s spectra in visible wavelengths, because of the Universe’s expansion which “stretched” the nebula’s ultraviolet light toward the visible part of the spectrum. “This is a very exceptional object: it’s huge, at least twice as large as any nebula detected before, and it extends well beyond the galactic environment of the quasar,” says Cantalupo.
According to the leading cosmological models of galaxy evolution and distribution of matter in the Universe, all the galaxies and galactic clusters that we observe today are just the luminous nodes of a vast interconnected network of cosmic filaments, spawning the entire Universe. All the primordial dark matter that existed after the Big Bang in time formed huge, invisible halos whose gravity attracted most but not all of the ordinary matter, making it to coalesce on top, like the icing on a cake. The left-overs of ordinary matter that weren’t pulled around these dark matter halos were dispersed as diffuse gas in the vast dark matter filaments that connect every single galaxy together.
Video Credit: Springel et al. (Virgo Consortium)/The Millennium Simulation Project
The observations by Cantalupo and his team are the first to show definite evidence for the existence of these filaments with the discovery of the gas around UM 287. The gas extends much further than the dark matter halos, which indicates it is not part of the quasar itself. “We have studied other quasars this way without detecting such extended gas,” says Cantalupo. “The light from the quasar is like a flashlight beam, and in this case we were lucky that the flashlight is pointing toward the nebula and making the gas glow. We think this is part of a filament that may be even more extended than this, but we only see the part of the filament that is illuminated by the beamed emission from the quasar.”
One surprising result from the study was that the observed nebula was approximately 10 times more massive than what computer simulations have predicted in the past. “We think there may be more gas contained in small dense clumps within the cosmic web than is seen in our models. These observations are challenging our understanding of intergalactic gas and giving us a new laboratory to test and refine our models,” Cantalupo said.
Even though the team’s results make for an exciting discovery, other scientists remain cautious. “The authors do make a convincing case that the emitting gas extends beyond the dark halo hosting the bright quasar, but that does not necessarily make it a large-scale filament,” observed Dr. Joop Schaye, professor of astronomy at the Leiden University in the Netherlands. The reason for this skepticism comes from the fact that this is the first time that such an observation was made, and more are needed to really confirm that what astronomers are seeing is indeed evidence of the Universe’s structure at the largest of scales. More observations would also help to better refine cosmological models and account for the discrepancy between theory and observations concerning the amount of mass inside cosmic filaments.
In the coming years, more refined searches will shed even more light on the distribution of matter between galaxies and galactic clusters. In the meantime, the recently published study by Cantalupo’s team represents a major breakthrough on our quest to solve one of the greatest cosmological riddles: how the Universe is really structured on the largest of scales.