A rare alignment of a quasar’s “flashlight” beam and a filament of the cosmic web illuminates the universe’s large-scale structure.
By now, most of us have seen the universe evolve — in simulations, that is. (See an example in the video below.) In these movies, gravity transforms the primordial soup into weblike structures of material, in which growing galaxy clusters are fed by gas that spools in along thin filaments of dark matter.
The simulations match the observed universe remarkably well. But although we can see galaxies throughout the universe, we rarely see the webs’ filaments. What little visible matter they contain is sparse gas — there are virtually no stars.
But Sebastiano Cantalupo (University of California, Santa Cruz and Lick Observatory) and colleagues think they’ve found a rare cosmic flashlight to illuminate a piece of the web.As the team reported online January 19 in Nature, the astronomers observed UM 287, a quasar that lived 3 billion years after the Big Bang. Using the 10-meter Keck I telescope in Hawaii, the team witnessed cold hydrogen emitting Lyman-alpha radiation underneath the spotlight of the quasar’s intense ultraviolet beam.
Lyman-alpha blobs (yes, that’s a technical term) have been found before, but their origin is still mysterious. Generally, they’re thought to be associated with a single galaxy. But this blob is twice as big as previous discoveries. And at 1.5 million light-years across, it’s far too long to be contained within UM 287’s host galaxy, or even the galaxy’s much larger dark matter halo. That’s why Cantalupo’s team thinks this blob may be a cosmic filament, aligned by chance with the quasar’s beam.Observers have seen hints of filaments before. Some reveal themselves indirectly when their gas absorbs light from distant quasars — different than the new study, which is of gas emission. And in a 2012 paper, Jörg Dietrich (University Observatory Munich, Germany) and colleagues reported the direct detection of a filament connecting two galaxy clusters, made visible by its gravitational distortion of background galaxies’ light and also by X-rays emitted from its hot gas. Unlike Dietrich’s study, Cantalupo’s study is looking at relatively cold gas.
“I believe the authors make a convincing case that they have indeed observed relatively cold filamentary gas,” Dietrich says. He adds that his team’s results and those in the new study are complementary, because they deal with gas in different states — one hot, one cold. “This will lead to a fuller understanding of filaments and their make-up on all scales.”
The UM 287 observations already point to a potential flaw in simulations. Going by its Lyman-alpha radiation, the blob stores an astonishing amount of mass — a trillion Suns’ worth — in its cold gas. But that amount is 10 times more than expected from cosmology simulations.
That’s not necessarily a surprise because “universe in a box” simulations tend to watch the cosmos evolve on extremely large scales, with “boxes” about 1 billion light-years across. “Small scales are not well resolved in simulations and gas processes on small scales are notoriously difficult to simulate,” Dietrich explains. So observations of individual filaments are the perfect laboratory for testing theory.