A pie in the Sky

Enrico Garaldi

On large scales, the Universe resembles a giant web. But the appearance of this cosmic web is very different when seen through different observables. To study the evolution of the Universe, astrophysicists use large supercomputers to run sophisticated numerical simulations, where the hydrodynamical and radiative transfer equations are solved on a grid. The movie shows some of the different looks of the cosmic web using a slice through the output from one of these simulations. The distribution of ordinary matter is given by the gas overdensity delta_b, shown in blue-to-yellow colors. The filamentary and self-similar structure constitutes the backbone of cosmic structures. However, this density can not be directly observed. What telescopes can see is the stellar light, that is shown here via the mass of stars M_* (black-to-yellow colors): galaxies form only in the densest knots of the web and therefore have a sparse distribution that, nevertheless, tracks the underlying matter field. While galaxies trace the dense regions, studying the light absorption by cosmic gas allows to determine the properties of diffuse low-density gas. For instance, its temperature T can be determined through the Lyman-alpha transition of neutral Hydrogen. This quantity is shown with a white-to-red colormap and features large warm regions that are heated up by the intense radiation coming from stars and accreting black holes. In a similar way, the absorption and emission lines from atoms heavier than Helium (‘metals’ in the astronomical jargon), allow to infer their abundance, called metallicity (Z), which is portrayed by a blue-to-grey colormap. Since metals are produced by exploding stars, they are mostly located in and around galaxies, where they are brought through galactic outflows driven by stellar winds. Finally, the future SKA telescope will enable a tomographic mapping of the distribution of the neutral Hydrogen fraction (x_HI) in the Universe using the hyper-fine transition of the Hydrogen atom). This quantity is depicted using a red-to-blue color scheme. The radiation from stars and accreting black holes turns the gas into a plasma everywhere but in the densest regions, where the large gas reservoir effectively shields some of this gas from the incoming radiation.

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